Satellite Laser Ranging Newsletter SLR Subcommission Of The CSTG

International Coordination of Space Techniques for Geodesy and Geodynamics

June 1995

CONTENTS

NEW SLR MISSIONS/PROGRAMS

• Asia-Pacific Space Geodynamics Project (APSG)
• Atomic Clock Comparison in Space Using Laser Ranging Techniques
• ADEOS/RIS
• ERS-2 Acquisition and Campaign
• Tether Physics and Survivability Experiment (TiPS)
• GFZ-1 Acquisition and Campaign

SLR MISSION UPDATES

• ERS-1 Tandem Mission
• LAGEOS-I and LAGEOS-II SLR Analysis at Center for Space Research
• TOPEX/Poseidon Precision Orbit Determination

STATION REPORTS

• Closing of Activities at Bar Giyyora
• FTLRS Status Report (June 1995)
• Helwan SLR Station Back in Operation
• NRL SLR Station Integrated at USAF PL SOR No. 7884
• The SAO-1/TLRS-1 1994 Collocation Experiment
• The Saudi Arabian Laser Ranging Observatory
• ZIMLAT: The New Zimmerwald Laser and Astrographic Telescope

SLR DATA AND ANALYSIS

• The IRV Force Model and Reference System
• Changes in Accessing SLR Full-Rate Data on the CDDIS
• Enhancements to SLR On-Site Data Availability
• WWW Home Pages for SGAPO and the CDDIS
• SLR Stations Operating During 1994

MEETING SUMMARIES

• EUROLAS Meeting Summary
• WEGENER/MEDLAS Program Summary
• WEGENER Proceedings from the St. Petersburg Meeting

EDITORIALS/OPINIONS

• Some Comments on the Global Network of SLR Stations, the Station Distribution and Performance

GENERAL

• Upcoming
• Publications
• Acronyms
• SLR Newsletter Information
• Mailing List Update

INTRODUCTION

This Newsletter is an activity of the Subcommission on Satellite Laser Ranging, a subcommission of the CSTG (International Coordination of Space Techniques for Geodesy and Geodynamics). The CSTG has been organized within the IAG, the IUGG, and COSPAR.

The purpose of this Newsletter is to facilitate the dissemination of information within the SLR community. Manuscripts and suggestions for articles to be published are welcome, as are other suggestions for improvement of the Newsletter.

As described in the first Newsletter (September 1986), the subcommission organization includes several subcommittees. The Newsletter provides a status report for some of the subcommittees. Other reports will be included in subsequent issues.

Please return the last page of this issue to be added to or removed from the mailing list.

NEW SLR MISSIONS/PROGRAMS

ASIA-PACIFIC SPACE GEODYNAMICS PROJECT (APSG)

Y. Shuhua/Shanghai Observatory

The Asia-Pacific region - particularly the western Pacific boundary zone including China, Japan and Southeast Asia (SEA), and the northern Indian Ocean boundary zone including Tibetan Plateau of China and SEA - is the convergence zone of four plates: Eurasian, Pacific, Philippine and Indo-Australian plates. This region is also the main-part of the tectonic system of new global mountain-building zones which is composed of the Round-Pacific mountain-building zone and the Alps-Himalayas mountain-building zone. This region is characterized by complex tectonics, violent motion, frequent and fierce earthquakes and volcanic activities. In this region, there are dense populations, rapidly developing economies, frequent and serious earthquakes, volcanic eruptions, sea immersion, etc. Therefore, this region is the most appropriate and the most urgent area for the study of crustal tectonic motion, local deformation, sea-level change and their effects on human environment. The main objective of the Asia-Pacific Space Geodynamics Project (APSG) is to unite with all forces of the region to perform cooperative researches on crustal plate motion, crustal deformation and sea-level change in this area, to provide basic information for the cause of formation for the major disaster events, and to enrich our knowledge of the planet Earth. All countries in the Asia-Pacific region are urged to join in this project, while countries external to this region are warmly invited to contribute. The project will promote international scientific exchange and cooperation and assist in raising the scientific research level of the developing countries in this region.

Primary Objectives of the Study

1. To measure and monitor, using space techniques, the relative motion between the Eurasian, Pacific, Philippine and Indo-Australian Plates including the plate tectonic motion near the boundaries as well as regional and local crustal deformation.
2. To study the evolution of crustal motion for the island-arc tectonic system in the western Pacific boundary zone and the mountain-building zone of Tibet and SEA and their forcing mechanisms.
3. To measure and monitor sea-level change in the Asia-Pacific region using space techniques including both satellite altimetry and regional tide gauge data, to study the characteristics and causes of the fluctuations of the global ocean surface as well as underlying mechanisms.
4. To investigate the variation of the spatial motion of the Earth as a whole and the mass motion within each layer of the Earth, including atmosphere, ocean, lithosphere, mantle and core, and their dynamic interactions.
5. To investigate the occurrence of natural disasters (earthquake, volcanic eruption, sea immersion etc.) and their relation with various Earth's motions, and to provide basic information on their underlying causes.

Preliminary Organization Form

The APSG project accomplishes its mission through the following components.

• Steering Committee
• Coordination Center
• Space Technique Development Center.
  • GPS Working Group.
  • SLR Working Group.
  • VLBI working Group
• Data Synthetic Analysis Center
  • Data Center
  • Analysis Center 1
  • Analysis Center 2 and so on
• Special Research Group 1:
  • Crustal Motion and Dynamics of the Tibetan Plateau
• Special Research Group 2:
  • Crustal Tectonic Motion of the Western Pacific Volcanic-Seismic Belt
• Special Research Group 3:
  • Western Pacific Eustatic Sea-Level Change

The Steering Committee (SC) consists of members who are representatives of some of the participating nations, agencies, and institutes. The Chairperson is a member of the SC and is elected by the Committee for a nonrenewable three-year period. Generally, the Chairperson will rotate among the participating countries. The SC exercises general control over the activities of the project including modifications to the organization.

The Coordination Center (CC) is responsible for the general management of the APSG consistent with directives and policies set up by the SC and provides the day-to-day liaison with external organizations. The primary functions of the CC are to coordinate APSG activities, organize APSG reports, meetings and workshops, and publish APSG documents and reports.

The Space Technique Development Center (STDC) consists of the GPS Working Group, the SLR Working Group and the VLBI Working Group. The STDC's responsibility is to develop and improve the software and hardware for GPS, SLR and VLBI, to organize APSG measurement campaigns for geodetic and geophysical research in the Asia-Pacific area and to coordinate these Working Groups' activities.

These Working Groups' tasks include the following: establish GPS, SLR and VLBI tracking networks in the Asia-Pacific area; organize measurement campaigns 1-2 times every two years; perform routine observations; format tracking data and transmit these data to the Data Center; and develop and improve the software and hardware for space techniques, such as GPS, SLR, VLBI.

The Data Synthetic Analysis Center (DSAC) is made up of the Data Center (DC) and Analysis Center (AC) 1, AC 2, and so on. The responsibilities of the Data Synthetic Analysis Center are to synthesize, analyze and compare the geodetic and geophysical information from different observation data (GPS, SLR, VLBI and satellite altimetry data etc.) and feed information on data quality back to the STDC.

The Data Center task is to collect formatted tracking data from the GPS, SLR, VLBI Working Groups or SRGs, to archive the data, to establish the data base, and to provide them to other APSG Working or Research Groups or Analysis Centers who need the data. Any institute or organization with the desire of becoming an Analysis Center in the Asia-pacific area may be so designated. They can perform any regional or global researches using APSG data and should present their research results to the DSAC.

Special Research Group 1: Crustal Motion and Dynamics of Tibetan Plateau

The Special Research Group (SRG) 1 will be in charge of the research on crustal motion and dynamics of Tibetan Plateau. Combining highly accurate actual measurement derived from space techniques, such as VLBI, SLR and GPS etc., with precise gravimetry (including superconducting and absolute gravimeters), ground leveling, seismological observations, ancient geomagnetic data and remote sensing images etc., the SRG 1 will gradually establish a more complete 3-dimensional quantitative model reflecting the present-day crustal motion of the Tibetan Plateau and its background field. Observation and research results will be reported to the CC.

Special Research Group 2: Crustal Tectonic Motion of Western Pacific Volcanic-Seismic Belt

The primary tasks of the Special Research Group 2 are the following:

• measure and monitor the crustal tectonic motion of the Western Pacific Volcanic-Seismic Belt by using space techniques, especially densified GPS observations;
• analyze the relationships between the crustal tectonic motion of the belt and the volcanic eruptions and earthquakes, then provide basic information for the underlying of the disasters in the Western Pacific Volcanic-Seismic Belt;
• forward the above research results to the CC

Special Research Group 3: Western Pacific Eustatic Sea-Level Change The main responsibilities of the Special Research Group 3 are to

• measure and monitor the western pacific eustatic sea-level change using space techniques, including SLR/VLBI, GPS and satellite altimetry and tide gauge data as well as absolute gravimeter data in the western Pacific area;
• obtain the displacement rate of each observation point relative to the fiducial point (SLR/VLBI stations) by analyzing the above various data, study the more refined features of present-day regional crustal motion and the intra-plate deformation in this area, measure the vertical motion of the crust where tide gauge stations are located, and separate the real sea-level fluctuation from the tide gauge observations;
• study long-term variability of relative sea-level and provide basic information on sea immersion in this area;
• report the above data and research results to the CC.

We welcome any country willing to contribute to join the APSG project. We also welcome other local or regional cooperative research projects into the frame of the project. Any other recommendations or proposals regarding the APSG project will be very much appreciated.

The APSG proposal will be presented at the IUGG XXI General Assembly in session G2, July 4 this year. A special meeting of APSG is arranged for the evening of July 6, to discuss the establishment of the APSG.

ATOMIC CLOCK COMPARISON IN SPACE USING LASER RANGING TECHNIQUES

E. Mattison/SAO

High-stability frequency references based upon quantum transitions in atoms (atomic clocks) play roles in science, technology and commerce ranging from testing Einstein's theory of equivalence to providing precise navigation through the Global Positioning System (GPS) to establishing international time scales. The atomic hydrogen (H) maser is the most stable frequency standard currently available for averaging intervals from 1 second to 104 seconds. H masers have been suggested for a variety of experiments in space, including tests of general relativity, searches for gravity waves, space-based Very Long Baseline Interferometry (VLBI) radioastronomy, precise time transfer among earth stations, and probes of the sun.

The Smithsonian Astrophysical Observatory (SAO) is currently engaged in a NASA- sponsored technology demonstration program, called Hydrogen Maser Clock (HMC), to design, construct and test in space a hydrogen maser that would be suitable for use in such suggested space experiments (Figures 1 and 2). The H maser will be carried to orbit by the Space Shuttle and will be attached to the outside of the Russian MIR space station for an extended period of operation and testing. MIR's orbit has an inclination of 51 degrees and an altitude of approximately 400 km.

A critical part of the HMC experiment is the use of laser ranging techniques to compare time kept by the space maser with time kept by clocks on the earth. The HMC instrument will be equipped with a pair of hemispherical retroreflector arrays, each of which contains a photodetector and a 10 ps resolution event timer. Each array contains 20 1-cm diameter reflectors, and together the arrays cover a spherical field of view. Laser pulses from Goddard Space Flight Center's MOBLAS-7 Laser Ranging Station (LRS) will be reflected from the retroreflectors. The times of a pulse's transmission from and re-arrival at the LRS will be recorded by an earth-based event timer slaved to an H-maser at the station; the average of these times will give the time of arrival of the pulse at the spacecraft in terms of the ground clock. The space-based photodetector and event timer will record the pulse's arrival at the spacecraft in terms of the space maser's time scale. By measuring the same event (arrival of a pulse at the spacecraft) in terms of both the space and earth clocks, we compare the two time scales and thus determine the performance of the space maser with the LRS H-maser and, by reference, with international time scales. A microprocessor that is part of the HMC instrument will record a host of maser operational parameters during the flight and telemeter the data to the ground. This housekeeping data, combined with the results of tests commanded from the ground during the mission and with the LRS-derived frequency information, will enable SAO scientists to evaluate the maser's frequency stability and to determine the role environmental effects play in the maser's performance.

An aim of the HMC program is to compare time between space and earth with a precision of 100 ps or better for a single pass of the spacecraft over the LRS, corresponding to a measurement of fractional frequency stability of approximately 1 part in 1015 from day to day. To correct the space maser's frequency for relativistic effects at this level of accuracy, the maser's altitude must be known to an accuracy of 1 meter, and its speed to an accuracy of 1 mm/second, averaged over the orbit. MIR's low orbit, extended geometry and variable attitude make it unfeasible to determine MIR's orbit by the use of laser ranging and dynamical calculations alone. As a consequence, relativistic corrections will be calculated from orbital data provided by a GPS receiver that will be part of the HMC instrument package.

ADEOS/RIS

N. Sugimoto/NIES

A large single-element retroreflector named Retroreflector In Space (RIS) will be on the Japanese Advanced Earth Observing Satellite (ADEOS). ADEOS will carry 8 international Earth-observing sensors including RIS. ADEOS has a sun-synchronous sub-recurrent orbit with a recurrent period of 41 days. The local time at the descending node is approximately 10:30. The inclination of the orbit is 98.6 degrees, and the altitude is 797 km. The launch of the ADEOS has been scheduled for February 1996.. Recently, however, NASDA informally announced a delay until August for technical reasons. Figure 1 and Table 1 show the spacecraft and list its characteristics.

RIS, a hollow cube-corner retroreflector with an effective diameter of 0.5 m, is an instrument provided by the Environment Agency of Japan (JEA) for experiments on laser long-path absorption measurement of atmospheric trace species. A new hollow retroreflector design was developed for RIS to optimize the pattern of the reflected beam. A spherical mirror with very small curvature is used for one of the three retroreflector mirrors. Velocity aberration caused by the satellite movement is compensated by the effects of the curved mirror and the spoiled dihedral angles between the curved mirror and the plane mirrors[1]. RIS has high reflectance in the wavelength range from 350 nm to 14 microns.

The National Institute for Environmental Studies (NIES/JEA) and the Communications Research Laboratory (CRL) plan the measurements of atmospheric trace species such as O3, CFC12, HNO3, CO, N2O, CH4 using pulsed CO2 lasers and their second and third harmonics from Tokyo.

The RIS science team is listed in Table 2. (The ADEOS Research Announcement is still open for late proposals.)

Besides the experiment planned by NIES and CRL from Tokyo, research themes are solicited by the ADEOS Research Announcement by NASDA and JEA. At present, the following research subjects using RIS from independent ground stations are selected.

1. "Laser Ranging and Atmospheric Measurements Using RIS from the UK" (PI) M. J.T. Milton (National Physical Laboratory, United Kingdom)
2. "Dualcolor Satellite Laser Ranging Measurements Using RIS" (PI) U. Schreiber (Research Establishment Satellite Geodesy of the Technical University of Munich, Germany)
3. "Precise Determination of the ADEOS Orbit by Laser Ranging Technique" (PI) A. Sengoku (Hydrographic Department Maritime Safety Agency, Japan)

NIES and CRL developed an active satellite tracking method for the experiment to be conducted from Tokyo. This method uses the image of RIS illuminated by a laser at 532 nm with a beam divergence of approximately 1 mrad. The large divergence is required because the error in the orbital predictions of ADEOS provided by NASDA based on telemetry is 1 km (3 sigma). However, to carry out experiments from the stations without the active tracking, the precise orbital prediction based on SLR will be required. For that reason, the RIS science team requests tracking support from the international SLR community. Also, NIES/JEA is asking NASA for in tracking, data distribution, and archiving services.

The construction and the tests of the RIS Flight Unit (RIS PFM) has been completed. The interferogram of the RIS PFM agreed very well with the theoretical one. The difference in the three dihedral angles are only 4 microrad, 1.7 microrad and 1.7 microrad. The ground tracks of ADEOS and the reflected laser intensity from the actual RIS in orbit are being calculated for the ground stations at various latitudes. The results will be reported later.

[1] A. Minato, N. Sugimoto, Y. Sasano, Appl. Opt. 31, 6015-6020 (1992).

ERS-2 ACQUISITION AND CAMPAIGN

F.-H. Massmann/GFZ

The ERS-2 Mission

The ERS-2 mission is the follow-on mission of ERS-1 (ESA Remote Sensing Satellite), which has been operating in orbit since July 1991. So the mission goals, the spacecraft and its payload as well as the orbital characteristics are identical with those of ERS-1. The ERS-2 satellite is a direct copy of the successful operating ERS-1 satellite with the following exceptions:

• The along-track scanning radiometer is now equipped with six channels in the infrared and visible parts of the spectrum. The three channels in the visible spectrum are new and are used to observe vegetation.
• The PRARE (Precise Range and Range Rate Equipment) instrument was modified and two units were installed on ERS-2. PRARE is a weather independent microwave instrument providing range and Doppler measurements with high precision as well as X-/S-band delays for ionospheric studies.
• The GOME (Global Ozone Monitoring Experiment) is an entirely new passive instrument which will monitor the ozone content of the atmosphere with a precision hitherto unobtainable from space.

ERS-2 was successfully launched by an Ariane-4 launcher from Kourou on April 21st 1995 at 01:44 UTC. Within the first few hours, all antennas were deployed successfully and by the beginning of May the platform and all instruments were checked and switched-on without major problems. On May 8, the three month ERS-2 Commissioning Phase started, during which the instruments are being calibrated and the procedures within the ESA ground segment will be validated. For PRARE and GOME, a six month Commissioning Phase will be undertaken.

During the ERS-2 Commissioning Phase, ERS-1 will remain the operational satellite, after which ERS-2 will then take over the operational tasks. During the first nine months (August 95 through April 96), ERS-1 will be operated in parallel with ERS-2 (the so called TANDEM-Mission, see separate article).

The ERS-2 Orbit

The orbit was chosen by ESA to have a 35 day repeat cycle throughout the mission (ESA 1994). After launch, a series of maneuvers were executed in order to position ERS-2 in the same orbit as ERS-1 but with ERS-2 being one day behind ERS-1, to achieve the 35 day repeat orbit and to synchronize ERS-1 and ERS-2 such that the ground tracks agree within +/-200 m. On April 28, 1995, the 35 day repeat orbit was achieved, and on May 12, 1995, the synchronization was perfectly done.

SLR Tracking

On the basis of the information from the Mission Management and Control Center in Darmstadt Germany, orbit predictions were generated by GFZ/D-PAF and distributed to the global SLR (Satellite Laser Ranging) network shortly before launch. Due to the number of maneuvers in the beginning of the mission, SLR tracking from the global network was delayed until after the maneuver of April 24, 1995.

The first observed ERS-2 passes were reported simultaneously from the SLR stations in Riga Latvia and Metsahovi Finland on April 24, 1995, at 18:50 UTC (before the maneuver). Since then almost the whole SLR network has started tracking, supported by GFZ/D-PAF with orbit predictions and time bias functions like those for ERS-1. Figure 1 shows today's status of the SLR tracking since launch (see also the article on the ERS Tandem Mission).

The SLR tracking started as planned on April 24, 1995 and, after a few days, additional stations joined the ERS-2 tracking community. Between 5 and 14 passes are being tracked by 4 to 10 stations per day. This is slightly less than for ERS-1 (86% of ERS-1 tracking data) and can partly be explained by the absence of some stations (Maidanak, Komsomolsk, Mendeleevo, Katzively). Thanks to the good cooperation between GFZ/D-PAF and the SLR network and due to the high level of competence of the SLR stations the ERS-2 SLR acquisition went smoothly.

PRARE Tracking

After the successful test on the Russian Meteor-3/7 spacecraft the PRARE system is now operating on the ERS-2 satellite. The PRARE2 system was successfully activated on April 26, 1995 and a few hours later the first contacts were established with the Master station in Oberpfaffenhofen and the Monitoring station in Stuttgart Germany. On April 28 the time synchronization was performed and on May 3 the first tracking data were stored in the space segment and later dumped to the Master station. Since then a number of tests have been performed with the space segment (Reigber 1995). All showed nominal behavior. The procedures of the Master station (decoding, preprocessing, generation of clock model, generation of normal points) were carefully checked. All tests showed a nominal behavior of the control segment. The next step will be the check of the calibration tables.

A set of 29 PRARE ground systems have been manufactured and sold to different institutes. Due to the late delivery of some stations, to date only a subset of systems have been installed and are operating either still for Meteor-3 or already for ERS-2. The actual network is displayed in Figure 2. The switch to ERS-2 for all the stations will be performed when the whole chain has been tested. A number of stations will come up in the very near future (Norway, Ascension Island, Austin/Texas etc.). The Commissioning Phase will last six months, but it is expected that calibrated values will be available earlier.

References

ESA 1994: ERS-2 High Level Operation Plan, ESA/EOPAG(94)11 Rev. 1, 29 July 1994.

Reigber, Ch.: PRARE2 Bulletin Board Info #1 and #2, GFZ Potsdam, May 1995

TETHER PHYSICS AND SURVIVABILITY EXPERIMENT (TiPS)

A. Peltzer, W. Purdy, and S. Coffey/NRL

The Tether (i) Physics and Survivability (TiPS) experiment is intended to be the first long term tethered satellite flight experiment. Two end bodies, shown in Figure 1, weighing 90 and 20 pounds respectively, will be connected by a four kilometer long tether. The system will be placed in a 550 nautical mile circular orbit with an inclination of 63.4&degree;. The tethered system will be jettisoned from a host spacecraft. The tether will be deployed after a period of separation from the host spacecraft. The TiPS experiment is presently planned to be operational in the spring of 1996.

There are two primary objectives to the TiPS experiment:

1. Evaluate the long term gravity gradient dynamics of a tethered system.
2. Gather data on the survivability of the tether in the space environment.

No tether experiment to date has been on orbit longer than about one month. As such, there is relatively little data on the long term dynamics of these systems. The dynamics of particular interest include in-plane and out of plane libration, system damping, orbital perturbations and environmental effects. The TiPS experiment is the first designed to maximize the survivability of the tether. The tether is predicted to have a mean life of 5 years before it is cut by orbital debris. The observed life of TiPS will help understand and validate these theoretical predictions. A schematic of the tether dynamics experiment is shown in Figure 2.

The TiPS experiment will have its dynamics measured by Satellite Laser Ranging (SLR) and ground based radars. The SLR will provide very accurate measurements of the positions of each end body. The length and libration angles of the system will be determined repeatedly over a long period of time from these measurements. Analysis of this data will be compared to theory to improve the understanding of tether dynamics. The SLR data will be available over Internet. Each end body will be covered with 18 uniformly spaced laser retroreflectors to enable the ground based laser ranging. The retroreflectors on one body will be coated to reflect light from 420 nm to 850 nm. The retroreflectors on the other body will be uncoated. These two types of retros will enable differentiation between the two end bodies. The laser ranging operations are planned to last for a period of at least 6 months after TiPS deployment. The U.S. Space Command network of radars will be utilized to support the laser ranging and to provide continuing observation of the system for the duration of the TiPS lifetime to determine if or when the tether has been cut.

GFZ-1 ACQUISITION AND CAMPAIGN

R. Knig/GFZ

General

The German GeoForschungsZentrum Potsdam (GFZ) awarded a contract for design, manufacture and launch of the satellite GFZ-1 to Kayser-Threde, Munich. GFZ-1 has been built under subcontract by the Russian Institute of Space Device Engineering, Moscow. Only one year elapsed between the signing of the contract to the satellite's release in space only one year elapsed.

GFZ-1 is a small passive satellite, spherical in shape and equipped with retro-reflectors. GFZ-1 is tracked by an international network of ground-based SLR (Satellite Laser Ranging) systems. SLR is performed with centimeter accuracy to obtain high-precision determinations of the satellite's motion. The objective is to improve the knowledge of the Earth's gravity field. GFZ-1 is the lowest orbiting geodetic satellite at present. The low altitude permits determination of the Earth's gravity field with increased sensitivity.

Technical Characteristics

• Massive satellite body made from bronze
• Mass 20.6 kg
• Diameter 21.5 cm
• retro reflectors (fused quartz prisms)

Orbital Characteristics

• Circular orbit
• Initial height 398 km
• Orbital inclination 51.6 degrees
• Nominal lifetime in orbit 4 years

Launch Scenario

The cargo spacecraft PROGRESS M-27 carrying GFZ-1 in its transport housing was launched April 9, 19:34Z, from Baikonur. PROGRESS M-27 docked at the MIR SS (space station) April 11, 1995, 22:01Z. GFZ-1 remained in its housing in the SS until April 18, when it was unpacked for a visual inspection. GFZ-1 was then installed in the lock chamber. The separation of GFZ-1 from the MIR SS was nominally conducted on April 19, 1995, at 19:12Z over the Southern Pacific shortly before the orbit crossed Southern America. The separation time of GFZ-1 from MIR was chosen so that the separation could be filmed with a video camera. The video signal was directly transmitted to the operations control center in Oberpfaffenhofen. The separation delta velocity of GFZ-1 with respect to MIR was given at 1.8 m/s opposite to the MIR flight direction. During the first minutes GFZ-1 flew behind MIR. Approximately 30 minutes after separation GFZ-1 overtook the MIR SS approximately 2 km below and from then on flew in front of the MIR SS with increasing distance.

SLR Tracking

The European SLR stations had the first opportunity to track GFZ-1 approximately half an hour after separation. The SLR stations Grasse in France and Graz in Austria reported good accuracy of the orbit predictions. Definite laser returns were received by the NASA station in Greenbelt, Maryland, on April 20, 00:21Z. The number of observed SLR passes for April 1995 can be seen in Table 1.

Table 1. Number of Passes in April 1995

			Sta.	19  20	21  22	23  24	25  26	27  28	29  30	Sum   Site Name
			------	--  --	--  --	--  --	--  --	--  --	--  --	---  ------------
			1864	 .   .	 .   1	 .   .	 .   .	 .   .	 .   .	  1  Maidanak2
			1884	 .   .	 .   .	 1   .	 1   .	 .   .	 .   .	  2  Riga
			1953	 .   .	 1   .	 .   .	 .   1	 1   .	 1   .	  4  S. de Cuba
			7105	 .   1	 .   .	 .   .	 .   .	 .   .	 .   .	  1  Greenbt ML
			7109	 .   .	 2   2	 1   1	 1   .	 .   .	 .   .	  7  Quincy
			7110	 .   .	 .   .	 1   1	 2   1	 .   2	 .   .	  7  Monu. Peak
			7210	 .   .	 1   1	 .   .	 .   .	 .   2	 .   .	  4  Haleakala
			7403	 .   .	 1   1	 .   .	 .   .	 .   .	 .   .	  2  ArequipaT3
			7836	 .   .	 1   3	 2   1	 1   1	 .   .	 .   .	  9  Potsdam
			7839	 1   2	 2   .	 .   .	 .   .	 .   .	 .   .	  5  Graz
			7840	 .   3	 .   2	 .   .	 2   .	 .   2	 .   .	  9  Herstmonceux
			======	==  ==	==  ==	==  ==	==  ==	==  ==	==  ==	===  ============
			Totals	 1   6	 8  10	 5   3	 7   3	 1   6	 1   0	 51  11 stations

During May 1995, GFZ-1 was in daylight most of the time at all stations in the Northern hemisphere. In June 1995, the passes will occur at night again. The day/nighttime tracking schedule for 1995 is shown in Figures 2 and 3 for the SLR stations Potsdam and Yarragadee as examples.

In Figures 2 and 3, nighttime is shaded. The passes are marked by crosses or squares, where the squares indicate passes being illuminated at least partly by the Sun and thus visually accessible.

The figures show that there are monthly periods between day and nighttime tracking. The acquisition becomes particularly difficult during the daylight. A good orbit prediction accuracy required to support daylight ranging can only be maintained if the SLR network acquires an adequate number of passes. Figure 3 shows opposite day/nighttime periods with respect to Figure 2. Thus, nighttime acquisition periods alternate between the stations in the Northern and Southern hemispheres. Therefore, the Southern stations play an important role in the mission.

Project Manager Technical Manager / Point of Contact Prof. Ch. Reigber Dr. Rolf Knig GeoForschungsZentrum Potsdam (GFZ) GeoForschungsZentrum Potsdam (GFZ) Div. Kinematics and Dynamics of the Earth Div. Kinematics and Dynamics of the Earth Telegrafenberg A17 GFZ/D-PAF, Postfach 1116 D-14473 Potsdam, GERMANY D-82230 Oberpfaffenhofen, GERMANY

SLR MISSION UPDATES

THE ERS TANDEM MISSION

F.-H. Massmann/GFZ

Introduction

The launch of ERS-2, while ERS-1 is still in orbit and in good condition, provides a unique opportunity for a parallel operation of two identical Earth observation satellites. After long discussions to cover the additional expenses, ESA has decided to operate both satellites in tandem for 3 plus 9 months. The tandem scenario is described below (ESA 1995). A detailed list of the potential tandem benefits can be found in ESA 1994a.

Status of the ERS-1 Mission

ERS-1 is still in very good condition and is expected to continue operating for at least another year. On July 17, 1995 it will be four years in orbit, although the planned lifetime was only 2-3 years. Since the end of March 1995, ERS-1 is again in the nominal 35 day repeat orbit. ERS-1 has been reasonably well tracked by the global SLR network. On the average, 12 passes from about 7 to 10 stations are available per day. This allows us to compute the orbit with an accuracy of about 10 cm radially.

Status of the ERS-2 Mission

ERS-2 was launched successfully on April 21, 1995 with an Ariane launcher. After passing the Launch and Early Operations Phase, in early May the ERS-2 Commissioning Phase was undertaken which will last until August 1995. Within this three month period, all instruments (for PRARE and GOME, 6 months) will be validated and calibrated; the data flow and the processing chains within the ERS ground segment will also be checked. ERS-2 will remain in the 35 day repeat cycle orbit for the entire mission. Both ERS satellites are in the same orbit with ERS-2 being one day behind ERS-1. On average, 9 passes from 5 to 9 stations are observed by the SLR stations per day.

Tandem Mission during the ERS-2 Commissioning Phase

During the ERS-2 Commissioning Phase, ERS-1 will continue to provide the nominal services to the users. There will be no distribution of ERS-2 data to the users during this period except to those institutions supporting the ESA calibration and validation activities. Within this phase, the compatibility of the ERS-1 and the ERS-2 SAR will be checked, and multi-temporal images with ERS-1 and ERS-2 data will be generated. Also is it planned to verify interferometry with ERS-2 data alone as well as with both ERS-1 and ERS-2 data together. The ERS-2 data products generated during the Commissioning Phase will be available for the users at the end of the phase. Figure 1 depicts the SLR tracking for ERS-1 and ERS-2 up to now. As can be seen, the number of passes for ERS-2 is about 86% that of ERS-1. This is partly caused by the absence of some stations in Russia and Uzbekistan.

After the Commissioning Phase, ERS-2 will be operated in accordance with the ERS-2 High Level Operation Plan (ESA 1994b) and provide nominal service to users, while ERS-1 will be dedicated to the Tandem Mission. During this period, different kinds of tandem operations are planned

• by the same instrument on both satellites over the same area on the ground;
• by the same instrument on both satellites over different areas on the same day;
• by different instruments over the same area at very short interval (&lt1h);
• by one instrument of ERS-1 over an area where mission rules normally prevent acquisition.

In order to support all user interests optimally, there will be a change in the offset between the two satellites from one day to eight days. The schedule is as follows

• a two week transition between the ERS-2 Commissioning Phase and the Tandem Mission (August 1995)
• three 35-day cycles at one day offset between ERS-1 and ERS-2 (until mid-December 1995)
• five 35-day cycles at eight days offset (until May/June 1996)

This implies intensive SLR tracking support for both satellites at least until June 1996. Afterwards, the ERS-1 payload will probably be switched-off, and the spacecraft will be followed only by S-Band tracking.

References

ESA 1994a: ERS-1/ERS-2 TANDEM Scenario; ESA/EOPAG(94)1, January 1994
ESA 1994b: ERS-2 High Level Operation Plan, ESA/EOPAG(94)11 Rev. 2, October 1994
ESA 1995: The ERS Tandem Mission: Status and Plans; ESA/PB-EO/DOSTAG(95)2, May 1995

LAGEOS-I AND LAGEOS-II SLR ANALYSIS AT CENTER FOR SPACE RESEARCH

R. Eanes/UTCSR

This note summarizes some results from the routine weekly analysis of LAGEOS-I and LAGEOS-II SLR quick-look (QL) data performed at the University of Texas Center for Space Research during 1994. Weekly analysis reports starting from 1995 are available via anonymous ftp from ftp.csr.utexas.edu in directory pub/slr/weekly or via the World Wide Web at http://www.csr.utexas.edu. Older reports can be obtained by requests to Richard Eanes by e-mail at eanes@csr.utexas.edu or fax at (512) 471-3570.

QL observations were obtained from 46 different sites during 1994. Twenty of these sites tracked more than 50 passes of either LAGEOS-I or LAGEOS-II, and 16 tracked more than 100 passes. Some statistics for these 16 systems are given in Table 1 below. The best systems produce 2-minute LAGEOS-I normal points with precisions better than 5 mm, pass bias RMS values under 10 mm and total residual RMS values below 20 mm.

The SLR residuals were obtained by using SSC(CSR)95L01 for station positions, EOP(CSR)95L01 for earth orientation and 3-day orbital arcs in which 5 orbital excitations were adjusted simultaneously with the 6 initial conditions. The orbital excitation time series contain important geodynamic information about Earth's gravitational field and its temporal variations due to tides and mass transfer in the atmosphere and oceans. The weighted RMS of these SLR residuals for the 122 arcs during 1994 is just 16 mm, but the information content of the data has not been completely removed at this stage in the analysis.

At CSR the residuals from this type of adjustment are then used to look for systematic variations of SLR system biases, diurnal and semidiurnal variations in the Earth orientation parameters and geocenter, and low frequency motion of the geocenter with respect to the terrestrial reference frame origin. One recent adjustment of this sort using LAGEOS-I data from October 1993 through May 1995 fit the global SLR data to 12 mm RMS. Improvements to be implemented in the near future for modeling and/or estimating vertical station motion from atmospheric pressure loading and residual ocean loading are expected to reduce the RMS of the SLR residuals to near the 10 mm level.

At this point in the cyclic race between SLR system accuracy and geodynamic modeling accuracy the modelers have almost caught up again. It will be important for the designers and operators of SLR systems to do everything possible to further reduce the ranging errors, in particular by improving calibration procedures to avoid system bias changes that are apparent in the observation records for many sites. As long as the feedback from analysts to observers continues to improve I am confident that this goal will be accomplished in the near future.

Notes:

Total RMS = RMS of SLR residuals from 3-day orbit fits

NP Precision = RMS of pass residuals after trend removal

Pass Bias RMS = Weighted RMS of pass range bias estimate

TOPEX/POSEIDON PRECISION ORBIT DETERMINATION

J.A. Marshall/NASA GSFC

Since its inception, the joint USA/French TOPEX/POSEIDON Mission (T/P) has been a driving force behind advancements in precision orbit determination. To get maximum scientific benefit from the satellite's two radar altimeters, mission requirements dictated that the error in the knowledge of the spacecraft's radial position should not exceed 13 cm root mean square (rms). Initially, gravity field mismodeling was the limiting error source. However, with gravity field improvements, nonconservative force model errors assumed an equal prominence. These models have reached a sufficient level of sophistication such that T/P ephemerides with 2-3 cm radial orbit accuracy are routinely produced via direct Satellite Laser Ranging (SLR) and Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS) tracking data. SLR data has been critical in improving these models and attaining this phenomenal level of precise orbit determination.

SLR observations have permitted major advances in gravity modeling [Nerem, 1994]. The laser geodetic satellites are of particular importance in the development of contemporary gravity models. By design, these solid, dense spheres reduce both the magnitude and complexity of their surface forces. Because separation and modeling of conservative and non-conservative forces acting on these satellites is more achievable than with complex satellite forms, they have provided the most important data for geopotential recovery. SLR observations acquired on STARLETTE, LAGEOS-1&2, AJISAI, and STELLA account for the dramatic improvements seen in the long wavelength static and tidal geopotential fields. SLR is the most highly weighted data-type within all of the current standard gravitational models.

Accurate modeling of nonconservative force, such as direct solar radiation pressure, albedo, atmospheric drag, and spacecraft thermal imbalances, can encompass extreme levels of complexity. It is therefore common to adopt sets of assumptions and a simplified satellite form representation which meet the specific requirements. Current research is focused on approximating the complex shape, attitude and surface optical properties as the equivalent combination of flat plates aligned in inertial space, according to the satellite's attitude control laws. Parameters associated with each plate can be tuned with satellite tracking data to reflect the actual on-orbit acceleration history. Such an approach was essential to reach the level of accuracy now obtained with a complex satellite like T/P [Antreasian and Rosborough, 1992; Marshall and Luthcke, 1994a, 1994b; Ries et al., 1993].

Even with these conservative and nonconservative force model improvements, evidence demonstrated that small errors in the T/P orbit remained. Fortunately, T/P carries four independent tracking data systems: SLR, DORIS, GPS, and the Tracking and Data Relay Satellite System (TDRSS). A comparison of ephemerides generated from these different tracking types provides a unique opportunity to assess the radial orbit error contained in the T/P orbits.

A traditional dynamic orbit determination methodology is used to compute the Precise Orbit Ephemeris (POE) found on the mission GDR's from the SLR and DORIS measurements. This approach is dependent upon, and inherently limited by, detailed modeling of the complete set of forces acting on the T/P spacecraft as well as all components of and corrections to the tracking measurements [Marshall and Luthcke, 1994a, 1994b; Nerem et al., 1993, 1994; Tapley et al., 1994a].

An experimental GPS receiver was flown as a demonstration of its satellite-satellite tracking capabilities and has yielded orbit accuracies comparable to those obtained from SLR/DORIS [Bertiger et al., 1994; Yunck et al., 1990]. The dense temporal and 3-dimensional spatial coverage of this data type permits the use of a reduced dynamic orbit determination strategy. In the GPS reduced dynamic approach, residual dynamic force modeling error is significantly reduced [Wu et al., 1991; Yunck et al., 1990, 1994]. However, accurate GPS satellite positions, antenna phase center locations, and an appropriate reference frame realization are all potential error sources for the reduced dynamic approach. Consequently, while still imperfect, the reduced dynamic ephemerides produced from the GPS data are likely to have errors which differ in character from those found within the NASA POEs (SLR/DORIS). Therefore, a comparison of ephemerides computed from these methods can effectively isolate the unique error characteristics of each.

This comparison resulted in three significant conclusions: First, further improvement of the gravity field and tide models was necessary. This entailed upgrading to the JGM-3 gravity field model [Tapley et al., 1994b] and a higher resolution tide model tuned with T/P altimetry data [Ray, 1994]. Second, the SLR residuals were well above the noise level and a factor of two increase of in their weight in the orbit determination solution relative to the DORIS data was warranted. Third, the SLR and DORIS data were of sufficient temporal density to support more frequent adjustment of empirical parameters which further reduce force modeling errors. These parameterization upgrades have resulted in a reduction of the T/P radial orbit error from 3-4 cm rms to 2-3 cm rms [Marshall et al., 1995]. An orbit error budget is presented in Table 1. Reference frame differences are now the limiting factor in agreement between the SLR/DORIS and GPS T/P orbits.

Future efforts to increase the T/P orbit accuracy will focus on nonconservative force modeling upgrades and reference frame definitions. Nonconservative force modeling improvement will most likely result from the simultaneous processing of all the tracking data types which will provide the dense tracking data coverage required. The success of this approach assumes a resolution to the reference frame discrepancies. Further developments in this arena depend on SLR since it provides the most accurate and unambiguous range measurements on an observation-by-observation basis and is the principal connection to the geocenter. Although this potential improvement may seem modest, it is nonetheless crucial to the oceanographic investigations which depend upon T/P altimetry data.

Antreasian, P. G., and G. W. Rosborough, Prediction of radiant energy forces on the TOPEX/POSEIDON spacecraft, J. Spacecraft and Rockets, 29(1), 81-90, 1992.
Bertiger, W. I., Y. E. Bar-Sever, E. J. Christensen, E. S. Davis, J. R. Guinn, B. J. Haines, R. W. Ibanez-Meier, J. R. Jee, S. M. Lichten, W. G. Melbourne, R. J. Muellerschoen, T. N. Munson, Y. Vigue, S. C. Wu, T. P. Yunck, B. E. Schutz, P.A.M. Abusali, H. J. Rim, M. M. Watkins, and P. Willis, GPS precise tracking of TOPEX/POSEIDON: Results and implications, J. Geophys. Res., TOPEX/POSEIDON Special Issue, Vol. 99, No. C12, December 15, 1994.
Marshall, J. A., and S. B. Luthcke, Modeling radiation forces acting on TOPEX/POSEIDON for precision orbit determination, J. Spacecraft and Rockets, 31(1), 89-105, 1994a.
Marshall, J.A. and S.B. Luthcke, Radiative force model performance for TOPEX/POSEIDON orbit determination, J. Astro. Sci., 42, 2, p. 229-246, 1994b.
Marshall, J.A., N.P. Zelensky, S.M. Klosko, D.S. Chinn, S.B. Luthcke, K.E. Rachlin, and R.G. Williamson, The Temporal and Spatial Characteristics of TOPEX/POSEIDON Radial Orbit Error, 2nd TOPEX/POSEIDON Special Issue, in press, 1995.
Nerem, R. S., B. H. Putney, J. A. Marshall, F. J. Lerch, E. C. Pavlis, S. M. Klosko, S. B. Luthcke, G. B. Patel, R. G. Williamson, and N. P. Zelensky, Expected orbit determination performance for the TOPEX/POSEIDON mission, IEEE Trans. Geosci. and Remote Sens., 31(2), 1993.
Nerem, R. S., F. J. Lerch, J. A. Marshall, E. C. Pavlis, B. H. Putney, J. C. Chan, S. M. Klosko, S. B. Luthcke, G. B. Patel, N. K. Pavlis, R. G. Williamson, B. D. Tapley, R. J. Eanes, J. C. Ries, B. E. Schutz, C. K. Shum, M. M. Watkins, R. H. Rapp, R. Biancale, and F. Nouel, Gravity model development for TOPEX/POSEIDON: Joint Gravity Model-1 and 2, J. Geophys. Res., TOPEX/POSEIDON Special Issue, Vol. 99, No. C12, December 15, 1994.
Ray, R.D., B.V. Sanchez, D.E. Cartwright, Some extensions to the response method of tidal analysis applied to TOPEX/POSEIDON altimetry, EOS, paper G31A-5, presented at the Spring Meeting of the AGU, Baltimore, MD, 1994. Ries, J. C., R. J. Eanes, C. K. Shum, and M. M. Watkins, Progress in the determination of the gravitational coefficient of the Earth, Geophys. Res. Lett., 19(6), 529-531, 1992.
Tapley, B.D., et al., Precision orbit determination for TOPEX/POSEIDON, J. Geophys. Res., TOPEX/POSEIDON Special Issue, Vol. 99, No. C12, December 15, 1994.
Tapley, B.D., M.M. Watkins, J.C. Ries, G.W. Davis, R.J. Eanes, S. Poole, H.J. Rim, B.E. Schutz, C.K. Shum, R.S. Nerem, F.J. Lerch, E.C. Pavlis, S.M. Klosko, N.K. Pavlis, R.G. Williamson, The JGM-3 gravity model, Annales Geophysicae, Vol. 12, Suppl. 1, p. C192, April 1994b.
Wu, S. C., T. P. Yunck, and C. L. Thornton, Reduced-dynamic technique for precise orbit determination of low earth satellites, J. Guid., Control and Dynamics, 14, 24-30, 1991.
Yunck, T. P., S. C. Wu, J. T. Wu, and C. L. Thornton, Precise tracking of remote sensing satellites with the Global Positioning System, IEEE Trans. Geosci. and Remote Sens., 28(1), 108-116, 1990.
Yunck, T. P., W. I Bertiger, S. C. Wu, Y. Bar-Sever, E. J. Christensen, B. J. Haines, S. M. Lichten, R. J. Muellerschoen, Y. Vigue, and P. Willis, First assessment of GPS-based reduced dynamic orbit determination on TOPEX/POSEIDON, Geophys. Res. Lett., 21(7), 541-544, 1994.

STATION REPORTS

CLOSING OF SLR ACTIVITIES AT BAR GIYYORA

M. Pearlman/SAO

Funding restrictions have caused the closing of SLR activities at the Bar Giyyora Station. This comes unfortunately after a successful upgrade to the system and very productive years of laser ranging over the last decade. Operations ceased in August 1994. NASA and ATSC personnel packed up the system in October, and it was returned to GSFC in late 1994.

NASA plans to replace the laser with a GPS receiver, maintaining the station as one of the global fiducial sites. Action is now underway to update the agreement between NASA and the Israeli Space Agency to include the GPS operations.

Our special thanks goes to Yehuda Ben Moshe and the station crew for a excellent job supporting the global SLR network.

FTLRS STATUS REPORT (JUNE 1995)

F. Barlier/CERGA, F. Pierron/Observatoire de la Cote D'azur

After modifications during the winter of 1994/1995 the French Transportable Laser System (FTLRS) is in testing at the Grasse Observatory to evaluate its capability prior to entering field operations.

Many problems in the mechanics of the mount and in the servo loop algorithm were solved during this time. We are now putting our efforts into tuning up the performance parameters of the station.

We have been successful in tracking low satellites (e.g., TOPEX and ERS-1). The data are in engineering analysis at the observatory and will be delivered to the community in a few weeks.

During this summer, a collocation campaign will be carried out with the Grasse fixed SLR station. Calibration activities on TOPEX and ERS-1 and -2 are also foreseen in the Mediterranean area in the months to come.

HELWAN SLR STATION BACK IN OPERATION

A. Novotny/Technical University of Prague

Helena Jelinkova and Miroslav Cech arrived in Helwan in late May and the Helwan SLR station is now in operation for the summer with a Egyptian-Czech team. Antonin Novotny, Karel Hamal and Ivan Prochazka plan to work at the station later this summer. Detailed technical information on the station is available on the World Wide Web (in HTML WWW format) at the URL address:

HTTP://WWW.DGFI.BADW-MUENCHEN.DE/EDC/HELWAN.HTML

Upgrade plans including a new receiving telescope, a SPAD receiving system, and full Internet capability are being planned for implementation over the next year.

Dr. Maher Y. Tawadros, Head of the Space Department of the National Research Institute of Astronomy and Geophysics, invites you to visit the station if you are in the area this summer.

NRL SLR STATION INTEGRATED AT USAF PL SOR NO. 7884

G. C. Gilbreath/NRL, A. Peltzer/NRL, M. Davis/ATSC, and R. Q. Fugate/PL

NRL has integrated a state-of-the-art SLR capability at the United States Air Force (USAF) Phillips Laboratory (PL) Starfire Optical Range (SOR).

• Telescope - 3.5 meter primary
• Laser - Nd:YAG, frequency doubled by Type II KD*P crystal - 300 mJ, 250 ps, 10 Hz
• Polarization aperture sharing
• Receiver - PMT/MCP, Tennelec CFD, SR 620 & HP5370A Time Interval Units
• Controller - 486 PC
• Time Base - GPS Receiver, Rb standard

• Presently calibrating system - internal, external, LAGEOS-I and LAGEOS-II
• Experimental station - intermittent not continuous operations
• Returns have been obtained from GFZ, TOPEX/POSEIDON, LAGEOS-I and -II, ETALON-I and -II, GPS-36
• Currently analyzing calibration/satellite data

If SLR data on other satellites is requested, the USAF requires NRL to have written permission from the specific satellite owner/government before lasing is permitted. The satellites mentioned above are the ones we have permission to track.

Point of Contact: Amey R. Peltzer

THE SAO-1/TLRS-1 1994 CO-LOCATION EXPERIMENT

G. Bianco/ASI, A. Cenci/Telespazio

Introduction

A collocation experiment was conducted at Matera to check the ranging capabilities of the SAO-1 (MATLAS) and TLRS-1 laser ranging stations. SAO-1 is a second-generation SLR system installed in 1983 at the Italian Space Agency's (ASI's) Space Geodesy Center (CGS), located in Matera, Italy thanks to cooperation between the Smithsonian Astrophysical Observatory (SAO) and the ASI and to an agreement between NASA and ASI. The station is operated by Nuova Telespazio. TLRS-1 is a third-generation transportable SLR system developed by NASA, that is now under the management of ASI and operated by a Telespazio team. SAO-1 was previously collocated in 1986 with the transportable SLR stations MTLRS-1 and MTLRS-2; in that occasion no significant bias was detected on LAGEOS. The results of this new collocation will also be a valid reference against which one can check the TLRS-1 system following its upgrade. This new experiment started in early May, 1994, at the ASI/CGS, and ended in early June, 1994, when TLRS-1 left the to support the 1994 WEGENER/MEDLAS campaign. SAO-1 was operated by Telespazio and TLRS-1 by ATSC with the support of the Telespazio personnel. The collocation experiment was performed following the guidelines and requirements described in "The 1994 Matera Collocation Plan for MATLAS and TLRS-1", published in April 1994 and available on request.

SAO-1 Technical Outline

The laser emitter is a ruby system consisting of an oscillator and an amplifier stage. A Pockels cell and a Brewster stack are used for the Q-switch process. The laser power supply control unit permits firing rates up to 30 pulses per minute (ppm). The oscillator output is coupled to the pulse chopping system which produces a 3 nsec wide output pulse. This is a krytron-activated Pockels cell with appropriate polarizers for the necessary transmission and isolation. A Blumlein circuit provides the proper high-voltage pulse to operate the Pockels cell, and a PIN diode triggers the system. The pulse chopper output is coupled into the amplifier through a Galilean telescope, with a 12.7 cm objective lens and a minimum divergence of 120 arcseconds. Photodiodes mounted near the output of the oscillator and amplifier pick-up scattered light from outgoing pulses to monitor output energy and to trigger, by means of a "start" signal, the ranging system electronics. The ranging system electronics consist of a clock, a firing control, a range-gate control, a processing system for the start and stop pulses, a time interval unit, and a data handling system which feeds an on-line mini-computer. The clock is slaved to a cesium master oscillator. It controls the firing rate and provides the epoch of observation. The firing time can be shifted manually by multiples of 0.001 s to a maximum of +/- 3 s, to account for an early or late arrival of the satellite at a given point in its predicted orbit. The range-gate control unit provides a gate to the counter and the pulse processing system, operating with a window up to 10 s. The time interval counter is a Nanofast, with a resolution of 0.1 ns; it is triggered on and off by outputs from the pulse processing system. The pulse processing system records the width and area of the outgoing laser pulses, the area of the oscillator pulse and the pulse shape of the received echo. The width and area of the outgoing pulse are measured by a discriminator and a pulse integrator, which form the "start channel" part of the pulse processor. The "stop channel" consists of an analog pulse detection system. A matched filter tuned for the laser pulse is followed by a differentiator and a slope triggered low threshold discriminator acting basically as a zero-cross detector. The alt-azimuth static mount has a pointing accuracy of +/- 30 arcsec. The system is driven by stepping motors in a computer controlled open-loop mode. Pointing angles and range-gate settings are entered in the system on a point-by-point basis in real time from the on-line mini computer. The receiving telescope is a 50.8 cm aperture, folded reflecting system coupled with an Amperex 2233 photomultiplier tube having a quantum efficiency of 4% at 6943 Angstrom, 4e+7 gain and 2 ns rise time. The computing system consists of a DEC-VAX/VMS 3900 and a HP Vectra 486 Personal Computer. The PC performs all the real-time monitor and control and data acquisition functions. The VAX is connected to the PC via a RS 232 Port and provides a detailed post pass analysis and data quality assessment as well as the data archive.

TLRS-1 Technical Outline

The transmitter source is a Nd:YAG mode-locked laser; mode-locking is achieved using a dye cell and an acousto-optic modulator. An IR pulse, generated by the mode-locked oscillator and selected by a pulse slicer, is amplified by a double-pass amplifier and a final amplifier. A second harmonic generator converts the beam into a 532 nm wavelength. A single telescope system, with a Coude frame beam director, is designed for use both as optical transmitter and receiver, by means of a mechanical T/R switch. Two stepping motors drive the beam director taking feedback from two incremental optical encoders. The outgoing pulse carries an energy of about 100 mJ and has a beam divergence of about 12 arcsec. The firing rate is 5 pps. Two constant fraction discriminators detect the start pulse from a photodiode in the laser subsystem and the stop pulse from a microchannel plate (PMT) tube. The Time Interval Unit is a HP5370B, with a resolution of 20 ps. A NOVA 3 computer, via a CAMAC interface, exchanges data and commands with all the other subsystems. The timing system is based upon a Cesium Frequency Standard synchronized by a GPS timing receiver, a Time Code Generator fed by the Cesium 5 MHz output, and a CMOS clock connected to the NOVA via CAMAC.

Observations

According to the collocation plan, we have adopted the definition that two simultaneous passes are overlapping if a minimum of 35% of the total possible overlapping normal points were available from the two systems. During the test period, SAO-1 tracked 82 passes (11 LAGEOS-I, 29 LAGEOS-II and 42 AJISAI), while TLRS-1 tracked 63 passes (13 LAGEOS-I, 27 LAGEOS-II and 23 AJISAI). Of these, 35 passes were overlapping according to the above definition: 8 LAGEOS-I and 12 LAGEOS-II (most of them observed during night because SAO-1 cannot track LAGEOS' in daylight), as well as 15 AJISAI. Some problems should be underlined:

• TLRS-1 sometimes did not acquire the satellite at the beginning of the pass and, for this reason, the overlap interval with the SAO-1 pass was sometimes very short;
• SAO-1 lost some passes because of a PMT failure; it was necessary to replace it with a spare one;
• bad weather conditions extended the collocation schedule.

Timing

The clock closure between the two system was measured, generally on a daily basis, with an accuracy of 100 ns and maintained to 500 ns level (it was slightly higher in the last week).Both the systems have a Cesium beam frequency standard for time keeping and a GPS FTS 8400 for synchronization with UTC.

Data Flow

Full-rate SAO-1 data of for each tracked satellite pass were transmitted daily, in MERIT II format, using INTERnet, to the NASA SLR Network communication system for distribution. Regarding TLRS-1, in addition to the normal data flow through 9-track tapes, full-rate data were sent as soon as possible to the NASA SLR Network in the LMT-88 byte format using INTERnet. The tapes with the raw data were read with the tape drive and stored on the VAX 3900 of the CGS, then the data sent via network to ATSC. NASA SLR Headquarters processed the data and made them available in MERIT II format for use by the CGS.

Data Analysis

Each pass over Matera, simultaneously tracked by the two stations, has been analyzed using GEODYN-II to produce the range residuals of the observations. A polynomial analysis of these orbit residuals is then performed using POLYFIT, a program developed by Telespazio. The first step of the analysis is the computation of a polynomial fit to the TLRS-1 orbit residuals; this reference curve is used to generate the polynomial residuals for the two stations. A second polynomial fit, performed independently for each station, is then necessary to properly edit the data with a 3-sigma editing criterion. After the selection of the common part of the pass, the interval is divided into 5-minute bins. This bin width is a compromise between the necessity to detect short period trends and to maintain reasonably good normal point statistics. For each station, a mean of the polynomial range residuals belonging to overlapping bins with more than four observations is evaluated. The difference between the SAO-1 and the TLRS-1 pass mean is the relative bias of the pass. The global bias resulting from the collocation test is the weighted mean of the biases for those passes with more than 25 overlapping observations for both stations and more than one overlapping bin.

Results

The following table summarizes the results in terms of the computed bias of SAO-1 with respect to TLRS-1 for each satellite.

Although the number of passes and observations is not very high, the experiment showed that the bias between the two systems using the two LAGEOS satellites is at or below a cm.

The AJISAI Bias

Different considerations hold for the AJISAI results. The mean range bias from the analysis was 5.7 cm (SAO-1 ranging longer than TLRS-1) and this value is consistent with the result of the Arequipa/TLRS-3 collocation which involved another SAO-type system very similar to SAO-1. One possible contribution to this bias is the difference in return signal strength during calibration and during satellite tracking. The system delay is evaluated using the calibration data, taken before and after the pass using an external target, and applied to the pass observations. Often, more than one pass is acquired between a pre-calibration and a post-calibration and often one of these passes is on LAGEOS. In this case, the calibration is made simulating the condition of the system during a LAGEOS pass (single-photon level energy) and, as a consequence, the system delay is correct for LAGEOS but wrong for the lower satellites for which the return energy is higher (multi-photon level energy). In order to partially correct for this error, we take note of the fact that with the 3 msec pulse width, the return rms decreases with increasing signal strength. An algorithm has been developed to relate the variation of the system delay with the variation of the rms of the acquired data (the difference between the rms of the calibration and the rms of the pass). Application of the algorithm reduces the pass biases and maps directly with the weighted mean of the single pass bias, causing a reduction of about 1 cm. Other checks were made, for example some controls on the attenuation table, but additional investigations are necessary to clarify the source of the remaining 4.7 cm bias (e.g., the correlation between the different pulse length for MATLAS and TLRS-1 and the AJISAI target depth signature).

Conclusions

In spite of problems which made the test period longer than expected, the experiment showed that the mean range difference of MATLAS versus TLRS-1 is less than 1 centimeter on LAGEOS; the bias of the order of 5 cm, found ranging to AJISAI and consistent with the Arequipa/TLRS-3 results, needs deeper investigations.

THE SAUDI ARABIAN LASER RANGING OBSERVATORY

M. Al-Dail/KACST

In 1986, the King Abdulaziz City for Science and Technology (KACST), an agency of the Kingdom of Saudi Arabia, and the U.S. National Aeronautical and Space Administration (NASA) signed an Agreement of Cooperation in Geodynamics.

To implement the Agreement, KACST commissioned the development of the Saudi Arabian Laser Ranging Observatory (SALRO). In January 1988, the Australian Trade Commission (AUSTRADE) and its contractor, the Electro Optic Systems (EOS) Pty Limited of Australia was selected to develop the SALRO.

SALRO was described as a state-of-the-art system capable of ranging to LAGEOS with a precision of less than one centimeter. The EOS design specifications included a 75 centimeter Contraves Goetz telescope with Coud optics, a Nd:YAG laser, and a Microchannel Plate detector. The system was to be housed in a single trailer.

The SALRO was fabricated in Australia. Following a Critical Design Review in January 1989, initial systems testing at the Tidbinbilla Tracking Station, and a pre-ship review in April 1991, the SALRO was shipped by water transportation in late 1991 to the Goddard Geophysical and Astronomical Observatory (GGAO) for collocation tests with MOBLAS-7.

SALRO arrived at the GGAO in February 1992. However, a series of hardware problems including recoating of the telescope optics and a decision to use a SPAD as the primary detector, delayed the start of collocation testing until late June 1994.

The collocation tests, completed in March 1995, confirmed almost all of the SALRO performance specifications. With the SPAD, and with pre-processing software not optimized for the SPAD, the SALRO accuracy in ranging to LAGEOS satellites was greater than one centimeter. However, it was agreed that the system capability was presently comparable with all of the current scientific needs and that performance would continue to improve once the system was optimized. A Preliminary Acceptance Test (PAT) Review Board of international experts recommended shipment of the SALRO to Saudi Arabia, with the proviso that further tests and optimization be conducted prior to Final Acceptance Tests (FAT).

SALRO arrived at its permanent site in June 1995. The SALRO station is approximately 40 kilometers from Riyadh, the capital of Saudi Arabia. The KACST operating crew are housed in a separate building next to the SALRO trailer. EOS will operate the SALRO for two months after installation and check-out is completed. The FAT is to be completed in late summer 1995. It is expected that by the end of 1995 the SALRO should join the global network of laser stations providing data for the study of the solid Earth and for the other programs requiring precise SLR tracking.

ZIMLAT: THE NEW ZIMMERWALD LASER AND ASTROGRAPHIC TELESCOPE

W. Gurtner/AIUB

The nearly 20 years old Laser Telescope in Zimmerwald can no longer respond to all the requirements of modern Satellite Geodesy. Daytime operation is impossible, ranging is limited to low orbiting satellites (LAGEOS), pass interleaving is slow, and the accuracy is not sufficient.

During the last few years, development and testing of an astrographic tracking CCD on moving objects (comets, minor planets, geostationary and other satellites, space debris) has been underway on the SLR telescope at Zimmerwald. However, neither the optical quality nor the tracking accuracy of the telescope are sufficient to make use of the full potential of this method.

Since 1991 the Astronomical Institute has been evaluating a new telescope for both satellite laser ranging and astrographic tracking. In March 1994, a new instrument was ordered from Telas, Cannes, a Joint Venture between the French companies Aerospatiale and Framatome.

With this new telescope, with a 1 m primary mirror, a secondary and tertiary mirrors will be shared between satellite laser ranging and astrographic tracking of moving objects through a beamsplitter/mirror arrangement. The transmitted laser beam is expanded into a ring, enters the telescope through a coude path and leaves it concentrically around the secondary mirror. The transmit/receive switch is a pierced mirror with no moving parts. There are three receiver ports prepared for each of the two possible wavelengths.

Four camera ports, three of which are reserved for CCD cameras of different focal lengths and one for a TV guiding camera for SLR, are available on a rotatable instrument platform vertically mounted on one side of the telescope. They will be equipped with the following reduction optics:

The telescope will allow either independent SLR or astrographic mode of operation or, with a slight reduction in imaging quality, even a simultaneous tracking mode. Switching between different satellite tracks can be rapidly performed thanks to excellent angular velocities and accelerations of the mount:

The Laser system, the electronic equipment, the station computer, and the tracking software will be upgraded as well:

Laser: Titanium Sapphire, wavelengths: 423 nm, 846 nm
Counter: Stanford SR620
Detectors: SPAD; Hamamatsu Photomultiplier

The installation of the new system on site will start in June 1995. It is expected to become operational before the end of 1995. The system is financed by the Federal Office of Topography, the Swiss National Science Foundation, the University of Bern, and the Canton of Bern.

SLR DATA AND ANALYSIS

THE IRV FORCE MODEL AND REFERENCE SYSTEM

A.T. Sinclair/RGO

This note has been finalised in consultation with Richard Eanes, Brion Conklin and Rolf Koenig.

The IRV format, coordinate system and force model were adopted by CSR, University of Texas in about 1982 as a means of providing predictions for Lageos, following a system previously in use by NASA. A description is given in a paper “Lageos Ephemeris Predictions” by B.E. Schutz, B.D. Tapley, R.J. Eanes and B. Cuthbertson in the proceedings of the Laser Ranging Workshop, Austin, 1981. Even though the force model in particular is now somewhat dated the system has been retained unchanged, as it serves its purpose perfectly well, and the CSR software (IRVINT) and software written by the RGO that followes the same force model and reference system are in wide use through the SLR network, either used directly or as the basis of other software packages. The IRV system has now been adopted for all satellites, but it is not clear where, if at all, the parameters for these other satellites have been documented. This note is intended to clarify the present situation, and provide a starting point for discussion of updating the system, should this be considered necessary.

Force Model

GEM10 gravity field. Degree and order 7 for LAGEOS, ETALON, GPS, GLONASS. Degree and order 18 for other satellites.

GM = 0.39860044 * 10¹⁵m³s^-2 for all satellites

a_e= 6378145 m.

Lunar and Solar perturbations are computed from simple precessing-ellipse orbit models. No drag or solar radiation pressure forces are included.

Note. An earlier suggestion that a slightly different value of GM should be used for the CSR IRVs for the ETALON satellites is incorrect.

Reference System

The IRVs (inter range vectors) are referred to a pseudo body fixed reference frame, the true equator and Greenwich meridian of 0h of the day for which they apply. They are not referred to the CIO equator, which differs from the true equator by the polar motion transformation.

The action of the integrator used to generate satellite positions from the IRVs can be regarded as a black box by most users, but, in outline, what it does is the following :

1. Convert the IRV velocity to an inertial frame:
   
   and IRATE is given with the IRVs.
2. Calculate ₀ = GMST at 0^h UTC of the day required, ignoring the correction from UTC to UT1. Rotate IRV position and velocity through - ₀ about the z-axis to refer them to the mean equinox of 0^h.
3. Integrate numerically the force equations in the near-inertial frame of the true equator of date and mean equinox of 0^h of the day. At a general point in this integration at time T seconds from 0^h the Earth’s position relative to the integration frame is obtained by a positive rotation through the angle . This is needed in order to calculate the force due to the gravity field, which is then rotated back through the same angle, to be used in the numerical integration.
4. The integration gives the positions at steps through the day of the satellite relative to the true equator of date and mean equinox of 0^h. The position at a general time T seconds from 0^h is converted to the true equator of date and Greenwich meridian by a positive rotation through the angle , and these are the positions delivered to the user.

At this stage the user could apply polar motion if desired to refer the positions to the CIO equator. It may also be desirable to make a correction to the nominal “Greenwich meridian” to which these positions are referred. When forming the IRVs the prediction centre has to adopt predicted values of UT1-UTC. For the Lageos and Etalon satellites the predictions are generated for a year or more ahead, and over these periods the predicted values of UT1-UTC can be significantly in error, and this will affect the nominal Greenwich meridian to which the satellite positions are referred. The University of Texas supplies with their IRVs for the Lageos and Etalon satellites a file of the values of UT1-UTC used, and so the user can correct the reference frame of the final satellite positions if the discrepancy of the predicted UT1-UTC is significant. This effect is unlikely to be significant for lower satellites, for which the predictions are generated for shorter time periods.

Comments

There are some approximations introduced by the integrator program. In stage 2 the correction UT1-UTC is neglected in the rotation to the equinox. This has just a very small effect on the calculation of the lunar and solar perturbations. It does not affect the calculation of the gravity field force, as the same rotation is reversed before this is calculated. In stage 3 the gravity field force is calculated relative to the true equator, whereas the actual orientation of the Earth is described by the CIO equator. However the tuning program makes the same approximation, and thus the IRVs have been tuned to accomodate this approximation.

For several years the IRVs issued by ATSC (formerly Bendix) have been referred to the CIO equator instead of the true equator. The effect of using these with the Texas or RGO software was to cause an along-track run-off of the orbit over each day, which for example amounted to about 180 metres for Topex. From about November 1994 the ATSC IRVs have been changed to refer to the true equator, and so this problem has been removed.

The original version of the software package PC TIVAS announced by ATSC in CDDIS Bulletin August 1994 requires that the IRVs should be referred to the CIO equator. ATSC have produced a revised version of PC TIVAS that uses IRVs referred to the true equator, and it is expected that this will be placed in CDDIS at about June 1995.

The original IRV programs from the University of Texas used the FK4 expressions for GMST. It would not matter much if a reconstruction program were to use the FK5 expression instead, or if it were to make the correction from UTC to UT1 in calculating GMST (provided the same ₀ is used at stages 2, 3 and 4), as these would only have a small effect on the calculation of the lunar and solar perturbations. In fact it is known that the original Texas program contains a slip in the subroutine RAOGU, where the paramter AL1 has the value 1.720279266007D-2 instead of 1.7202791266D-2, this being the coefficient of T in the GMST expression, usually written as 8640184.542^s/cy, expressed as radians/day. It is unlikely that other reconstruction packages would duplicate this slip, and so this small difference exists in the community between some IRV reconstruction packages. The only significant consequence of this is if attempts are made during testing of software to compare the outputs of IRV packages by comparing positions referred to the equinox, and then these small differences of transformation to the equinox must be taken into account.

As mentioned above, the lunar and solar positions should be computed from simple models of precessing ellipses. This is not in fact a particularly good model for the lunar motion, and the PC-TIVAS software uses a much more elaborate model in a subroutine called DIANA, which evaluates long series of terms from the “Improved Lunar Ephemeris”, with a claimed accuracy of 2 arcsec. This is not necessarily an advantage, if the IRVs in use have been tuned to the simpler lunar model, but the effect of using a different lunar model from that for which the IRVs have been tuned is fairly small, only about 10 metres for Lageos, and smaller for lower satellites.

CHANGES IN ACCESSING SLR FULL-RATE DATA

C. Noll, J. McGarry/NASA GSFC B. Conklin, W. Decker, D. Edge, D. Gorbitz, V. Husson/ATSC

Introduction

NASA, together with AlliedSignal Technical Services Corporation (ATSC), has been studying ways to improve the handling of SLR full-rate data from the global network. The CDDIS contains a rich historical archive of SLR data, dating back to 1975. NASA wants to continue to add to this archive and to make full-rate data available to the global user community as has been the request, while decreasing the cost associated with handling the full-rate data. To this end, NASA is striving to streamline the processing of this valuable data set. With a redesign of the data flow, NASA has simplified and automated data processing and at the same time continues to provide a more rapidly-available product to the user community. NASA has reduced, and plans to eventually eliminate, special full-rate products, such as those for the TOPEX/Poseidon and ERS-1 orbit determination, while continuing to satisfy the requirements of these missions. This article addresses changes to the flow and storage of SLR full-rate; future streamlining efforts plan to address quick-look (normal point and sampled) data as well.

Previous Full-Rate Data Products

The global network currently tracks sixteen satellites on a routine basis and provides full-rate data directly to ATSC or to the CDDIS, mainly through the EuroLAS Data Center (EDC) located at the Deutsches Geodtisches Forschungsinstitut (DGFI) in Munich Germany. Prior to data from 1995, ATSC merged the entire global data set into monthly increments by satellite; data were interleaved by time and provided in MERIT-II format. The CDDIS then distributed these data sets to users via 9-track tape, 4 mm tape, or electronically. ATSC also generated normal points from full-rate data; these data are available from the CDDIS electronically or via tape. The CDDIS distributed the A (or initial) release of the full-rate data sixty days after the end of the observation month. The B (or second) release, containing additional or updated passes, was typically available six months after the end of the observation month.

Revised Full-Rate Data Products

Where possible, full-rate data are sent electronically from the site, nominally on a daily basis, to be processed by ATSC (and EDC). These data are stored in compressed files on the CDDIS by satellite, by station, and by day. Figure 1 shows the current directory structure for these on-line files. Filenames have the form: stat_yymmdd_v.satname_Z where stat is the four-digit station number, yy is the two-digit year, mm is the two-digit month, dd is the two-digit day of the first observation of the pass, v is the one-character version of the data, satname is the satellite name, and _Z indicates a compressed file. As an example:

7105_950101_A.LAGEOS1_Z

contains LAGEOS-I data from MOBLAS-7 at station 7105 where the first observation of all passes were taken on January 01, 1995 (compressed format)

7105_950101_B.LAGEOS1_Z

contains any late arriving or updated LAGEOS-I data from MOBLAS-7 at station 7105 where the first observation of all passes were taken on January 01, 1995 (compressed format)

These data were compressed using the UNIX compression algorithms; software for IBM PC and VAX systems are available to decompress the files. Interested users should contact the CDDIS staff for account access information.

Automated procedures ship these files to the CDDIS with a three day delay. For any data still delivered to ATSC or CDDIS via magnetic tape, procedures are executed on the data sets to create these daily satellite-station files in the appropriate formats. Data will be retained on-line on the CDDIS for at least six months. Data more than six months old will be made available to users from the CDDIS via special request. Data arriving at ATSC or CDDIS later than six months after the observation day will be retained on-line for some period to allow user access. Notices will appear in an on-line information file to alert the user community of late arriving data, problem data, and other special information.

Replacement of full-rate SLR data will be kept at a minimum. However, late-arriving or seriously flawed data will be made available in the day/station/satellite file format and noted by an increment in the version label in the filename. Thus, users of a particular day's worth of data for a station must apply any files labeled with version B or higher to the original A version of the file. Version A of files will not be modified; any additional or replacement data will be placed in a new file with an incremented version code (e.g., B, C, etc.).

Concluding Remarks

Only full-rate SLR data (MERIT-II format) and on-site normal point data (CSTG format) are now available as data products. Studies by ATSC have determined that on-site normal points are of the same quality as normal points generated from full-rate data. Normal points calculated from MERIT-II full-rate data will no longer be produced.

Monthly SLR full-rate release tapes will no longer be generated by ATSC on a routine basis (after the release of the December 1994 Version B data set). The CDDIS, however, will study the need to create monthly merged full-rate data sets, allowing for a sufficient time delay to capture as much late-arriving data as possible. Therefore, users should be aware that the monthly full-rate data tapes will no longer be available from the CDDIS under the current two- (A release) and six-month (B release) delay schedule.

NASA began these procedures with data observed on January 01, 1995. In addition to stations in the NASA network, a few cooperating SLR stations have begun supplying data to the CDDIS in this new fashion. Comments, questions, and suggestions on this new data system should be directed to Carey Noll.

ENHANCEMENTS TO SLR ON-SITE DATA AVAILABILITY

C. Noll, J. McGarry/NASA GSFC B. Conklin, D. Edge, J. Horvath, V. Husson/ATSC

SLR data generated on-site has been available on-line from the CDDIS since late 1991. Currently, the CDDIS supplies these data to users in daily files, each file containing all data received during the last 24 hours. Therefore, the file may contain data from several days. Sometime after the end of the month, the CDDIS creates a monthly, time-sorted file from these individual daily files.

The format of the data is CSTG normal point format. The CSTG format is designed to accommodate both normal points and sampled/engineering data points for engineering purposes. Furthermore, the CSTG format provides a header record containing information for the entire pass and data records containing time-dependent information. Normal point data records are preceded by the indicator '99999' and a header record; sampled or engineering data are preceded by the indicator '88888' and a header record. ATSC currently converts data from SLR systems which do not at this time produce CSTG normal points; the normal point indicator is set to 0 for all data converted from other formats (e.g., CSTG sampled, STDN, or SAO formats). Previously, ATSC has forwarded only the normal point data to the CDDIS for those systems providing both normal point and sampled data. However, as of September 1, 1994, any sampled data received from the global SLR network has also been forwarded to the CDDIS. The normal point and sampled data are made available to users in separate files having the naming convention:

where yy is the two-digit year, mm is the two-digit month, dd is the two-digit day , and sat is the three-character satellite name; the yymmdd time tag refers to the date the data was delivered to the CDDIS. Previously, ATSC has provided a service to convert data into normal points for those SLR stations only capable of producing SAO, STDN, or CSTG sampled data. As of February 1, 1995, however, these conversions are no longer performed. The NEW_QL file now contain true normal point data only; the NEW_EN file include only sampled data in CSTG format.

SLR stations are encouraged to submit sampled data through their data channels for availability to the user community. These data are useful to analysts in diagnosing system problems, etc.

SPACE GEODESY RELATED WORLD WIDE WEB HOME PAGES NOW AVAILABLE

C. Noll/ NASA GSFC

Two space geodesy related INTERnet resources have been available for the past several months on the World Wide Web (WWW). For those users unfamiliar with the Web, the WWW is simply a method for exploring resources available over the INTERnet. The WWW is a distributed, hypertext-based information system and publishing tool developed at the Center for European Laboratory for Particle Physics, CERN, in Geneva Switzerland. The WWW allows users to navigate through hypertext links to obtain information. Users specify a link to be followed and a new data object is retrieved and presented to the user through client software running on their local computer (PC, workstation, etc.). This link can be as simple as clicking highlighted text or can be specified by entering a Uniform Resource Locator (URL). A WWW client, or browser, is an application that interfaces with the user and asks for documents from a server as the user requests them. Both client and server software (required for groups to display their own WWW pages to others over the network) for a variety of platforms are freely available. An example of one of these packages is MOSAIC, available from CERN and the National Center for Supercomputing Applications (NCSA) at the University of Illinois. Text, graphics, images, and sound can be displayed (and heard) through the WWW, providing the user's client software supports these.

The CDDIS WWW home page is shown in Figure 1; the URL to view this page is: http://cddis.gsfc.nasa.gov/cddis.html. This page provides information about the CDDIS and its data holdings, the Space Geodesy Program, the Dynamics of the Solid Earth investigation, and the current space geodesy techniques (SLR, GPS, and VLBI). Furthermore, the CDDIS home page provides links to other areas of interest at NASA and internationally, such as the home page for the International GPS Service for Geodynamics (IGS). Various documents sponsored by the SGP can also be browsed through the WWW. These documents include the NASA Space Geodesy Program: Catalogue of Site Information, the Space Geodesy Personnel and Networking Directory, and previous issues of the CDDIS Bulletin. Updates are made to the appropriate pages of these documents, often automatically, as they become available.

The NASA/GSFC Space Geodesy Altimetry Projects Office (SGAPO), under the direction of Dr. John Degnan, has implemented a WWW home page (see Figure 2). The URL to view this page is http://cddis.gsfc.nasa.gov/920_1/sgapo.html; alternatively, this page is linked to the CDDIS home page (and vise-versa). This page provides general information on this office, current programs and satellites supported, the SLR technique, and research activities. The recently-published SLR brochure is also accessible through this WWW application. The SGAPO home page was designed as part of a Code 920 division effort to provide information on scientific programs and research here at GSFC through the WWW. Many of the SGAPO pages are under development; users should stay tuned for further enhancements. The SGAPO and CDDIS staff welcome comments and suggestions on the contents of these pages.

SLR STATIONS OPERATING DURING 1994

C. Noll/NASA GSFC

Table 1 and Table 2 on the following pages list the SLR stations operational during 1994 and summarize the full-rate data for 1994 archived in the CDDIS as of June, 1995; these sites are also shown in the map in Figure 1. All full-rate Version A data sets have been released; the latest B release of the 1994 full-rate data set is for the month of June.

Table 1. SLR Stations Operating During 1994

			Site Name	     Location				       SOD	   System
			-------------------  ----------------------------------------  ----------  --------
			Arequipa	     Arequipa, Peru			       74031303	   TLRS-3
			Bar Giyyora	     Bar Giyyora, Israel		       75300204	   MOBLAS-2
			Beijing		     Beijing, Peoples Republic of China	       72496101	   BEJLAS
			Borowiec	     Borowiec, Poland			       78113802	   POLLAS
			Cabo san Lucas	     Cabo san Lucas, Baja, Mexico	       78821404	   TLRS-4
			Cagliari	     Cagliari, Italy			       75486201	   CAGLAS
			Changchun	     Changchun, Peoples Republic of China      72371901	   CHALAS
			Easter Island	     Easter Island, Chile		       70971208	   TLRS-2
			Ensenada	     Ensenada, Baja, Mexico		       78831404	   TLRS-4
			Evpatoria	     Evpatoria, Ukraine			       18675301	   EVPLAS
			Grasse		     Grasse, France			       78353102	   GRASSE
			Graz		     Graz, Austria			       78393402	   AUSLAS
			Greenbelt	     GGAO, GSFC, Greenbelt, MD		       71050724	   MOBLAS-7
			Greenbelt	     GGAO, GSFC, Greenbelt, MD		       79205513	   SALRO
			Haleakala	     LURE Obs., Mount Haleakala, Maui, HI      72102312	   HOLLAS
			Helwan		     Helwan, Egypt			       78314601	   HELLAS
			Herstmonceux	     Royal Greenwich Obs., Great Britain       78403501	   RGOLAS
			Katzively	     Katzively, Crimea, Ukraine		       18931801	   CRMLAS
			Komsomolsk-na-Amure  Komsomolsk-na-Amure, Russia	       18685901	   KOMLAS
			La Grande	     La Grande, Quebec, Canada		       74111402	   TLRS-4
			Maidanak	     Maidanak, Uzbekistan		       18645401	   MD2LAS
			Makura Saki	     Makura Saki, Japan			       73231701	   HTLRS
			Matera		     Matera, Italy			       75411102,03 TLRS-1
			Matera		     Matera, Italy			       79394101	   SAO-1
			McDonald	     McDonald Observatory, Fort Davis, TX      70802419	   MLRS
			Mendeleevo	     Mendeleevo, Russia			       18706301	   MENLAS
			Metsahovi	     Kirkkonummi, Finland		       78053301	   FINLAS
			Monument Peak	     Mount Laguna, CA			       71100411	   MOBLAS-4
			Oga		     Oga, Japan				       73211701	   HTLRS
			Orroral		     Orroral Valley, Australia		       78432502	   NLRS02
			Potsdam		     Potsdam, Germany			       78365801	   GFZLAS
			Punta sa Menta	     Punta sa Menta, Cagliari, Italy	       75451104	   TLRS-1
			Quincy		     Quincy, CA				       71090815	   MOBLAS-8
			Richmond	     Perrine, FL			       72951403	   TLRS-4
			Riga		     Riga, Latvia			       18844401	   RIGLAS
			San Fernando	     San Fernando, Spain		       78244501	   SPNLAS
			Santiago de Cuba     Santiago de Cuba, Cuba		       19532001	   CUBLAS
			Sarapul		     Sarapul, Russia			       18716401	   SARLAS
			Shanghai	     Shanghai Obs., Peoples Republic of China  78372805	   CHILAS
			Simeiz		     Simeiz, Ukraine			       18734901	   SIMLAS
			Simosato	     Simosato Hydrographic Observatory, Japan  78383601	   SHOLAS
			Wettzell	     Wettzell, Germany			       75971507	   MTLRS-1
			Wettzell	     Wettzell, Germany			       88341001	   WLRS
			Wuhan		     Wuhan, Peoples Republic of China	       72362901	   WUHLAS
			Xrisokalaria	     Xrisokalaria, Greece		       75251107	   TLRS-1
			Yarragadee	     Yarragadee, Australia		       70900513	   MOBLAS-5
			Yigilca		     Yigilca, Turkey			       75871504	   MTLRS-1
			Zimmerwald	     Bern, Switzerland			       78104801	   ZIMLAS
			===================  ========================================  ==========  ========
			Totals:		     41 systems at 45 distinct sites

MEETING SUMMARIES

EUROLAS MEETING (MONDAY, MARCH 20/TUESDAY, MARCH 21)

M. Pearlman/SAO and A. Sinclair/RGO

Meetings of EUROLAS are held approximately annually, and are usually arranged as splinter sessions at some other scientific meeting. On this occasion a self-contained meeting was arranged, as there were several urgent technical matters that required fairly detailed discussion. The meeting was hosted by the EUROLAS Data Center at DGFI in Munich, and there were 38 participants, including 3 from NASA and 2 from NRL. John Luck had hoped to be present to represent WPLTN, but unfortunately was not able to be so. Andrew Sinclair, the President of EUROLAS, chaired the meeting.

The technical matters that were discussed in detail were:

1. Satellite signatures and center-of-mass corrections, particularly for systems operating at single-photon return levels, and the effects of data processing methods.
2. Time-walk effects in avalanche photo-diode detectors which depend on receive-energy level and laser pulse width.
3. The revision of the format for station formed normal points.

Item 2 and to some extent item 1 have been previously considered by a small working group led by Georg Kirchner, and their results together with some other new work were presented at the Canberra Workshop. The purpose of this meeting was to bring all these results together and make recommendations for appropriate operational procedures. A new factor, as shown in the Canberra Workshop paper by Reinhart Neubert, is that the mean reflection point of the LAGEOS satellites is about 8 mm further into the satellite than the standard ground-test value, and the consequences of this for systems operating at the single-photon level were analyzed in detail. The result was to decide upon operating procedures and data processing methods which are appropriate for the characteristics of the data obtained by single photon systems, avoid time-walk effects, and for which the satellite center-of-mass correction values are sufficiently close to the standard values that for most purposes no further correction will be needed.

Item 3 continued the discussion of the proposal presented by Van Husson at Canberra for a revision of the data format, which had run out of time at that meeting. A consensus viewpoint was reached, and a draft of the revised format based on this consensus has been prepared, for examination by NASA and WPLTN, and eventual consideration by CSTG. The revised format provides for ranging at more than one wavelength, provides for wavelengths greater than 1 micron, provides additional statistical information to compensate for the eventual phasing out of full-rate data, and provides additional information on the system hardware and mode of operation so that adjustments to the center-of-mass value can be made if desired and appropriate for the particular system (but these will be only at most a few mm).

A full report of the meeting and its conclusions and recommendations, including copies of the technical papers presented, is being prepared, and will be widely distributed. Meanwhile the notes below prepared by Mike Pearlman give an excellent summary of the meeting. Contact him if you would like the report.

Data and Data Processing

Seemueller reported that the EDC is expanding its FTP capability to facilitate data transfer to the Center and to provide access to station documentation. In general, the World Wide Web can be very slow, particularly during peak hours. They suggest sending traffic during off hours. They continue to have data transfer problems from the Russian stations. Bianco mentioned that they are working on a similar system to access station documentation.

Noomen reviewed the DUT data processing system and their experience over the past year. Items that they have in mind for improvements in the future include: improved treatment of the earth rotation parameters, increased frequency of analysis, more refined models (JGM-3, ocean loading, etc.), and working directly with normal equations. Noomen mentioned that he believes that his 10 day arcs on LAGEOS I and II are verifying performance at the 2-3 cm level. His criteria for including data is about 8 cm in range bias and 40 microseconds in range bias.

Support for C.I.S. Countries

The European Community is providing funding to the C.I.S. through the International Association for the Promotion of Cooperation with the Scientists from the Independent States of the former Soviet Union (INTAS). In response to a EUROLAS proposal, some funding has been made available to support Katsively and Maidanak. The funds will go for the purchase of timing receivers, PCs, and a few other small items.

Reduction of Full-Rate Data Operations

The CDDIS has announced a new policy regarding the availability of SLR full-rate data (daily deliveries of satellite files). At the same time, NASA is announcing that as part of its cost reduction program, it will be reducing data processing services, and in particular, reducing or possibly deleting all full-rate data activities. ATSC will no longer compute normal points at Headquarters.

Most data users have made the transition to field-generated normal points as their data source, and very few are using the full-rate product. It is probably time to discontinue full-rate data flow and merely request that the stations maintain their own full-rate files for some specified period of time in case a user has a specialized need. The only remaining issues with the field generated normal points are after-the-fact timing corrections and meteorological data corrections. However, with the now general use of GPS timing receivers at the most accurate SLR stations and reasonable care and redundancy with the meteorological measurements, these issues should not limit the use of the SLR data in current applications.

It was agreed that we should cease the operational flow of full-rate data from the stations, except in special cases of programmatic need. As a caveat of this policy, those few stations that do not compute normal points will probably be left by the wayside.

Global Distribution of SLR Network

The potential reduction of NASA SLR activity will have a fundamental impact on global SLR activities. NASA reported that a program being proposed to reduce personnel requirements, and yet maintain almost all of the station operations, was being given favorable consideration. This brought great relief. We also reported that the letters from overseas groups are probably having a strong positive influence at NASA.

It was recognized, however, that although the current crunch was coming at NASA, an equally powerful ( maybe even more powerful ) concern should be felt by the European groups. The SLR network is in desperate need of better global distribution, and yet nearly half the stations in the world are located within a radius of 1,000 miles. How long will supporting agencies continue funding such redundancy?

Normal Point Data Distribution Update

At the moment, there is an issue about how the investigators will know when a station has reissued normal points. It was agreed that the Data Centers should issue an update catalog (not an updated catalog!) to investigators along with new data.

Standardized SPAD Data Processing

The EUROLAS Meeting addressed the issue of standardizing the processing formula for low signal level operation of the SPAD. The return distribution of the SPAD has the "trailing edge" and is accordingly not symmetrical. The users have great concern that the application of the standard 3 rms iterative screening procedures, which assume a normal data distribution, are aliasing their data.

In the discussion, the following key points were noted:

• The high signal strength MCP operation effectively ranges to the front of the retroreflector distribution, while the low level SPAD ranges more toward the "center" of the array distribution.
• The standard center-of-mass correction for LAGEOS (251 mm) was measured for the MCP with high signal strength returns.
• The corresponding center-of-mass correction for the SPAD in low signal strength operation is 243 mm. If the SPAD data is to be used directly, some accommodation for this difference must be made or the final range value will be short by 8 mm.
• Since that SPAD is "ranging deeper into the satellite", its ranges will be longer than those from the MCP by 8 mm.
• The peak of the return distribution is the "most likely value" and, in the absence of a symmetrical distribution, still gives a well identified reference point on the return distribution. Corrections to the peak on returns from LAGEOS will decrease the range by 5-6 mm.

After extensive discussions on SPAD performance, satellite center-of-mass corrections, and satellite return signatures, the SPAD users agreed that the data must be tightly screened to get over the effects of the "trailing edge". By extrapolating the data to the peak of the return distribution, they can select a unique reference point and they can nearly compensate for the difference in the center-of-mass corrections. They agreed on a two step processing procedure:

• An iterated 2.5 rms screening procedure to convergence to calculate normal points for each pass.
• A correction to the peak will be calculated for each pass using a very tight screening procedure ( 1 rms), with the correction being furnished in the data header, and applied to the data. The appropriate flag should be set.
• The same procedure would be used for ground calibrations.

Standardized SPAD Operations (Signal Strength)

To overcome the signal strength range dependence of the SPAD, it must either be operated in the low signal strength regime (ideally, single photon), or signal-strength-dependent corrections must be measured and applied. It was agreed that the SPAD community should specify a signal strength criteria that strikes the right balance between (1) reduced range bias and (2) healthy data yield.

The EUROLAS members agreed that return signal strength bias should be limited a few mm or below and that return rate should be used as the measure of signal strength. Since there are more than one source and model of geiger mode avalanche photodiode, it was agreed that each group should measure through ground calibrations the characteristics of his own photodiode to determine the breakpoint at which the range bias is introduced and the jitter characteristics. Each group should adopt a maximum return rate with tolerable jitter and no time walk. This should typically be in the range of 25% - 30% return rate. This should be used as an upper bound for all ranging operations.

SLR Data Format

The latest proposed SLR data format was presented at the CSTG meeting in Canberra. In particular, in the light of (1) reduction and probably termination of the full-rate operations and (2) the emergence of multiple wavelength ranging, the new format was being proposed to include some additional statistical information on ranging and calibration data and the capability to handle additional range data. There was general agreement in Canberra, but several suggestions were made at the time to both simplify it and improve flexibility.

EUROLAS agreed to revise the proposed format, incorporating the Canberra comments, and to give it a bit more scrutiny. The format has been endorsed with slight modifications.

WEGENER/MEDLAS PROGRAM

M. Pearlman/SAO

One additional WEGENER/MEDLAS SLR occupation session is being considered for 1995 in order to (1) complete unfinished surveys (provide a third occupation), (2) provide an additional occupation where system problems have degraded an occupation and left results in doubt, or (3) resolve situations where the trend indicated by the SLR data is unclear or markedly different from that indicated by GPS. GPS surveys will continue at these sites on a regularly scheduled basis.

Representatives from IfAG, DUT, ASI, GFZ, and NASA (ATSC, Hughes/STX, and SAO) have reviewed the data sets and the analysis results to date and have agreed on the following list of candidate sites in order of priority:

Karitsa

Only two SLR occupations to date; requires a third occupation.

Katavia

The horizontal trends are well defined, however the SLR and the GPS show quite different trends in the vertical. This could be a critical SLR/GPS comparison and provide some interesting geophysics.

Dionysos

There are four SLR occupations, but the last one in 1992 appears to be out line with the previous three in 1986, 1987, and 1989. There is additional motivation for this occupation because the station will be one of the global fiduciary sites.

Roumelli

There are four SLR occupations but the SLR station vector is not conclusive, in part, due to systems problems during the 1992 occupation.

Roumelli is the only site that has Nelson piers. A ground target accommodation would have to be made at the other sites if they were to be occupied by TLRS-1.

There was also interest in Basovizza, where only two occupations have taken place. Since one of the occupations was marginal and the weather introduces a serious concern, it was agreed that this would not be included in the short list, but that ASI should try to fit it into its TLRS-1 schedule if possible.

Status of the Systems

TLRS-1 is now at Matera awaiting conversion to 50 Hz. power. It is anticipated that the system will be ready for deployment to one site, probably Roumelli, by August. MTLRS-1 is at Wettzell preparing for a collocation, after which it should be available for deployment, also probably in August. MTLRS-2 has begun operations at Kootwijk and should be ready for the field this summer or fall.

Plan

Of the high priority sites identified, only Roumelli has the Nelson piers required for TLRS-1 occupation. Aside from Roumelli, the sites would be occupied in order of priority as MTLRS-1 and MTLRS-2 become available.

Final WEGENER/MEDLAS Conference

With this final occupation campaign, the WEGENER/MEDLAS activity will draw to a close. A final MEDLAS conference is being planned for 1996, probably in Frankfurt. The proceedings of the conference should constitute the final report.

WEGENER PROCEEDINGS FROM THE ST. PETERSBURG MEETING

M. Pearlman/SAO

The Abstracts, Presentations, and Reports from the Sixth General Assembly of WEGENER held at St. Petersburg in June 1994 has been sent out . A few copies are still available if yours did not arrive. Contact Mike Pearlman at SAO for further information.

EDITORIALS/OPINIONS

SOME COMMENTS ON THE GLOBAL NETWORK OF SLR STATIONS, THE STATION DISTRIBUTION AND PERFORMANCE

P. Wilson/GFZ

The recent discussions over the future of SLR may not pass unnoticed by those who believe themselves not to be directly involved or threatened by the potential consequences, for in fact all of those engaged with the technique are under scrutiny and without a global network the technique will only be present as a historical footnote. Other techniques will follow the same route when the time comes. We have historical precedences that are less than 30 years old (ballistic cameras, NNSS and others). To date SLR operates with the lowest level of standardization and the least discipline of any of the international networks. This may have created a pleasant working atmosphere in the past, but this era has come to an end and if SLR is to survive as a necessary part of the tracking scenario, its efficiency must be increased and its cost reduced.

It is suggested that the key to future survival is to be sought in the deployment of a truly global network of about 20 evenly distributed stations producing a wealth of high resolution (1 cm), well calibrated data. These systems already exist - at least in their basics - or they are already being manufactured, though many are currently located in the wrong places. Serious efforts should be made to re-locate some of the best systems and technological activity in the immediate future should be directed towards upgrading the performance of the instruments that are or will shortly be available to improve their ranging resolution, their reliability and, in particular, reduce their operational cost (crew requirement), in order to obtain better return for the available resources. More effort must be put into the development of automation and, insofar as it will be possible, remote control and operation.

The scientific output from the technique to date has been impressive, but with GPS moving into many areas which have previously been the realm of SLR it is necessary to reconsider our management and the application of the technique. With the height component taking more attention, SLR offers a unique capability for resolving the seasonal variations in the position of the geocenter (the real origin of the vertical datum) and the continued tracking of the LAGEOS satellites has not yet revealed all long term components that are of interest. The routine orbit determination for other satellites will continue to be a task - at least as long as there is a network to respond.

GENERAL

UPCOMING MEETINGS

July 02-14, 1995

International Union of Geodesy and Geophysics XXI General Assembly

Boulder, Colorado USA

Contact:

AGU Headquarters

2000 Florida Avenue, N.W.

Washington, DC 20009 USA

July 04, 1995

Space Geodetic Measurement Sites (SGMS) Subcommission

Boulder, Colorado USA

Contact:

John M. Bosworth and Richard J. Allenby

Code 920

NASA/GSFC

Greenbelt, MD 20771 USA

July 06, 1995

Asian Pacific Space Geodynamics (APSG) Project Meeting

Boulder, Colorado USA

Contact:

Ye Shuhua

Shanghai Observatory

Chinese Academy of Sciences

80 Nandan Rd., Shanghai, 200030

PEOPLES REPUBLIC OF CHINA

July 12, 1995

Fundamental Geodynamic Reference Service (FGRS) Meeting

Boulder, Colorado USA

Contact:

Ivan I. Mueller

Ohio State University

Department of Geodetic Science

1958 Neil Avenue

Columbus, OH 43210-1247 USA

Nov. 15-17, 1995

Workshop on Global Sea Level Change

Miami, FL USA

Contact:

Chris Harrison

University of Miami/RSMAS-MGG

4600 Rickenbacker Causeway

Miami, FL 33149 USA

Dec. 11-15, 1995

American Geophysical Union Fall Meeting

San Francisco, California USA

Contact:

AGU Headquarters

2000 Florida Avenue, N.W.

Washington, DC 20009 USA

May 06-10, 1996 DD>European Geophysical Society

The Hague, The Netherlands

Contact:

ftp: linax1.mpae.gwdg.de

Username/Password: EGS96

May 20-24, 1996

American Geophysical Union Spring Meeting

Baltimore, Maryland USA

Contact:

AGU Headquarters

2000 Florida Avenue, N.W.

Washington, DC 20009 USA

July 14-21, 1996

31st COSPAR Meeting

Birmingham, England

Contact:

Postfach 49

37189 Katlenburg-Lindau GERMANY

Dec. 09-13, 1996

American Geophysical Union Fall Meeting

San Francisco, California USA

Contact:

AGU Headquarters

2000 Florida Avenue, N.W.

Washington, DC 20009 USA

PUBLICATIONS

These publications have appeared since the last Newsletter:

Newsletters:

Crustal Dynamics Data Information System Bulletin

Published by: NASA/Goddard Space Flight Center

Last Issue: Vol. 10, No. 5, June 1995

Contact:

Carey E. Noll

Code 920.1

NASA/GSFC

Greenbelt, MD 20771 USA

Space Geodetic Measurements Sites (SGMS) Subcommission Newsletter

Published by: NASA/Goddard Space Flight Center

Last Issue: Vol. 5, No. 1, March 1994

Contact:

Richard Allenby

Code 920.1

NASA/GSFC

Greenbelt, MD 20771 USA

Proceedings:

NASA Conference Publication 3283, Satellite Laser Ranging in the 1990s, Report of the 1994 Belmont Workshop, Belmont Conference Center, Elkridge, Maryland, February 1-2, 1994

Published by: NASA/Goddard Space Flight Center

Contact:

John J. Degnan

Code 920.1

NASA/GSFC

Greenbelt, MD 20771 USA

Sixth General Assembly of WEGENER, St. Petersburg, Russia, June 20-24, 1994

Published by: Smithsonian Astrophysical Observatory

Contact:

Dr. Michael R. Pearlman

Smithsonian Astrophysical Observatory

60 Garden Street

Cambridge, MA 02138 USA

Other:

Space Geodesy Personnel and Networking Directory, January 1995

Published by: NASA/Goddard Space Flight Center

Contact:

Carey E. Noll

Code 920.1

NASA/GSFC

Greenbelt, MD 20771 USA

NASA Technical Memorandum 4482, NASA Space Geodesy Program: Catalogue of Site Information, March 1993

Published by: NASA/Goddard Space Flight Center

Contact:

Mark A. Bryant and Carey E. Noll

Code 920.1

NASA/GSFC

Greenbelt, MD 20771 USA

UPDATE 1, NASA Technical Memorandum 4482, NASA Space Geodesy

Program: Catalogue of Site Information, September 1993

Published by: NASA/Goddard Space Flight Center

Contact:

Mark A. Bryant and Carey E. Noll

Code 920.1

NASA/GSFC

Greenbelt, MD 20771 USA

ACRONYMS

SLR NEWSLETTER INFORMATION

This Newsletter has been reviewed by the Steering Committee of the SLR Subcommission of the CSTG (see September 1986 issue). Preparation and publication of the Newsletter has been done by the Goddard Space Flight Center, and has been edited by John J. Degnan. Addresses of contributors and individuals mentioned in this issue are:

			Dr. Mohammed Al-Dail				 Van Husson
			Inst. of Astronomical & Geophysical Research     AlliedSignal Technical ServicesCorp./SLR
			King Abdulaziz City for Science an Technology	 Goddard Corporate Park
			P.O. Box 6086					 7515 Mission Drive
			Riyadh 11442  SAUDI ARABIA			 Lanham, MD 20706  USA
									 E-mail:  dsgvsh@cdslr1.atsc.allied.com
			Dr. Francois Barlier
			CERGA/GRGS					 Dr. Rolf Koenig
			Avenue Nicolas Copernic				 GeoForschungsZentrum Potsdam (GFZ)
			F-06130 Grasse	FRANCE				 Div. Kinematics and Dynamics of the Earth
			E-Mail:	 barlier@ocar01.obs-azur.fr		 GFZ/D-PAF, Postfach 1116
									 D-82230 Oberpfaffenhofen, GERMANY
			Dr. Giuseppe Bianco				 E-mail:  koenigr@dfd.dlr.de
			Agenzia Spaziale Italiana (ASI)
			Centro Geodesia Spaziale			 J. Andrew Marshall
			P.O. Box 11, 75100 Matera (MT)	ITALY		 NASA/GSFC
			E-mail:	 bianco@asimt0.mt.asi.it		 Code 926
									 Greenbelt, MD 20771  USA
			Alberto Cenci					 E-mail:  andy@viper.gsfc.nasa.gov
			Telespazio S.p.A.
			Via Tiburtina, 965				 Franz-Heinrich Massmann
			00156 Rome  ITALY				 GeoForschungsZentrum Potsdam (GFZ)
			E-mail:	 tpz3@icnucevm (BITnet)			 Div. 1, Kinematics and Dynamics of the Earth
									 German Processing and Archiving Facility,
			Brion Conklin					 Postfach 1116
			AlliedSignal Technical Services Corp./SLR	 D-82230 Oberpfaffenhofen, GERMANY
			Goddard Corporate Park				 E-mail:  fhm@dfd.dlr.de
			7515 Mission Drive
			Lanham, MD 20706  USA				 Edward Mattison
			E-mail:	 dsgbpc@cdslr1.atsc.allied.com		 Smithsonian Astrophysical Observatory
									 60 Garden Street
									 Cambridge, MA 02138  USA

			Mark Davis					 Jan McGarry
			AlliedSignal Technical Services Corp./SLR	 NASA/GSFC
			Goddard Corporate Park				 Code 920.1
			7515 Mission Drive				 Greenbelt, MD 20771  USA
			Lanham, MD 20706  USA				 E-mail:  mcgarry@cddis.gsfc.nasa.gov
			E-mail:	 dsgmad@cdslr1.atsc.allied.com
									 Carey E. Noll
			Winfield M. Decker				 NASA/GSFC
			AlliedSignal Technical Services Corp./SLR	 Code 920.1
			Goddard Corporate Park				 Greenbelt, MD 20771  USA
			7515 Mission Drive				 E-mail:  noll@cddisa.gsfc.nasa.gov
			Lanham, MD 20706  USA
			E-mail:	 dsgwmd@cdslr1.atsc.allied.com		 Dr. Antonin Novotny
									 Technical University of Prague
			Dr. John J. Degnan				 Department of Physical Electronics
			NASA/GSFC					 Brehova 7
			Code 920.1					 115 19	 Prague 1  CZECH REPUBLIC
			Greenbelt, MD 20771  USA			 E-mail:  novotny@troja.fjfi.cvut.cz
			E-mail:	 jjd@ltpmail.gsfc.nasa.gov
									 Dr. Michael R. Pearlman
			Richard J. Eanes				 Smithsonian Astrophysical Observatory
			University of Texas at Austin			 60 Garden Street
			Center for Space Research/C0605			 Cambridge, MA 02138  USA
			Austin, TX 78712  USA				 E-mail:  pearlman@cfa.harvard.edu
			E-mail:	 eanes@csr.utexas.edu
									 Amey R. Peltzer
			David Edge					 Code 8123, B58 R127
			AlliedSignal Technical Services Corp./SLR	 U.S. Naval Research Laboratory
			Goddard Corporate Park				 4555 Overlook Ave SW
			7515 Mission Drive				 Washington, DC 20375  USA
			Lanham, MD 20706  USA				 Email: peltzer@nrlfs1.nrl.navy.mil
			E-mail:	 dsgdre@cdslr1.atsc.allied.com
									 Francis Pierron
			Robert Fugate					 Observatoire de la cote d'azur
			Starfire Optical Range, Phillips Laboratory/LIG	 Satellite Laser Ranging Station
			3550 Aberdeen Avenue S.E.			 06460 Saint Vallier de Thiey  FRANCE
			Kirtland AFB, NM 87117-5776  USA		 E-Mail:  pierron@slr.obs-azur.fr
			E-mail:	 fugate@plk.af.mil
									 Dr. Andrew T. Sinclair
			Dr. G. Charmaine Gilbreath			 Royal Greenwich Observatory
			Code 8123					 Madingley Road
			U.S. Naval Research Laboratory			 Cambridge, CB3 0EZ  UNITED KINGDOM
			4555 Overlook Ave SW				 E-mail:  ats@ast.cam.ac.uk
			Washington, DC 20375  USA
			E-mail:	 gilbreath@ncst.nrl.navy.mil		 Dr. Ye Shuhua
									 Shanghai Observatory
			Denise Gorbitz					 Chinese Academy of Sciences
			AlliedSignal Technical Services Corp./SLR	 80 Nandan Rd., Shanghai, 200030
			Goddard Corporate Park				 PEOPLES REPUBLIC OF CHINA
			7515 Mission Drive				 E-mail:  xytan@fudan.ac.cn
			Lanham, MD 20706  USA

			Werner Gurtner					 Nobuo Sugimoto
			Astronomical Institute				 National Institute for Environmental Studies
			University of Berne				 16-2 Onogawa
			CH-3012 Bern  SWITZERLAND			 Tsukuba, Ibaraki 305  JAPAN
			E-mail:	 gurtner@aiub.unibe.ch			 E-mail:  sugimoto@sun51b.nies.go.jp

			Julie Horvath					 Dr. Peter Wilson
			AlliedSignal Technical Services Corp./SLR	 GeoForschungsZentrum Potsdam
			Goddard Corporate Park				 Telegrafenberg A17
			7515 Mission Drive				 D-14473 Potsdam  GERMANY
			Lanham, MD 20706  USA				 E-mail:  wilson@gfz-potsdam.de
			E-mail:	 dsgjeh@cdslr1.atsc.allied.com

Suggestions, questions, or contributions for future issues should be sent to:

Dr. John J. Degnan
NASA/GSFC
Code 920.1
Greenbelt, MD 20771 USA

Telephone: 301-286-8470
Fax: 301-286-0213
Internet: jjd@ltpmail.gsfc.nasa.gov
DECnet: CDDIS::DEGNAN

MAILING LIST UPDATE

If you would like a hard copy of this Newsletter, please contact:

Responsible Government Official: Carey Noll
NASA's Privacy Policy and Important Notices

Send us your comments

Last modified date: Friday, November 05, 2004
Author: Carey Noll
Maintained by: Carey Noll