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CDDIS Bulletin - August 1994
Volume 9 No. 6
In this issue:
About the cover: The map on the cover of this issue of the CDDIS Bulletin shows the current global network of permanent GPS tracking stations supporting the International GPS Service for Geodynamics (IGS). The primary objective of the IGS is to provide a service to support, through GPS data products, geodetic and geophysical research activities. The IGS collects, archives, and distributes GPS tracking data and products to a wide variety of users. The data from a majority of the sites shown, as well as the generated data products (precision orbits and Earth rotation parameters), are available from the IGS Global Data Centers (CDDIS, IGN, and SIO).
Carey Noll, Jan McGarry/NASA Brion Conklin, Win Decker, David Edge, Denise Gorbitz, Van Husson/ATSC
Introduction NASA, together with AlliedSignal Technical Services Corporation (ATSC), is 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 hopes to simplify and automate data processing and at the same time provide a more rapidly-available product to the user community. NASA would also like to reduce and eventually eliminate special 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.
Current Full-Rate Data Products The global network currently tracks thirteen 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 Geodätisches Forschungsinstitut (DGFI) in Munich Germany. ATSC merges the entire global data set into monthly increments by satellite; data are interleaved by time and provided in MERIT-II format. The CDDIS then distributes these data sets to users via 9-track tape, 4 mm tape, or electronically (see the June 1994 issue of the CDDIS Bulletin). ATSC also generates normal points from full-rate data; these data are available from the CDDIS electronically or via tape. The CDDIS distributes 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, is typically available six months after the end of the observation month.
Proposed Full-Rate Data Products The diagram shown in Figure 1 illustrates the proposed flow of SLR full-rate data. Where possible, full-rate data will be sent electronically from the site, nominally on a daily basis, to be processed by ATSC (and EDC). These data will be stored in compressed files on the CDDIS by satellite, by station, and by day. Figure 2 shows the proposed directory structure for these on-line files. Filenames will 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)
Automated procedures will ship these files to the CDDIS with a three day delay. For any data still delivered to ATSC or CDDIS via magnetic tape, procedures will be 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) will be 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. 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.
To further aid users in the evaluation of this new system, a sample data set has been placed on-line on the CDDIS. The latest release of the August 1993 data are now on-line in individual satellite/station/day files (under main directory SLR_DATA:[SLRFR]). 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. NASA plans to begin these procedures with data observed on January 01, 1995. Comments, questions, and suggestions on these plans should be directed to Carey Noll by October 01, 1994.
Carey Noll, Jan McGarry/NASA Brion Conklin, David Edge, Julie Horvath, Van 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). Currently, ATSC forwards 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 will also be forwarded to the CDDIS. The normal point and sampled data will be made available to users in separate files having the naming convention:
NEW_QLyymmdd.sat SLR normal point data
NEW_ENyymmdd.sat SLR engineering/sampled data
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. Initially, ATSC will continue to convert data into normal points for those SLR stations only capable of producing SAO, STDN, or CSTG sampled data. Therefore, the NEW_QL files will contain all on-site passes received, regardless of format, converted into CSTG normal point format. However, as of February 1, 1995, these conversions will no longer be performed. The NEW_QL file will contain true normal point data only; the NEW_EN file will only include 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.
B. Conklin, M. Davis, J. Horvath, V. Husson, U. Rao, G. Su/ATSC
The SLR Subcommission of the CSTG has recommended that SLR acquisition data and on-site SLR data products be standardized. In keeping with this philosophy, NASA's intent is to (1) phase out the production of SAO (mean Keplerian) elements, and (2) work toward the goal of having all global SLR stations producing field generated normal points as soon as possible. To this end, Allied Signal Technical Services Corporation (ATSC) has developed two PC programs, Tuned Inter-range Vector Acquisition Software (TIVAS) and Generic Normal Point Software (GNPS). SLR stations, which currently do not use tuned inter-range vectors or do not have on-site normal points capability, can use these software packages to make the transition to the new formats.
Both of these programs will be available via anonymous ftp from the CDDIS in the directory ANON_DIR:[SOFTWARE.SLR]. These software packages consist of executables, documented source code, user's guides, sample input files, sample output files, and README files. These programs require at least a 386 DX Personal Computer (PC), 4 Mbytes of Random Access Memory (RAM) and MS-DOS version 3.1 or above.
The PC TIVAS is a FORTRAN 77 PC program designed to produce inter-range vectors, inertial vectors, look angles, and ranges using daily tuned inter-range vectors as input. This program will facilitate satellite acquisition above the -20 degrees local horizon. Prediction parameters can be produced at the desired frequency for any SLR system by adjusting the integrator step size and by using the provided interpolator.
The PC GNPS is a FORTRAN 77 PC program designed to produce CSTG-formatted normal points using MERIT II formatted full-rate data as input. The software is generic, since it produces normal points for any SLR system and any currently supported SLR satellite. The Herstmonceux definition of a normal point is the basis for the normal point generation algorithms used in the code.
The original GNPS written in 1988 in VAX FORTRAN 77 produced only LAGEOS-1 two minute normal points in the MERIT II format. Upgrades to the software since 1988 include the production of normal points on other non-LAGEOS SLR satellites and a Poisson filter to edit "outliers" (non-satellite returns) in "noisy" passes. The CSTG accepts the current VAX-based version of this software as the global standard for SLR normal point production. The normal points produced by the PC GNPS compare to the sub-millimeter level with the normal points produced by the VAX GNPS.
The expected delivery date of the TIVAS executable, documented source code, user's guide, sample input files, sample output files, and README files is August 5, 1994; the expected delivery date of the GNPS executable, user's guide, sample input files, sample output files, and README files is also August 5, 1994. The GNPS documented source code will be provided at a later date.
If you to not have the computer capabilities described above but would like to use one or both of these software packages, or if you need technical assistance or have questions regarding these software packages, please contact ATSC as soon as possible. Any questions or comments should be directed to:
Brion Conklin (e-mail address dsgbpc@cdslr1.atsc.allied.com),
Mark Davis (e-mail address dsgmad@cdslr1.atsc.allied.com), or
Van Husson (e-mail address dsgvsh@cdslr1.atsc.allied.com).
Data from several 1992 and 1993 DOSE investigation experiments have been submitted to the CDDIS by JPL. The tables below summarize these data sets. Users interested in obtaining data from any of these GPS experiments should contact the CDDIS staff.
Table 1. GPS Experiment Information
SLR/VLBI/GPS
Experiment Name Exp. Type Investigators No. Sites Markers Occupied
Baja 1993 DOSE Tim Dixon 21 Ocotillo, CA;
Ensenada, Mexico
Mammoth Lakes 1992 DOSE Frank Webb 8 Mammoth Lakes,
Owens Valley, CA
Mojave 1993 DOSE Meghan Miller 7 Black Butte,
Mojave, CA
Mojave-Mammoth 1993 DOSE Meghan Miller
Frank Webb 14 Black Butte, Ely,
Mojave,
Owens Valley, CA
Ventura 1993 DOSE Andrea Donnellan 26 None
Table 2. GPS Experiment Data Summary
Data No. Size
Experiment Name Start Date End Date Source Files (Mb) Logs?
Baja 1993 05-Apr-93 20-Apr-93 JPL 275 18 Microfiche
Mammoth Lakes 1992 15-Sep-92 18-Sep-92 JPL 84 7 Microfiche
Mojave 1993 02-Mar-93 06-Mar-93 JPL 50 10 Microfiche
Mojave-Mammoth 1993 06-Aug-93 25-Aug-93 JPL 271 44 Microfiche
Ventura 1993 02-Nov-93 10-Nov-93 JPL 138 92 No
This section is designed to give brief notices of special events, new data sets, or problems encountered in recent months. The CDDIS staff believes the user community should be informed of these problems in order to ensure that the best results possible are produced from data issued by the archive.
MTLRS-1 occupied the site at Karitsa, Greece (site occupation designator 75201501) from March 18 through May 13, 1986. Peter Sperber (IfAG) reports that due to a software error in the MTLRS-1 site installation program in 1986, the orientation correction of the system, which was determined by star observations, was not saved in the site installation file. This oversight caused an orientation error of approximately 2.1 degrees in the computation of the eccentricity vector from the telescope to the main marker. This error shifted the coordinate solution for Karitsa in 1986 by approximately 8.5 cm to the east. The new corrected vector is now available in the eccentricity data base from Danny van Loon (and the CDDIS). Using this correct vector, the site velocity of Karitsa, computed from the laser observations in 1986 and 1989 is approximately zero, compared to the Eurasian plate.
The CDDIS recently installed a new 3.5 Gbyte disk drive. This drive will mainly be used for SLR data archiving, both quick-look and full-rate. In the near future, these data sets will be moved to new, more consistent disk and directory locations as follows:
Table 3. Proposed Disk and Directory Locations for On-Line SLR Data
Current Disk Location
New Disk Location
Type of SLR Data and Root Directory and Root Directory
Full-rate data SLR_DATA:[SLRFR.satname] No change
Normal points from
full-rate data DIS_DATA:[SLRDATA] SLR_DATA:[SLRNPT.satname]
Quick-look data DIS_DATA:[satnameDATA] SLR_DATA:[SLRQL.satname]
where satname is AJISAI, ERS1, ETALON1, ETALON2, GLONASS, GPS35, GPS36, LAGEOS1, LAGEOS2, METEOR3, MSTI2, STARLETTE, STELLA, or TOPEX. These changes should be implemented by September 01, 1994; users will be notified by electronic mail where possible.
Since the June 1994 issue of the Bulletin, the CDDIS has archived data from the following new GPS tracking sites:
Table 4. New GPS Tracking Sites
Mon.
Name Site Name Data Source Receiver Type Start Date End Date
CARR Carr Hill/Parkfield, CA JPL Rogue SNR-8000 28-May-94 --
CASA Mammoth Lakes, CA JPL Rogue SNR-8000 26-Nov-93 --
CAS1 Casey, Antarctica AUSLIG Rogue SNR-8100 05-Jul-94 --
DAV1 Davis, Antarctica AUSLIG Rogue SNR-8100 05-Jul-94 --
HOB2 Hobart, Tasmania AUSLIG Rogue SNR-8100 06-Jul-94 --
LBCH Long Beach, CA JPL Rogue SNR-8000 26-Jul-94 --
MAC1 Macquarie Island, Ant. AUSLIG Rogue SNR-8100 05-Jul-94 --
OATT Oatt Mountain, CA JPL Rogue SNR-8000 19-Jul-94 --
WLSN Mount Wilson, CA JPL Rogue SNR-8000 15-Jul-94 --
CASA is a semi-permanent GPS site and is part of the volcano monitoring project under DOSE. The principal investigator, Frank Webb (JPL) has initially requested a 180 day delay in the public release of the data. The LBCH, OATT, and WLSN sites were installed by JPL in support of the Southern California GPS Array Project; NASA plans to install twelve additional sites in the near future.
On the cover of the June 1994 issue of the CDDIS Bulletin, a map of Russia was shown identifying various SLR, VLBI, and GPS sites. Some of the GPS sites were designated as currently operational and supporting the IGS. As of time of publication, these sites are not existing IGS sites but plans are to install receivers in the near future. The author regrets any confusion this map may have caused the reader.
The CONT94 experiment was a fifteen-day session of continuous VLBI observations held in January 1994. Three independent networks consisting of 23 globally distributed VLBI sites observed during this experiment. Seventeen of these sites were co-located with GPS. Table 5 below summarizes the networks and sites observing during CONT94.
Table 5. CONT94 VLBI Experiment
Network Site Name GPS Receiver
R&D Network Fairbanks, AK Permanent
Kokee Park, HI (20 meter) Permanent
Westford, MA Permanent
Los Alamos, NM (VLBA) Temporary
Fort Davis, NM (VLBA) Temporary
Onsala, Sweden Permanent
Wettzell, Germany Permanent
VLBA Network St. Croix, U.S. Virgin Islands Temporary
Hancock, NH None
North Liberty, IA Permanent
Pie Town, NM Permanent
Kitt Peak, NM None
Owens Valley, CA None
Brewster, WA None
Mauna Kea, HI None
NAVEX-G Network Hobart, Tasmania, Australia Permanent
Kauai, HI (9 meter) Permanent
Green Bank, WV Temporary
Kashima/Misuzawa, Japan None
Matera, Italy Permanent
Fortaleza, Brazil Permanent
Algonquin, Canada Permanent
Hartebeesthoek, South Africa Permanent
GPS data from the permanent receivers are available routinely from the CDDIS. The data, both raw and RINEX, from the temporarily-installed GPS sites, Green Bank, Fort Davis, Los Alamos, and St. Croix, are now also available to interested users through the CDDIS.
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