[127] Because the Mercury orbital flight program required effective ground control during the unmanned and manned phases, a worldwide tracking and telemetry network was developed. Early in the project, the requirements for the network in terms of systems, installation, site locations, testing, and training for network personnel were established. Maximum utilization was made of existing facilities, but additional stations had to be implemented because of a strategic need at certain points along the orbital ground track. In addition to the telemetry and tracking facilities, two important centers were established, those of the Mercury Control Center, which was the focal point for all flight control activities, and the Computing and Communications Center. System reliability and provision for ease of maintenance were primary guidelines during the network implementation. Because of the unique spacecraft tracking task, an acquisition aid device was developed to assist in the location and tracking at first contact with the spacecraft. As the telemetry, tracking, and computation functions of the network were being installed, the network was staffed to support even the early ballistic flight program. As the scope and complexity of the missions increased, the network was expanded and modified to accept the changing and more demanding flight control and monitoring requirements. In addition to the tracking and data reception capabilities of the network, a multi- frequency air-to-ground reception and remoting provision was necessary during the manned flight program. A requirement had been established to provide continuous tracking and communications during the launch phase, as well as voice communications with the astronaut within maximum prescribed time intervals. Throughout the Mercury-Atlas orbital flight program, the Mercury Worldwide Network provided adequate and timely support in each of its charged responsibilities. Voice communications, telemetry, and tracking were satisfactory for effective flight control and monitoring, and the computation and data handling facilities provided timely support during the critical retrofire and reentry phases of each of the maimed orbital flights.

Meeting mission objectives required that a worldwide tracking and ground instrumentation system be developed to provide a continuous flow of information to be used for mission control. The intent in this paper is to describe the evolution of the network in support of the various Mercury missions. Specifically, the paper discusses the development of network requirements and systems; installation, test, and training; the network configuration and later changes made in response to mission requirements operations, and performance.

The task of implementing a tracking and ground instrumentation system was given to the NASA Langley Research Center (LRC). LRC formed the Tracking and Ground Instrumentation Unit to manage and direct this effort. This unit in turn utilized industrial firms to assist in determining the approach to be taken [128] in meeting the requirements in certain critical areas and to augment the NASA team.

Basically, network systems were required to provide all functions necessary for ground control and monitoring of a Mercury mission from launch to landing. The function of the network was to end when the spacecraft had landed and the best possible information on the location of the landing point had been supplied to the recovery teams.

At the outset, the following functional requirements were established:

(1) Provision of adequate tracking and computing to determine launch and orbital parameters and spacecraft location for both normal and aborted missions.

(2) Voice and telemetry communications with the spacecraft with periods of interruption not to exceed 10 minutes during the early orbits, contact at least once per hour thereafter, and communications to be available for at least 4 minutes over each station.

(3) Command capability to allow ground-initiated reentry for landing in preferred recovery areas and to initiate abort during critical phases of launch and insertion.

(4) Ground communications between the ground stations and the control center.

Safety of the Mercury spacecraft and its occupant was made a dominant consideration. Speed and efficiency of installation were essential to meet the planned operational dates. Although no compromises with safety were made, economy was an important consideration in the overall plan.

Stations were selected on considerations of the flight plan and on the character of the spacecraft electronic systems consistent with the basic requirements. Because of factors relating to the earth's rotation and the lack of suitable geographic locations, certain compromises had to be made in selecting the total number and locations of the stations required for a three-orbit mission. These compromises resulted in gaps, primarily on the third pass, greater than the desired 10 minutes. For stations selected, see figure 8-1.

Two Centers were also required:

The Mercury Control Center (MCC), to be located at Cape Canaveral, was to provide equipment necessary to allow control and coordination of all activities associated with the Project Mercury operation.

The Computing and Communications Center, to be located at the Goddard Space Flight Center (GSFC), Greenbelt, Maryland, was to provide for communications control, switching and distribution; also, it was to provide all computations necessary to monitor and control the mission from launch to landing.

Such an arrangement of stations, supported by appropriate instrumentation, would provide for tracking, command, and monitoring capabilities in the highest probable abort phase of launch through insertion and for the critical reentry phase after orbital flight. It also allowed the maximum use of facilities at the National Ranges and of equipment at the Australian Department of Supply facilities at Woomera, Australia. The participating countries and ranges were as follows:

The U.S. Department of Defense provided use of facilities at the Atlantic Missile Range, ! Pacific Missile Range, White Sands Missile Range, and the Eglin Gulf Coast Test Range.

Australia allowed the use of certain existing facilities and construction installation, and operations of the required new facilities. These arrangements were made through the Australian Department of Supply and were implemented by the Weapons Research Establishment.

United Kingdom permitted the construction of stations in Canton Island and Bermuda.

[129] Nigeria agreed to the lease of land and permission to construct a station in Tungu and Chawaka.

Spain agreed to provide the land for the Canary Island station.

Basic equipment design and implementation criteria for this program were the result of several major considerations. One of these was economics: existing facilities were to be used wherever they met the Mercury location requirements. Thus, at six locations, a major part of the equipment, including most of the network's tracking radars was already available. Another major consideration was time. Maximum use of existing proven equipment was dictated by the necessity to avoid the long-lead times required for research and development. But the primary consideration, overriding all others, was the safety of the astronaut. Some of the design requirements stemming from this consideration follow:

(1) Reliability of components and units was required to be designed and engineered into every element of the equipment configuration, and adequate testing, was required to prove this reliability.

(2) Despite rigid reliability requirements of units, redundancy was to be used extensively throughout each system and always at any critical point. Likewise, diversity was to be added to redundancy. Thus, a very reliable system was to be physically duplicated and then to be partially duplicated again by the use of an alternate frequency, location or some other means of achieving diversity.

(3) Wherever possible, the network system should have the ability to verify its own proper functioning . Suitable monitoring and display devices were thus required.

There were also other requirements resulting from "overlapping" of two or more systems. One of these concerned interference. Determined efforts were made to minimize interference to non-Mercury users of radio frequencies; to reduce mutual interference between Mercury equipment so that there was no degradation of system performance under normal equipment operation; and, to minimize interference from non-Mercury sources by carefully selecting station locations and equipment placement. Interference studies and field measurements were to be undertaken as required. Radiated noise measurements were to be made at all sites.

Particular attention had to be given to system integration problems and to simplifications which might be possible; for example, without compromising reliability, the possibility of reducing the number of antennas at a given site by use of antenna-sharing systems had to be considered.

Finally, all equipment had to be able to withstand the environmental conditions found in such diverse climates as those of the desert at Woomera and the "salt air" of Bermuda.

To provide mission support, the equipment of the network had to provide the following major functions:

(1) Ground radar tracking of the spacecraft and transmission of the radar data to the Goddard computers

(2) Launch, orbital, and reentry computations during the flight with real- time display data being transmitted to Mercury Control Center ( MCC )

(3) Real-time telemetry display data at the sites

(4) Command capability at various stations for controlling specific spacecraft functions from the ground

(5) Voice communications between the spacecraft and the ground, and maintenance of a network for voice, teletype, and radar data communications.

Development of the individual systems to meet these requirements is described in the following paragraphs. Some systems have been discussed in earlier publications (refs. 1 and 2); so they are only briefly described here. whereas other systems, especially systems requiring extensive design, are covered in more detail.

Mission requirements dictated the need for continuous radar tracking during launch and insertion to monitor the launch phase and to establish the initial orbital parameters on which the go-no-go decision would be based. During orbital flight, additional tracking data would be required for a more precise determination of the orbital parameters and time of retrofire for the [130] desired landing point. As nearly continuous tracking as possible was necessary during the less predictable reentry portion of the flight to provide adequate position data on the spacecraft's landing point.

To obtain reliability in providing accurate trajectory data, the Mercury spacecraft was equipped with C-band and S-band cooperative beacons. The ground radar systems had to be compatible with the spacecraft radar beacons.

The FPS-16 radar (fig. 8-2) in use at most

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national ranges was selected to meet the C-band requirement. Although it originally had a range capability of only 250 nautical miles, most of the FPS-16 radar units selected for the project had been modified for operation up to 500 miles, a NASA requirement, and modification kits were obtained for the remaining systems. In addition to the basic radar system, it was also necessary to provide the required data-handling equipment to allow data to be transmitted from all sites to the computers. Details on data flow and computation are discussed subsequently in the computer section.

The FPS-16 system originally planned for the network did not have adequate displays and controls for reliably acquiring the spacecraft in the acquisition time available. Consequently, a contract was negotiated with a manufacturer to provide the instrumentation radar acquisition (IRACQ) modifications. An essential feature of this modification is that it examines all incoming video signals, verifies the target, and automatically establishes angle-only track. Once the spacecraft has been acquired, in angle range, tracking in the automatic mode can be achieved with relative ease. Other features of the IRACQ system included additional angle scan modes and radar phasing controls to permit multiple radar interrogation of the spacecraft beacon. The addition of a beacon local oscillator wave meter permitted the determination of spacecraft-transmitter frequency drift.

Early in the installation program, it was realized that the range of the Bermuda FPS-16 should be increased beyond 500 miles. With the 500-mile- range limitation, it was possible to track the spacecraft for only 30 seconds prior to launch-vehicle sustainer engine cut-off (SECO) during the critical insertion phase. By extending the range capability to 1,000 miles, the spacecraft could be acquired earlier, and additional data could be provided to the Bermuda computer and flight dynamics consort This modification also increased the probability of having valid data available to make a go-no-go decision after SECO.

The Verlort radar (see figs. 8-3(a) and 8-3(b)) fulfilled the S-band requirement with only a few modifications. Significant ones were the addition of specific angle-track capability and additional angular scan modes. At Eglin Air Force Base the MPQ-31 radar was used for S-band tracking by extending its range capability to meet Mercury requirements. The data-handling equipment was essentially the same as for the FPS-16. Coordinate conversion and transmitting equipment was installed at Eglin to allow both the MPQ-31 and the FPS-16 to supply three-coordinate designate data to the AMR radars via Central Analog Data Distributing and Computing (CADDAC) .

After implementation these radar systems performed as planned and only minor modifications were made.

Once the types of radars to be used were determined, it became evident that these narrow-beam, precision-tracking units would have difficulty in initially acquiring the small, high-speed spacecraft. Without externally supplied dynamic pointing data, the spacecraft would pass through the radar beam so quickly that the basic radar circuits and/or operators would have very little time in which to recognize the target and switch into automatic tracking.

Two basic types of solution to the radar-acquisition problem were considered. One was the use of an on-site analog computer which [131] would be supplied with predicted spacecraft time and position data by teletype from the Goddard computers. The on-site computer would then generate dynamic-tracking data along the predicted orbit and supply it to the radar during the passage of the spacecraft. This approach was rejected because of the cost and development time necessary to provide suitable analog computers and because it was felt that complete dependence on teletype data for acquisition would not provide sufficient overall tracking system reliability.

The second solution to the problem was a new development called the "active acquisition aid." This device was designed to receive the

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spacecraft telemetering signals and automatically track the spacecraft in angle with sufficient accuracy to provide suitable pointing data to the radar.

The hardware to meet these requirements was developed around refurbished and modified SCR-584 radar pedestals, antenna, and receiver components. The major units of the final configuration used for Mercury are shown in figure 8-4, and figure 8-5 shows the acquisition aid antenna installation at Guaymas, Mexico.

Performance analysis. Tests of the first systems delivered showed two major performance deficiencies. The first of these stemmed from the fact that the spacecraft-telemetering transmitter bandwidth was substantially wider than had been anticipated; the acquisition aid receiver was consequently unable to achieve phase lock. This deficiency was corrected by [132] adapting another existing detector design to the Mercury equipment.

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The second major performance problem was that the equipment could not meet tracking accuracy specifications on a continuous basis. Two principal factors contributed to the accuracy problem. The predominant one, especially at low and medium elevation angles, was that of multipath signal reception. The lesser factor was the inherent coarseness of the quadhelix antenna array and other RF components. Redesign of the antenna would have pushed beyond the state of the art and probably would have de] eyed the program. Use of another, existing antenna with less beamwidth and therefore less multipath susceptibility would, of course, have meant some sacrifice of one of the most desirable advantages of the system: that of being able to cover large areas of space in a short period of time.

Fortunately, early experience with the radars, particularly the FPS-16 which, equipped with the IRACQ modification, can lock on a target very quickly, indicated that the accuracy requirements of the acquisition aid could be relaxed: analysis of tracking requirements showed that with proper alinement, the equipment would provide sufficiently accurate data to the radars. The specified accuracy for the active acquisition aid was thus relaxed to require only tracking within the beamwidth of the particular radar with which it worked ( ±0.5° for the FPS-16 and ±1.0° for the Verlort) for 2 seconds out of every 5 instead of ±0.5° on a continuous basis.

With these changes, the initial performance deficiencies of the system were alleviated. However, in the course of the project, a number of other modifications to the equipment were found necessary to improve reliability, ease of maintenance, and ease of operation. Installation of hermetically sealed RF components, waterproof connectors, better antenna limit switching and mechanical limit stops, and bias regulators for the RF amplifiers was made to improve reliability. Test points and grounding switches in the voltage-controlled oscillator (VCO) and a connector board with many of the system test points in one convenient location were installed to improve the ease of maintenance. Changes to the antenna handwheels, relocation of controls, and installation of mode switches were made to increase the ease of operas ion.

In conclusion, it should be noted that although a number of problems of varying degrees of seriousness were encountered with the acquisition aid-most of them stemming from the necessity of developing a new system in an extremely short time-the equipment successfully fulfilled its intended function. Rarely during the latter Mercury missions did one of them fail to acquire and track the spacecraft shortly after horizon time and thereby aid the radar in acquiring an automatic track.

Requirements. Early in the design of the Mercury system it was considered mandatory to receive information on a real-time basis and to provide for instantaneous computation and display of mission data from lift-off to landing. To meet these requirements, new data transmission equipment and computer peripheral gear were required. A new concept in large-scale, real-time data processing was required to tailor computations to a computer cycle and to manage the priorities of the computations performed automatically.

In all phases of the Mercury mission, it was vital that the many different forms of calculations be performed with exact precision and the data be made available almost instantaneously. For example, in a matter of seconds after launch-vehicle cut-off and spacecraft insertion into orbit, the computers were required to furnish data based on tracking information for evaluating whether or not the mission should be permitted to continue.

Before the Bermuda submarine cable was installed, it was decided to supplement the Goddard-Cape Canaveral complex with a secondary computing station at Bermuda. Installed there was an IBM 709 computer that received the inputs of the Bermuda FPS-16 and Verlort radars. The role of Bermuda was twofold: it served as a backup remote control center during the launch phase and as a tracking site thereafter. Specifically, it performed the following computing tasks:

(1) Provided all the necessary trajectory information to drive the display devices in the Bermuda control center.

(2) Computed an independent go-no-go at insertion based on Bermuda data.

(3) Computed retrofire times to be used in the event of an abort to land the spacecraft in one of the designated recovery areas.

(4) Computed refined landing points for several abort cases.

(5) Computed orbital characteristics.

(6) Sent postinsertion conditions to Goddard.

After the submarine cable was installed in April 1962, the Bermuda computer was removed and all the computations listed above were programed in the Goddard computers.

System description. Since the computing system was described in a prior publication (ref. 2), only a brief review is presented here.

During a mission, radar data from the network stations are transmitted by way of data circuits (ref. 2) to the communications center (fig. 8-6). Here, real-time equipment places the radar data from each tracking station automatically in the core storage of the computers. Two IBM 7091 computers operating independently but in parallel, process the data. Should a computer malfunction during the mission, the other computer can be switched on-line to support the mission while the malfunctioning computer is taken off-line and repaired.

The computers provide trajectory information necessary for the flight control of the mission. At MCC, about 18 digital displays, 4 plotboards and the wall map (fig. 8-7) are driven by the computers. This map shows the present position of the spacecraft and the landing point which would be achieved if the retrorockets were ignited in 30 seconds.

Development of new equipment. To implement a real-time computing system of the complexity of the one considered for Project

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[134] Mercury, it was necessary to design some specialized equipment. An example is the IBM 7281 Data Communications Channel (DCC) which automatically accepts inputs from a large number of data sources, places the information quantities directly at the disposal of the computer, automatically accepts calculated output data from the computer, and makes the information immediately available for transmission to many destinations.

For early missions, a duplexed configuration of IBM 7090 computers was connected by a DCC to radar stations, and sources comprising the real- time tracking and instrumentation system. For the MA-9 mission, a Triplex configuration of IBM 7094 computers, which were updated from the IBM 7090 configuration, was used.

Test and evaluation techniques. Any system as complex as the Mercury network had to be thoroughly tested under conditions as close to actual operating conditions as possible. It had to be certain that the units and subsystems were functioning properly and that all elements were functioning together as a complete system. Thus, it was necessary to devise computer-controlled tests to check out all computer-related elements of the total system. Called CADFISS (Computation and Data Flow Integrated Subsystem) testing, this worldwide network test concept was employed in Mercury launch countdowns to determine final tracking and data processing system readiness.

Performance analysis. A brief analysis of how the computing and data system performed during the manner orbital Mercury missions is presented.

Table 8-I shows FPS-16 and Verlort radar performance. Both radars approached their design limits while tracking an orbital target The values were derived by fitting the date to the equations of motion. The data were far better than expected. Note that, up until the MA-9 mission, the standard deviation in elevation for the FPS-16 is twice that in azimuth, probably as a result of refraction errors. An improved correction for refraction was incorporated into the Mercury programs for MA-9. This is not apparent in the Verlort; apparently the much higher noise level concealed the refractive error. In many cases the data from certain FPS- 16 and Verlort radars were better than the 0.1 mil and 1.0 mil criteria.

A comparison of the single-station FPS-16 orbital determination with the single-station Verlort solution shows that the FPS-16 is roughly four times as accurate in position and eight times as accurate in velocity determination.

The accuracy of the Mercury integration scheme, atmospheric mode], and tracking data is demonstrated in table 8-II. The orbit, as determined by multiple station solution, was integrated forward to compare with newer tracking data. The vector changes in position and velocity were averaged and are presented in table 8-II.

The accuracy of the total system is demon strafed by the calculation of time-to-fire retrorockets. The spacecraft timing system is such that the rockets are fired at the integer second. With the spacecraft traveling at 5 miles per second, the landing point is known only to ± 2.5

[135] miles. The recovery forces are able to estimate their position to about ±2 miles. Thus, the total uncertainty may be approximately ±5 miles. Table 8-III shows the landing points predicted for the four manned missions. The center column shows the landing point established by radar tracking. The tracking information in MA-7 and MA-6 provided landing points within 15 to 20 miles of that reported by the recovery forces. This difference may have resulted from lift experienced by the spacecraft in reentry. The predictions for MA-8 and MA-9 are well within the area of uncertainty and show a nearly perfect retrofire and reentry.

Several years ago, a prediction such as that shown in table 8-III would have appeared very optimistic for the performance of the manned space- flight network. In considering performances as a whole, the network can be said to have performed considerably better than originally anticipated. The network tracking and computing system has successfully predicted the spacecraft landing points, and at all times has provided accurate information on the astronaut's position. For all of the Mercury missions, the network and computing system performed their basic functions normally and without exception.

Mission	Change in position, yd	Change in velocity, ft/sec
MA-6	265	0.9
MA-7	266	1.1
MA-8	217	1.0
MA-9	a220	a 1.6
	b1,040	b 4.5

^a First three passes.

^b Mission average- no data on 15 of 22 passes.

Because the telemetry system has been described in reference 2, this section briefly describes only the design approach, modifications, and performance. To help orient the reader, a typical antenna installation at a telemetry station is presented in figure 8-8, and display and control consoles aboard a telemetry ship are presented in figure 8-9.

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Design approach. Obviously, the ground-station design requirements were established to be compatible with the spacecraft's telemetry characteristics. The basic type of telemetry system chosen early in Project Mercury was PAM/FM/FM. This system was chosen because it could provide the needed information [136] and was a reasonably well proven state-of-the-art type which could be implemented on the ground stations with commercially available hardware. Implementation guidelines used are as follows:

(1) Two independent links were to be used to gain reliability. The equipment at each station was to provide independent receiving systems for the two links from the spacecraft. Separate preamplifiers, receivers, diversity combiners filters, subcarrier discriminators and the associated monitor and control equipment were to be provided. Separate monitoring of the data from the subcarrier discriminators of each system with commutated data not decoded was to be provided to permit the operator to select the telemetry system output to be displayed at a main control console.

(2) At the stations which were to have command transmitters, separate decoding and display equipment was to he provided for the two telemetry links. (This arrangement was necessary to provide reliability in determining that the proper commands were received at the spacecraft.) . At all other sites, only one set of decommutation and output data display equipment was to be provided, with appropriate switching to the output of either receiving system.

(3) Provisions were to be made for separate magnetic tape recordings of the received outputs from each telemetry system to permit playback and reassessment of the data following a pass These recordings also were to provide a permanent record of the data with an overall accuracy of 1 percent.

(4) Data-output display equipment was to be provided with the appropriate meters, lamp indicators, and direct writing records.

(5) Continuous data on IRIG channels 5, 6, and 7 were to be recorded and displayed on direct writing strip chart recorders with an accuracy of 2 percent of full scale. Each of these channels was also to be provided with a suitable events-per-unit time display. (This provision was needed by aeromedical personnel to monitor the astronaut's heart action and respiration. )

(6) Individual data outputs of the analog quantities handled on the commutated subcarrier (PAM) were to be displayed on meters with an accuracy of 2 percent of full scale.

Display of the events data carried on the commutated subcarriers was to be in the form lights. Appropriate translation equipment was to be provided to display the time measurements as in-line decimal digits in hours, minutes, and seconds.

(7) Monitor displays were to be provided to permit the operator to assess the outputs of both receiving systems at a station and to select the system to drive the final data output displays.

(8) A permanent recording system capable of rapid processing and display was to be provided to record all subcarrier discriminator outputs, all decommutated analog quantities, and received signal strength.

(9) The overall system-accuracy requirement was that system error not exceed 2 percent under field conditions.

System performance. The telemetry and display system performance was outstanding throughout the project. During controlled flight, coverage time was generally horizon to horizon. Missions which had periods of drifting flight caused occasional signal dropouts due to nulls in the spacecraft antenna pattern. During reentry phases, both telemetry links were attenuated by the ionized sheath created by in tense heat and ablation of the heat shield and reception was completely lost for periods of 3 to 5 minutes.

System accuracy (to the displays) of 2 percent, as originally implemented, was met satisfactorily. Summary data from remote sites which included the degradation factors of 2-percent meters, meter parallax, short mission meter scales (e.g., utilizing 50 percent of full meter scale deflection), and reading error were generally within ±3 percent of full-scale meter deflection.

A system was required at each site to permit direct communications with the astronaut. This system, termed the air-ground system, would comprise all of the ground-based transmitting, receiving, control, and antenna equipment required to establish two-way voice communications with the Mercury spacecraft. General requirements included communications reliability, ease of rapidly restoring system operation in case of failure, and the use of proven [137] off-the-shelf equipment to reduce both delivery time and costs. The following paragraphs describe the specific requirements for this system, the system modifications,, and a summary of system performance.

Requirements. To provide a highly reliable system of communications which would be able to overcome difficulties arising from spacecraft equipment failure, atmospheric disturbances, and ground-equipment breakdown, the following specific requirements were established:

(1) Complete voice transmission and reception facilities for both HF and UHF operation were to be provided, with the HF equipment to serve as a backup facility for the UHF..

(2) Standby UHF transmitters were required for backup purposes at all stations.

(3) Standby HF transmitters were required for backup use at certain critical stations.

(4) Remote and local transmitter control was required for all transmitters.

(5) The means for operating these transmitters on tone modulation as well as voice was required.

(6) At those sites equipped with command transmitters, a voice- modulation capability for the command transmitters was required as an emergency mode of operation.

(7) A means was required for individual operation of the UHF, HF, and emergency-voice modes as well as simultaneous use of the UHF and HF or the UHF, HF, and emergency voices modes.

(8) At sites were transmitting equipment was to he installed in vans provisions for moving the van from the transmitting antenna to a receiving antenna were required in case of transmitting antenna or pedestal failure.

(9) To offset space-fading effects and also to provide built-in equipment backup facilities, dual space and polarization-diversity equipment was required for UHF reception, and duel-space diversity equipment was required for HF reception. This stipulation, then, required that two complete and identical sets of antennas, transmission lilies, and receiver elements for both the HF and UHF equipment be furnished at each site.

(10) Circular polarization of UHF transmitting and receiving antennas was required to offset signal attenuation caused by any skew attitude of the spacecraft antenna with relation to the ground antennas.

(11) Recording facilities were required for all transmitted and received audio.

(12) Varied distribution of all received audio and transmitter sidetones was required through monitor speakers and the station intercom system in order to satisfy the site operating requirements.

Performance. UHF was used for primary voice communication throughout the project with very satisfactory results.

Because of wave propagation, HF communication proved too intermittent to he used as more than backup communication and could not he considered as a reliable means of extending communication beyond station horizon. The HF quality improved somewhat, however, after a dipole antenna was installed on the MA-8 and MA-9 spacecraft.

A photograph of the air-ground antenna and transmitter van installed at Guaymas, Mexico is shown in figure 8-10.

Requirements. The criteria for the command equipment followed the general guide lines for all Mercury equipment. The basic requirement was the transmission of commands from certain stations to the spacecraft in order to provide a command backup for the manually controlled or internally programed events in the spacecraft. The range coverage of the command system was to be limited only by line-of-sight conditions to the spacecraft. The minimum normal range of the systems was originally set at 700 nautical miles.

This equipment was to employ a suitable coding technique to provide high reliability with particular attention to prevention of incorrect commands because of noise, interference, or transmitting equipment failures. All command sites would have dual FRW-2, 500-watt transmitters. The command antenna was to have at least 18-db gain, circular polarization, and to be steerable.

Modifications. Bermuda, having coverage of the critical insertion phase, required the ability to "brute force" command signals to the spacecraft regardless of the spacecraft [138] antenna position. A 10-kw RF power amplifier was to be provided for that purpose. Likewise, monitoring facilities that would provide failure sensing of this power amplifier were required. If failure occurred, antenna transfer to the operational 500-watt transmitter would be done automatically. Three existing sites already had this high power and failure switching capability.

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It was necessary to remove the standard coder controller of the FRW-2 and substitute coder control Units designed to be compatible with the coding technique employed in the spacecraft equipment and the input requirements of the FRW-2 coder KY-171/URW coder which was part of the FRW-2. Furthermore, the coder controllers were to be capable of remote activation and rapid changeover to any one of several codes which might be desired.

During the implementation phase of the program, ancillary equipment consisting of control and monitoring facilities was designed and fabricated. This equipment was necessary to provide the desired fail-safe features and degrees of flexibility this program required. Furthermore, at sites equipped with command vans, provisions were made to allow the transmitter van to be moved to the receiver antenna pedestal in case the command antenna pedestal failed.

Mission requirements made major command equipment additions necessary. The need for additional command coverage became apparent when the program was expanded beyond three orbital-pass missions. Consequently, dual 10 kw command facilities were installed on the Rose Knot Victor telemetry ship. The basic equipment furnished was identical to that furnished previously to the land-based stations. Temporary dual 500-watt command facilities were also added to the Coastal Sentry Quebec Ship. Here again, the basic equipment furnished was identical to existing land equipment.

Another major change in the command configuration was the MCC- Bermuda tone remoting system which became practical only after submarine cable circuits were available between Bermuda and Cape Canaveral.

Performance. As with the other systems, the command equipment functioned as planned throughout the project.

Introduction. Operation of this system was discussed in reference 2; therefore, it is only briefly reviewed in the present paper. Again the basic design criteria were used: reliability, cost, and speed of implementation.

Requirements. A primary requirement for the tracking network was that the stations be tied together with an adequate and reliable communications center. This center was to act as the heart of a communications system which would perform the following functions:

(1) Transmit acquisition information from the computing center to the tracking and telemetry stations.

(2) Transmit commands and instructions from the MCC to the stations.

(3) Transmit digital tracking data from the tracking stations to the computing center.

[139] (4) Transmit telemetry summary messages from the stations to the MCC.

(5) Provide high-speed data transmission between the computing center and the MCC for display purposes.

(6) Provide voice communications capability between certain stations and the MCC.

(7) Transmit mission teletype traffic throughout the network.

Both teletype and voice circuits were required. The teletype circuits usually operated at 60 words per minute and provided for transmission of all of the required types of information except high-speed tracking data and, of course, voice communications. These two were handled by voice- quality circuits with a pass band of 280 to 2,800 cps.

The network that was established to meet these requirements is illustrated in reference 2.

Because these channels traverse extremely long distances and employ a variety of transmission media, such as land lines of various types, submarine cables, and HF radio, it was necessary that the design arrangement and operating technique preserve their transmission capability. The chief factors involved were overall attenuation, bandwidth, distortion, noise, return loss, and echo.

Modifications and Performance. Following are some of the major changes made after the initial configuration was established:

(1) The HF link to Bermuda was dropped after the cable became available, and two highspeed data circuits from Bermuda to Goddard were added.

(2) The network was expanded to include the switching, conferencing, and monitoring (SCAMA) voice capability to Canary Island, Kano, Zanzibar, Canton Island, the Rose Knot Victor, and the Coastal Sentry Quebec.

(3) Zanzibar became a primary HF link for the Coastal Sentry Quebec.

(4) HF backup to Guaymas was added.

The Mercury communications network included 102,000 miles of teletype lines, 60,000 miles of telephone lines, and 15,000 miles of high-speed data lines.

The ground communication system operated very satisfactorily for all missions. Performance figures for the MA-7 and MA-8 missions are listed in table 8-IV.

A timing system was required to provide timing signals for all recorders in a common format, binary-coded time signals for radar data, strobe pulses for radar interrogation, and outputs for driving wall clocks and displays. The system was to have the capability of synchronizing with WWV timing with a resolution accuracy to within 0.001 second. The stability of the timing system was to be such that the local timing oscillator drift would not exceed 0.001 second in 48 hours.

The timing system which had been developed for the scientific satellite tracking stations was selected since it had proved to be reliable and accurate under actual field operating conditions.

The timing system performed satisfactorily throughout the Mercury Project, and only minor modifications were necessary to correct component failures and increase reliability.

It was apparent at the outset that rapid and flexible voice communications (intercom) would be needed within each station. Station personnel who would need such communications were (1) the flight controllers, who would monitor the flight status of the spacecraft and the overall conduct of the mission and who would advise and assist the astronaut in making decisions as required, and (2) the maintenance and operations personnel, who would provide technical support to the flight controllers in the operation of the various tracking, telemetry, and communications systems.

The intercom system had to have the capability of interconnecting several different consoles or positions in a conference type circuit (loop) whereby several people would be able to carry on a discussion, with others being able to "listen in" or be called on for comments or information. Also, because of the varied activities of different positions, there had to be several of these conference loops so that simultaneous conversations could be carried on with each loop usually isolated to one system or activity. The system also had to connect to outside lines so that the flight director could have immediate contact With any of the flight controllers at any station through the worldwide communications network.

After implementation by using standard components, only a few minor modifications to the intercom system were necessary to obtain proper, reliable operation. The system met the project requirements in a first- rate manner.

Mercury Control Center. The primary function of MCC was to provide a means of centralizing control and coordination of all the activities associated with a Mercury mission. Figure 8-11 is a view of the operation room of MCC. Mission control and coordination were conducted from MCC beginning at approximately 10 to 12 days before lift-off and continuing through the launch, orbital, reentry, and recovery phases. Communication, display, and control capability for MCC operation was provided in the various consoles, which are shown in figure 8-12. Many of the positions contained duplicate displays and controls to provide redundancy which was considered essential to the Mercury Project.

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Bermuda Control Center. In the earlier phases of the project, this secondary control center was required because the critical orbital insertion point of the spacecraft would be at a marginal distance and low- elevation angle from MCC, which might give unreliable data and would allow little time for MCC to determine go-no-go conditions. In addition, since Bermuda's vital tracking data needed for establishing insertion parameters had to be relayed by HF, a more fail-proof arrangement was needed. The Bermuda Control Center had the following basic functions:

(1) To command an abort in the event of critical spacecraft equipment failure or pilot difficulty late in the launch phase.

(2) To command an abort as directed by MCC in the event of certain propulsion or guidance system malfunctions.

(3) To control the mission independently in the event of communications failure with MCC

Figures 8-13 and 8-14 show a view of the center and an equipment layout.

After the submarine cable to Bermuda was available, it was possible to remote the control data safely to MCC. The Bermuda station functioned as a remote station for the MA-9 mission with a minimum of flight-control staff.

The development of a simulation system was established primarily to answer the need for an active training device for mission flight controllers. A secondary use for the simulation

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system was the familiarization of the maintenance and operating personnel with the mission support required of them for a particular flight.

The simulation system was designed in two parts: the first and major part was the addition of specialized instrumentation and control consoles at MCC that could be used by instructors to provide the stimulus necessary to activate the MCC operational consoles; the second part was a separate remote-site simulator for the purpose of training flight controllers who would be ultimately assigned to stations other than the control center.

Within a general requirement to furnish adequate instruction manuals for the network equipment, detailed specifications for individual manuals were prepared and the overall organization of this family of documentation was developed. The detailed specification called for new manuals to be prepared in accordance with the best commercial practices and established minimum content requirements for the acceptance of existing, off-the-shelf manuals. The most notable feature of the overall organization of the manuals was the concept of system manuals and equipment manuals. Equipment manuals covered individual units and subsystems, such as communications receivers, audio line amplifiers, and radar sets; and system manuals

provided information on how the individual units and subsystems tied together to form the major network system. Altogether, approximately 450 separate manuals with copies totaling nearly 50,000 were supplied for use on the network.

The installation of ground instrumentation equipment actually began with the efforts of the teams who selected the sites for the remote stations. The general area for each station had been determined from the planned orbit charts, but selected areas required on-site inspection for the evaluation of local problems and land availability. Each station had to be considered from cost, adaptability, and accessability standpoints. Every attempt was made to use existing facilities, but where these were not available below the orbital paths, sites were chosen which presented the fewest problems while satisfying the necessary criteria.

The Project Mercury tracking stations required considerable laud area to provide necessary isolation (seperation) between transmitting and receiving antennas. The equipment covered a very wide range of frequencies and required specific terrain configurations to operate at maximum efficiency. It was determined that five of the stations and the control center could be located on national ranges where use could be made of existing facilities. One new station was to be located in Texas and two on shipboard. The remaining eight would have to be established on foreign or overseas territory.

Selection of the foreign locations was accomplished by two teams. The first, a management team which had representation from the U.S. Department of State, was to determine and resolve, if possible, all difficulties of a general nature such as political considerations, preference of local officials as to station location, and currency problems. In addition, contact was made with local contractors, material suppliers, and service companies. Labor sources were also investigated and data on living conditions were obtained. The management team selected [143] a preferred and an alternate location for each station.

Data gathering was the prime function of the technical survey teams. Project personnel spent several days at each prospective site checking soil conditions, topography, water, sewage disposal, communications, transportation, electric power, and climate. A comprehensive report prepared on each site provided the basis for station selection and was used thereafter as a guide for equipment design and location.

The tight schedule made it impossible to stagger construction at the various stations. Although first construction operations were not started until April 29, 1960, all stations were under construction by midsummer, and construction was completed at the last station in Kano, Nigeria, in March of 1961.

Most buildings were constructed of prefabricated galvanized sheet metal supported by rigid steel frames. In addition to the buildings housing electronic equipment, most stations contained power buildings, cooling towers, air handlers, water chillers, and hydropneumatic tanks. Diesel generators were installed to produce power to back up commercial power.

Extreme precision was necessary in the positioning of every radar antenna. Each unit had to be surveyed to determine true latitude and longitude with exact interrelation, and angles were established with a maximum allowable deviation of 6 seconds.

As construction of facilities was still underway at some stations, the equipment and the installation teams were arriving. The number of installers on a site team varied between 5 and 25, depending on the amount of equipment to be installed. A typical team consisted of the site manager, the team crew chief, a lead man for a subsystem or a combination of subsystems, several technicians, and one or two subcontractor advisors for specialized areas such as the acquisition system. Each team was also supported by a logistics man.

All installation team leaders were authorized to work with the local labor unions and utilize the local labor market to perform certain jobs beyond the capabilities of the installation team and its facilities.

Two depots -one on each coast of the United States - were established to provide logistics support for the overseas stations and to handle the customs details involved in such shipments. The depots served as staging areas for overseas shipments, whereas equipment destined for stations in the United States was shipped directly from the manufacturer. More than 1,000 tons of cargo were processed through the depots, most of it in preassembled units. A rigid receiving and inspecting system was set up at each station to check in all equipment before it was turned over to the installation team.

Spare parts provisioning was another logistics consideration. There had to be a reasonable on-site repair capability. Each industry team member supplied a 2-year supply of spares unique to his equipment and a list of recommended common item spares. From these lists a combined list of common item spares was drawn up to eliminate duplications. Common item spares were procured in accordance with the combined list and shipped to each site.

Thus, the concept of a network of stations became a reality with equipment and logistic support. The scope of design, construction, installation, and activation for the Mercury Network is shown in figure 8- 15.

Figure 8-16 shows construction underway at Kano.

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Demonstration site. The necessity of testing and evaluating the ground instrumentation equipment as a complete system prior to its installation on a worldwide basis was recognized in the early planning stages of the Mercury Project. Equipment from more than 10 major manufacturers plus numerous subcontractors was involved, and it had to be determined that all interrelated problems had been solved and that the equipment would perform as a system.

The selection of NASA Wallops Station, Wallops Island, Virginia, as a test site was determined primarily because of its availability and its proximity to Langley Research Center at Langley Air Force Base, Virginia, and the Goddard Space Flight Center at Greenbelt, Maryland. A complete tracking station was installed, with the Mercury data conversion and acquisition equipment connected to the existing FPS-16 at the Wallops Station Launch Complex.

Representatives from the suppliers of equipment conducted tests at Wallops under NASA supervision. As a result of these tests, many changes were made to equipment in the prototype stage prior to worldwide deliveries. Also developed at the Demonstration Site were test procedures that were used throughout the network for acceptance testing of on-site equipment.

The test procedures were of four types:

(1) Mercury Unit Tests (MUT) were developed to provide acceptance of self-contained equipment such as the R-390 HF voice receiver or the Ampex FR-100B tape recorder. The unit tests covered every measurable aspect that could influence the reliability of minimum performance expected of the unit.

(2) Mercury System Tests (MST) were developed to provide acceptance of a complete system. These tests checked the action of each interfaced relay as well as system performance.

(3) Mercury Integrated Tests (MIT) were developed to provide acceptance of the station as an integrated complex. These tests assured successful interface of systems. They also revealed RF interference problems.

(4) Mercury Dynamic Tests (MDT) were developed to test the equipment under simulated operating conditions. As ground station equipment was installed and evaluated at the Demonstration Site, the need for a method of closely simulating spacecraft tracking soon became apparent. Small leased aircraft were used to check the tracking accuracy of the new acquisition aid, and it was found that certain modifications were necessary for the equipment to meet specifications.

Instrumented aircraft. As a result of these and other special aircraft tests, it was decided that aircraft would be obtained and completely instrumented with actual spacecraft electronics (see fig. 8-17) to serve three functions:

(1) To qualify each ground system prior to worldwide equipment delivery so that compatibility between ground and airborne systems was assured.

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[145] (2) To provide a complete checkout of each station in the network so that operational readiness was determined.

(3) To provide continual testing and training throughout the Mercury Project.

Prior to station assignment, selected senior engineers received specialized equipment training and later helped to install the equipment at the Demonstration Site. After assignment, these senior engineers were responsible for making their equipment operational and for indoctrinating the other team members. Training was largely accomplished by working with the equipment during installation and by playing an active role in conducting acceptance tests. As time allowed semiformal classes were held in theory and maintenance.

Formal training. Installation technicians were technically capable of performing maintenance, but operational requirements posed the need for a refinement of the team concept and a regimented reaction to the demands of mission accomplishment. Transition from installer and maintenance technician to operator was accomplished by a rigorous training program that included: formal indoctrination lectures on spaceflight matters and on Project Mercury; on-the-job training combined with classroom drills covering operation of the equipment local-station simulated missions: and network simulations using countdowns, live communications, and telemetry tapes.

The maintenance and operation capability of station personnel had to be continually upgraded, and replacement personnel had to be provided. Likewise, the station had to be exercised as an entity to assure that it could work as a cohesive unit during a mission.

Training center. To upgrade individual capabilities and to provide replacement personnel, a training center was established at the Demonstration Site. The primary long-term objective of the Engineering and Training Center was to sustain or improve the level of competence of the personnel manning the Mercury network stations through a comprehensive training program in each of the equipment subsystems making up the station. It was also designed to give the necessary high- level training to replacement personnel so that network proficiency would not suffer from personnel attrition.

To supplement the training received at the center, cross-training packages of lesson guides, equipment exercises, and examinations were developed for use at all the Mercury network stations. These were used for training of personnel in secondary areas of responsibility to enhance the overall capability of each team at the stations.

Up to this point, network requirements and systems development and implementation have been discussed. The types of systems available at each site are listed in table 8-V. To illustrate how a Mercury station was arranged, a little drawing of the Hawaii station layout is shown in figure 8-18.

Changes for MA-7. The second manned orbital flight, MA-7, was also planned as a three orbital pass mission. The network configuration was the same as that for MA-6 except for minor exceptions; there was no Atlantic Ship, and the Indian Ocean Ship was repositioned in the Mozambique Channel, off the east coast of Africa.

Changes for MA-8. The MA-8 mission was planned to be a six orbital pass mission with landing to be made in the Pacific Ocean. For this mission, the former Atlantic Ship had a command system installed and was redesignated as the Pacific Command Ship (PCS) for positioning south of Japan. Three additional ships, the Huntsville, the Watertown, and the American Mariner, were made a part of the network and positioned near Midway to get reentry data.

Changes for MA-9. Since it was decided to extend the length of the MA-9 mission to 22 orbital passes, it was necessary to modify the network so that adequate support could be provided. The following describes the changes that were required:

Equipment:

(1) All command sites were provided with additional command capabilities to give the site flight controllers the capability to turn on the spacecraft's telemetry transmitter, radar beacons and an astronaut alarm. Other command changes included the addition of a complete system aboard the Coastal Sentry Quebec (CSQ) and an increase of the Rose Knot Victor (RKV) command power from 600 watts to [147] 10 kilowatts. Figure 8-19 shows the two ships in the port of Baltimore for modifications.

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(2) Mercury tracking site clocks showing "spacecraft elapsed time" and "time to retrofire" were modified to extend their reading time.

(3) Additional equipment was installed at California and Bermuda, allowing biomedical data to be sent (over land lines) to MCC display consoles.

(4) A telemetry automatic processing system that used a small general purpose computer (AN/UYK-1) was installed at Bermuda. The system was designed to accept PAM/FM/FM frames of 88 parameters every 800 milliseconds in real-time and generate special and regular summary messages. The output data were in a format which represented selected parameters in engineering units. A running tolerance check of all parameters was included and selected data were stored for postpass analysis.

(5) Receivers were installed at MCC, Canary Island, and the CSQ for reception of the slow scan TV picture from the spacecraft. The installation at MICC and on the CSQ included record and display capabilities, whereas the installation at CYI was for record only.

(6) An additional IBM computer was added to the computer complex at GSFC, and the two 7090's already in operation were converted to 7094's.

Communications:

(1) The radio links to BDA were discontinued since the submarine cable was now operational.

(2) Communications to the CSQ at the new location were handled through a radio link which could operate through either Honolulu or Bassendean and thence by the usual path.

(3) Communications to the RKV were handled by RF links to Honolulu and New York.

(4) A new circuit was added to relay the Range Tracker data through Honolulu.

(5) The mission message format was changed to improve circuit operation and to facilitate accumulation of more data.

(6) New equipment arrangements were instituted at Goddard to permit CADFISS and operational programs to be conducted simultaneously.

Relocation of ships: The Coastal Sentry Quebec was relocated to the approximate position of 28°30'N. latitude and 130°00'E. longitude. The primary purpose of this location was to provide adequate retrosequence command backup during the 6th, 7th, 21st, and 22nd orbital passes.

The Rose Knot Victor was relocated to the approximate position of 25°00'S. latitude and 120°00' W. longitude. In this position, it provided optimum command coverage for passes not covered by other network sites. The RKV provided coverage with its 10-kw command transmitter during the 8th and 13th orbital passes.

Additional support: To provide the necessary coverage to support a mission of this duration it was necessary to add the following tracking facilities:

(1) The Range Tracker (C-band radar equipped ship) was stationed at 31°30' N. latitude and 173°00' E. longitude to provide reentry radar coverage for the 4th, 7th, and 22nd orbital passes.

(2) The Twin Falls Victory (C-band radar equipped ship) was stationed in the vicinity of 31°3' N. latitude and 75°00' W. longitude for reentry radar coverage for the 2nd and 17th orbital passes.

(3) The Ascension Island station provided FPS-16 radar tracking during the fourth orbital pass. Also provided were telemetry recording, air- ground relay, and ECG remoting.

(4) The East Island, Puerto Rico, station provided FPS-16 radar tracking.

[148] (5) The Antigua Island station provided telemetry recording, air- ground relay, and ECG relay.

(6) Air-ground voice facilities were provided at Wake Island, Kwajalein Island, and San Nicholas Island. The Wake and Kwajalein sites provided an extension for the Hawaii airground facilities. California had additional coverage provided by the San Nicholas installation.

Time at the tracking station is generally divided into mission periods and nonmission periods. The mission period for Mercury comprised some 10 days prior to launch and the actual flight time. The nonmission period was the time between missions used for personnel training, equipment modification, testing, and checkout. The operations activities during the mission period are explained in the following paragraphs, with the MA-9 mission used as an example.

The MA-9 precountdown period for all network stations was scheduled as follows:

F-7 day-Orbital mission simulation and reentry simulation

F-6 day-Orbital mission simulation and reentry simulation

F-5 day-Two reentry simulations

F-4 day-Detailed system tests

F-3 day-Equipment maintenance

F-2 day-Orbital mission simulation

F-1 day-Patching check and equipment maintenance

These various activities are described in the following paragraphs.

Simulations. To the station, the simulations were full-dress rehearsals for the missions.

With the entire network participating and all onstation systems in operation, authentic dry runs were conducted, complete with built-in emergency situations which had to be detected, analyzed, and acted upon in "real time" by the flight controllers and station personnel. Authenticity was gained by the use of taped inputs to the telemetry displays and events recorders and by the use of a communicator reading from a prepared script over the intercom loop that would ordinarily carry the real astronaut's voice. In addition to anticipated problems of spacecraft equipment malfunctions, the ground team had to cope with such remote possibilities as simulated heart attacks of the astronaut in flight.

Simulations would ordinarily cover launch and three orbital passes and might or might not cover reentry. Each simulation would take from 4 1/2 to 6 1/2 hours. Prior to MA-8, a full 18-orbital-pass mission was simulated in anticipation of MA-9 as a means of pointing out any major problem areas in personnel scheduling, sleeping, and eating plans.

Detailed system tests. The detailed system tests (DST), mentioned earlier as being performed on F-4 day, were a group of standard procedures used to check and measure thoroughly the operational performance of each of the station subsystems. Since the same test was used for corresponding systems at all stations, and since results of previously run DST's were recorded, the current status of any subsystem could be easily evaluated by the DST performed just prior to the mission.

The DST procedures consisted of two parts: the instructions and the data sheets. Meter readings, voltage and current measurements, standing-wave ratios, and various other parameters were recorded on the data sheets which were returned to Goddard for analysis immediately after the mission. On the station, the cumulative results of the DST's were used in the determination of the station status, which was a factor in the decision to proceed with or delay the launch.

Maintenance day. F-3 day and F-1 day were left open for last-minute maintenance details, particularly in correcting any equipment deficiencies detected during the DST's. Final briefings were also held to correct any procedural problems pointed up by the previous simulations.

The network countdown began 5 hours and 50 minutes prior to the scheduled launch. This time was devoted to computer and data flow checks, teletype checks, voice checks, and brief system tests. The Network Countdown document specifically scheduled each of these activities, and designated the stations and equipment [149] positions to which a particular operation was applicable. The brief system test was a shortened version of the DST and was designed to lend assurance that equipment performance had not significantly deteriorated since the DST was run 4 days previously. Whereas the DST may have taken 12 or more hours, most DST's could be performed in less than 2 hours.

The Network Countdown also contained the "plus-count," a scheduling of pertinent activities to be performed before acquisition of the spacecraft and during the pass.

After launch of the spacecraft, a time period of from about 5 minutes (at Bermuda) to 90 minutes (at Eglin) would elapse before the spacecraft passed over the station. The actual pass, the time from which the spacecraft appeared above the horizon until it was lost below the horizon, averaged about 7 minutes. Average time between passes was about 85 minutes. This time was devoted to equipment calibrations-setting up known levels and annotating the recorders so that later analysis would have known standards and preparation for the next pass.

Prepass calibrations were begun 45 minutes before the start of the next pass. Twenty-five minutes prior to the pass the first acquisition message would be received. This was a teletype message sent from the control center advising the station of the time and coordinates at which it could expect to acquire the spacecraft. These figures were derived by the computers at Goddard based on the real-time radar data from the last station passed over by the spacecraft. The information permitted the acquisition and radar operators to train their antennas to the spot where the spacecraft would first be "sighted." A second acquisition message was received 5 minutes prior to the spacecraft passage to communicate any inflight deviations during the intervening 20 minutes.

Acquisition would ordinarily take place within a few seconds of horizon time. Because of the wide beamwidth of the antenna used by the active acquisition aid, this system ordinarily was the first to acquire the target. At radar sites, the S-band and C-band radars would nonetheless search independently. At contact, all antennas were immediately slaved to the system which acquired first.

As the radar locked on target, it would then be set to track automatically, and, at operator discretion, it could be made the controlling system for the other antennas. At dual radar sites, data from the C-band radar-the most accurate of the two systems-was fed to the teletype for transmission to the computers at Goddard. If this radar lost track, data from the S-band radar were put on the line.

As soon as possible after the last pass over the station, the postlaunch instrumentation message was teletyped to the control center. It contained a tabulation of the times of acquisition and loss of signal for the various systems, the modes of operation, and a summary status report.

It was obvious that the length of the MA-9 mission would preclude the manning of all station equipments from launch to termination. The flight path was such, however, that all stations had periods when the spacecraft would not pass over them for three or more orbital passes.

Documentation guides. Three documents provided the major guideline for station personnel activities during the pass. The Network Operations Directive 61-1, was produced jointly by MSC, GSFC, and DOD and it set forth the general operating procedures for all systems so that a standard action would be used in a given circumstance at any station in the network.

The second document, the Data Acquisition Plan, gave detailed instructions for recorder setups, pen assignments, patching arrangements, and plotboard assignments and gave information for disposition of data records after the mission. A new Data Acquisition Plan was published prior to each launch. It was prepared by MSC with inputs from GSFC.

The third document was the Communications Operations Plan, prepared by GSFC. This was a detailed account of how the communications network was to function.

The Mercury network, throughout all orbital flights of the Mercury spacecraft, has clearly demonstrated its capability to keep track of a manned spacecraft and remain in communication [150] with the astronaut. These capabilities are the direct result of the many months of planning, instrumentation installation and checkout, training, and the highly efficient performance of the equipment and personnel at all network sites during the actual missions.

There were six orbital flights of the Mercury spacecraft, one unmanned (MA-4), one with

chimpanzee aboard (MA-5), and four manned (MA-6 through MA-9). The network performance continually improved during these missions as more and more experience was gained. This progress was typified by the peak performance demonstrated during the last Mercury mission, MA-9. It lasted for nearly 22 orbital passes (fig. 8-20) with the spacecraft landing in the planned landing area near Midway Island in the Pacific Ocean. There were some minor equipment failures associated with the Mercury network, but they did not materially affect mission Support or detract from the excellent performance demonstrated by the network throughout the flight.

A summary of network performance for the MA-9 mission is presented in the following paragraphs.

During the countdown on May 14, 1963, the radar at Bermuda failed to pass the CADFISS slew tests. Digital data were intermittently of poor quality in both the azimuth and range channels. Efforts to locate the trouble were ineffective, and the quality of the data gradually decreased. At T-15 minutes, the range data error exceeded the tolerable limits, and at T-13 minutes, the mission was postponed for 24 hours. Subsequent investigation revealed a faulty preamplifier in the azimuth digital-data channel and a faulty shift register in the range digital-data channel. The simultaneous failure of both components complicated the failure analysis.

On launch day there were no radar problems, and the C- and S-band beacon checks prior to [151] launch indicated no beacon problems. The network C-band radars tracked approximately 10 percent of the total mission time, which is 80 percent of the total time that the C-band beacon was turned on. The network S-band radars tracked 1.7 percent of the total mission time, which is 36 percent of the total time that the S-band beacon was turned on. The amount of radar data furnished to the Goddard computers was of sufficient quality and quantity to update the trajectories and it was determined that the orbital parameters did not decay an appreciable amount. Initial tracking reports indicated that the C-band beacon was not as good as it had been on previous mission because of the heavier than usual modulation off the beacon replies. The heavy modulation experienced by the MCC and Bermuda radars during launch seemed to lessen as the mission progressed.

In addition to the normal Mercury Network radar sites, the following sites were used for the MA-9 mission: Ascension Island, East Island, Puerto Rico, and the radar ships Twin Falls Victory and Range Tracker.

In general, the performance of the acquisition-aid systems at all stations was satisfactory and comparable to that of previous missions. Low-angle elevation tracking below approximately 15°, was accomplished manually because of multipath conditions at most stations. The only major acquisition-aid problem experienced during the mission was on the Coastal Sentry Quebec, where failure of the elevation antenna drive system occurred prior to the 6th orbital pass. However, the antenna was positioned manually from the 6th through the 8th passes, and the malfunction in the drive system was corrected in time for acquisition in the 9th pass.

The MA-9 countdown began at midnight on May 14, 1963. The Goddard computer, equipment, interface, CADFISS, and trajectory confidence tests were all satisfactory. During the countdown, while using the "B" computer, some dropout was observed at the MCC. The high-speed output subchannel on the "B" computer communication channel was interchanged with the plotboard high-speed subchannel.

At the request of the Flight Dynamics Officer, the powered flight phase was supported with the "A" and "C" computers, then switched to the "A" and "B" computers during orbital flight. The "B" computer gave no indication of dropout during the rest of the mission. Liftoff occurred at 08:01:13 a.m. e.s.t.

The Atlantic Missile Range (AMR) I.P. 7094 and the General Electric- Burroughs guidance computers provided excellent data throughout the launch. A "go" decision was indicated by all three data sources.

In the orbital phase, during the periods when the spacecraft C- and S- band beacons were on, the tracking data received from the network sites were excellent. During the mission, spacecraft weight change data resulting from fuel and coolant-water usage were manually put into the computers.

The retrofire time recommended by the Goddard computers was 33:59 :30 ground-elapsed time (g.e.t.), and retrofire was manually initiated at this time. After retrofire, the predicted landing point transmitted to the MCC from the Goddard computer was 27°22' N, latitude and 176°29'W. longitude. An attempt to refine this prediction with six frames of data acquired by the Range Tracker ship during blackout failed to yield a converged solution. The computed time of the blackout was from 34:08:16 to 34:22:30 g.e.t. The actual time of initial blackout was reported by the Range Tracker to be 34:08 :17 g.e.t. The actual landing point was reported by the recovery ship to be 27°22.6'N, latitude and 176°35.3'W, longitude.

Although several minor computer problems were encountered and corrected throughout the flight, at no time during the mission did the computers fail to drive the digital displays and plotboards at the MCC. In addition, performance of the high-speed lines between Goddard and the MCC was excellent.

For the first time, CADFISS tests were conducted during the mission to determine the operational status of major equipment subsystems at network sites. These tests were considered necessary since mandatory equipment at many sites did not operate for prolonged periods of time when the spacecraft was out of range. All of these test were successfully supported by the third Goddard computer while the other two [152] Goddard computers continued the operational support of the mission.

Two range ships, the Range Tracker and the Twin Falls Victory, were used to provide tracking data to the computers. The Range Tracker provided good tracking data during the 7th, 20th, and 21st orbital passes. During reentry the Range Tracker was poorly positioned with respect to the blackout zone and provided only six frames of data for this phase of reentry. An analysis of these data indicated a landing point which was about 3° or 180 nautical miles away from the correct landing point. Twin Falls Victory data readout was good on three passes.

The telemetry coverage for the mission was excellent. There were no major ground system failures, although some coverage was lost because of the manual switching procedure used onboard the spacecraft. In general, any deviation from nominal coverage can be attributed to spacecraft attitude or to the transmitters being turned off. The telemetry relay circuits from Antigua, California, Bermuda, and Ascension were satisfactory in all respects. During all passes were these stations when telemetry antennas were radiating, data were remoted to the MCC. During the third orbital pass, the telemetry was switched to the high-frequency link prior to the spacecraft's passing over Hawaii and remained on until it was over the California site, at which time telemetry was switched back to the low-frequency link. At all other times, the telemetry remained on low frequency. No telemetry system anomalies were noted during this period.

The air-to-ground communications were of good quality. The UHF system was used as the primary communications system except for the scheduled HF checks. During periods of communication, UHF coverage varied only slightly from predicted acquisition and loss times because of the nominal orbital trajectory. As expected, air-to-ground communications could not be established during the communications blackout period. An Instrumentation Support Instruction was transmitted to the network outlining the use of the UHF squelch circuit as defined in the network documentation. A premission checkout and the mission results indicated that proper use of the squelch circuit eliminated background noise from open UHF receivers during periods of silence. This change also resulted in a reduction of noise level on the Goddard circuit during air-to-ground transmissions.

Relay aircraft in the Atlantic Ocean area reported good UHF reception from the spacecraft and good relay transmissions to MCC on the 2nd, 3rd, and 17th orbital passes. A relay attempt on the 16th pass was unsuccessful because of a severe thunderstorm in the vicinity of the relay aircraft. Communications from the MCC to the spacecraft through the relay aircraft were not attempted on the 2nd pass, and they were unsuccessful on the 3rd pass because the spacecraft had passed out of range. However, the relay communications were successful on the 7th pass. Ascension and Antigua Islands in the Atlantic were also available for relaying communications between the spacecraft and the MCC. Relay through Ascension was successfully accomplished for a period of approximately 6 minutes during the third orbital pass. The Antigua voice relay was not used during the mission.

In the Pacific Ocean area, communications were successfully relayed from Hawaii through Kwajalein and Wake Islands on passes 3 and 19, respectively. A voice-operated relay from the MCC through the Range Tracker was attempted on the 20th orbital pass. However, this attempt was unsuccessful because the transmission was made on the MCC-Hawaii remote air-ground position instead of the Goddard Conference Loop. This error apparently placed a 1700-cps tone on the circuit to the Range Tracker and resulted in keeping the automatic voice relay continuously closed; however, several transmissions from the astronaut were received in the MCC. Another attempt to use the relay on the 22nd pass was ineffective. As in the MA-8 mission, satisfactory communications were established in the primary landing area between the spacecraft and Hawaii by using relay aircraft.

The reader is referred to appendix F for a transcript of the MA-9 air-to- ground voice communications.

[153] The command system for the MA-9 mission operated in a satisfactory manner, and the command control plan was followed very closely throughout the mission. Several malfunctions were noted at various sites, but command capability was never lost by any site during the time in which the spacecraft was passing over that site. The command carrier "on" indication from the Bermuda station to the MCC was delayed approximately 32 seconds on the first pass; however, it had no net effect on the mission since the onboard command receiver signal strength remained above the receiver threshold setting.

A total of 19 functions were transmitted from the command stations. All of these functions were received onboard the spacecraft with the exception of one telemetry "on" function from Muchea and the clock change from the Coastal Sentry Quebec. The telemetry "on" command from Muchea was not received because it was transmitted when the spacecraft was out of range of the 600-watt ground transmitter. The clock change from the Coastal Sentry Quebec was not received because the command tone was also sent before the spacecraft was within range of the ground transmitter.

The following ground-system malfunctions were experienced:

(1) The Rose Knot Victor had an intermittent problem in the beam power supply of the backup power amplifier. It was detected before lift-off and the equipment remained inoperative throughout the mission. The prime transmitter was used to support the mission.

(2) Guaymas had a failure in the filament transformer of the standby transmitter at 29:40:47 g.e.t. which damaged the power amplifier tube. The filament transformer and the power amplifier tube were both replaced and the equipment was operational by 32:05:47 g.e.t. The prime transmitter remained operational during this time.

(3) The Bermuda high-power transmitter came on with a 3.6-kw output but did not come up to full power. The station automatically switched to low power, 600 watts, at 00:06:31 g.e.t.

All regular, part-time, and alternate circuits of the network participated in the MA-9 mission. Critical coverage was continuously established on these circuits during preflight countdown until the end of the mission for Adelaide, Muchea, Honolulu, New York, Mercury Control Center, and GSFC. For other sites, critical coverage was dependent upon standby status critical coverage being allowed to lapse when the station was on a standby basis.

Upon review of the SCAMA log for the mission, it is apparent that this phase of communications was quite reliable. The few instances of poor readability were mainly a result of the station operation techniques and excessive background noise inside and outside the station.

Communications during the mission were nearly perfect. Every communication patch performed properly when needed. As anticipated, outages occurred on a few occasions when a station did not have the spacecraft "in view" or during otherwise unimportant communications periods.

Average total message delays during MA-9 approximated 2 minutes, compared with 3 minutes and 15 seconds for MA-8. This difference can be accounted for by the heavier traffic concentration of MA-8.

The MA-9 mission occurred during a period of high solar activity. Unlike MA-8, however, there were no geomagnetic disturbances and the propagation conditions were favorable.

The timing system performed satisfactorily at all stations except California. On passes 3, 4, 5, 16, 17, and 18, the serial decimal timing was in error in tens-of-seconds readout. The problem was corrected after pass 18 by replacing all tubes in the timing counter units and adjusting the phanastron in the time-comparison unit. During pass 20, the timing system was again defective since it indicated 21 hours rather than 20 hours.

1. Staff of NASA Manned Spacecraft Center: Results of the First United States Orbital Space Flight, February 20, 1962. Supt. Doc., U.S. Government Printing Office (Washington, D.C.).

2. Staff of NASA Manned Spacecraft Center: Results of the Second United States Orbital Space Flight, May 24, 1962. NASA SP-6, Supt. Doc., U.S. Government Printing Office (Washington, D.C.).

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