in Greenbelt, Maryland. Selection of which station tracks a given satellite at a given time is made by the NOCC based on requests from the Project Operations Control Centers (POCC) for unmanned spacecraft, and from the Mission Control Center (MCC) at the Lyndon B. Johnson Space Center (JSC) in Houston, Texas, for manned spacecraft.

Telemetry data received by STDN stations are either transmitted in near real time to GSFC, as discussed in Section 8.1.3, or are recorded on magnetic tapes and mailed to the receiving station. Range (position) and range-rate (velocity) data from the spacecraft are also acquired by radar or laser techniques at the tracking stations and relayed for use in orbit determination. Spacecraft command data are transmitted to the spacecraft in near real time or stored at the station for later transmission.

Computer facilities are located at each STDN station for processing spacecraft-associated data and performing local equipment test and control func-

Fig. 8-5. NASA Multiband Telemetry Antenna (26-m Diameter) at Rosman, North Carolina
Fig. 8-6. USNS Vanguard used for Spacecraft Tracking
Fig. 8-7. Advanced Range Instrumented Aircraft

tions. Data processing capabilities range from simple header generation to relatively sophisticated data compression operations [Scott, 1974]. One of the processing functions provided by STDN stations is time tagging, or attaching the Greenwich Mean Time (GMT) to processed data (see Section 8.3).

It is anticipated that in the future the ground-based communications network will be enhanced by satellite relay systems. For example, the Tracking and Data Relay Satellite System (TDRSS), scheduled to become operational in 1979, consists of two communications satellites in geostationary orbits which can relay telemetry

Fig. 8-8(a). NASA Tracking Stations at Rosman, North Carolina. Aerial view of tracking station showing seven antennas. See text for description.

data in real-time from other spacecraft which are not within the line of sight of any STDN station and which can also relay real-time commands from the tracking stations to the spacecraft. The two TDRSS satellites will be approximately 130 deg apart, at 41° and 171° West longitude. The inclination of their orbits will be between 2 arid 7 deg. A ground tracking station located within the continental United. States (presently planned for White Sands, New Mexico) will remain in constant contact with the Tracking and Data Relay Satellites {TDRS) providing telecommunication for orbital tracking data, telemetry data, and, in the case of manned spaceflight, voice communication (Fig. 8-9). This network will provide coverage of at least 85% of all orbits below 5000 km. For orbits above this altitude, the remaining STDN stations will provide coverage. To ensure reliability, a redundant TDRS will be placed in orbit midway between the two operational satellites and a fourth will be maintained on the ground for rapid replacement launch, if required. Redundant antenna systems will also be provided at the

Fig. 8-8(d). Range and range rate antennas.




Fig. 8-9. Tracking and Data Relay Satellite System (TDRSS), Scheduled for Implementation in 1979


Fig. 8-9. Tracking and Data Relay Satellite System (TDRSS), Scheduled for Implementation in 1979

primary STDN tracking site. The number of worldwide full-time tracking stations will be reduced to approximately five when the TDRSS is fully operational.

Two modes of operation are being considered for the TDRSS. The first, multiple access {MA), allows each TDRS to transmit telemetry and commands for as many as 20 spacecraft simultaneously. The disadvantage of MA is that the probability of transmission errors increases for spacecraft with altitudes greater than 5000 km. The single access (SA) method allows eatíh TDRS to transmit telemetry and commands to only two spacecraft at a time. Its advantage is its transmission efficiency for spacecraft with altitudes up to 12,000 km.

Tracking and data acquisition in the Soviet space program differs in several respects from NASA's program; detailed information on the Soviet program is both limited and somewhat dated. (See the U.S. Senate Report [1971] for a comprehensive discussion.) Although the United States has developed an extensive network of tracking stations in foreign countries, the Soviet Union relies primarily on stations within its own territory and on sea-based support. Because of the larger land area, stations within the Soviet Union can provide greater contact time than could a similar set of stations spread throughout the United States. Soviet references have been made to tracking stations in the United Arab Republic, Mali, Guinea, Cuba, and Chad.

At least 10 ships have been identified as working for the Soviet Academy of Sciences, the majority of which are involved in some phase of space operations. Among the most advanced of these are the Kosmonaot Vladimir Komarov and the Akademik Sergey Korolev. The latter is a space satellite control ship which was launched in 1971 and is described as the largest scientific research ship in the world, 182 m long and displacing 21,250 metric tons. The ships maintain contact with the Soviet Union via the Molniya communications satellites.

8.1 J Receiving Stations

NASA's STDN tracking stations are linked with each other and with GSFC and JSC by the NASA Communications Network (NASCOM). This system provides voice, data, teletype, television from selected stations, and other wideband communication. The network uses land lines, submarine cables, and microwave links. Redundant, geographically diverse routes are provided so that communication will not be lost if a primary route fails.

NASCOM leases full-time voice circuits (2-kHz bandwidth) to nearly all stations and control centers in its network. Most communication is routed through the GSFC Switching, Conferencing and Monitoring Arrangement (SCAMA). When these circuits are used for data transmission, the data format in Fig. 8-10 is used. The length of the data block may be any multiple of 12 bits, but the use of a

Fig. 8-10. NASCOM High-Speed Data Format

1200-bit block is encouraged so that it will be compatible with planned future STDN data-handling systems. A 48-bit message header normally follows the first 48-bit routing header, and the last 24 bits normally include a 22-bit algebraic code and 2 bits for flagging detected errors.

8.1.4. Transmission From the Receiving Station to Attitude Determination Computers

When telemetry data arrive at the receiving station, whether by NASCOM or mailed tapes, they are processed by a control center computer before delivery to an attitude determination processing computer. At GSFC, this function is performed by an Operations Control Center (OCC) for near-real-time data and/or by the Information Processing Division (IPD) for playback (tape recorded) data.

The processing performed by the OCC is minimal, since it is performed in near real time, and consists of stripping out data to be relayed to several destinations, one of which is the attitude determination computer. The sync pattern is examined and a quality flag is attached to the data, based on the number of incorrect bits in the sync pattern (Section 9.1). Sometimes the current GMT is attached to the data as well. The current date and the name of the tracking station which received the data are also inserted. The data are then transmitted to the attitude determination computer via a communication line controlled by a software package called the Attitude Data Link (ADL).

Processing performed by the IPD is more extensive, since the data need not be relayed immediately. Data are collected from tracking stations for periods of a day or more, and are then time ordered before transmission to the attitude determination computer. Segments of data which were incorrectly time tagged by the tracking station are detected and corrected. Other functions performed by the OCC are also performed by the IPD. The data are then transmitted to the attitude determination computer via a communication line under control of the ADL.

8.1.5 Transmission of Attitude Results and Spacecraft Commands

After the attitude determination computer processes the attitude data, it generates a definitive attitude history file, which is relayed to the IPD computers via the ADL and processed by a software package called the Telemetry On-Line Processing System (TELOPS). The data are then available for processing by experimenters. (For more detail on TELOPS, IPD, and their role in the data transmission process, see Gunshol and Chapman [1976].)

Commands may be uplinked to the spacecraft based on analysis of data on the attitude determination computer. Command requests, in engineering units, may be relayed from the attitude computer area to the OCC by voice (telephone lines). These requests are translated into coded commands by the OCC and transmitted to the tracking station via NASCOM. The tracking station then stores the command for later transmission or relays it to the spacecraft immediately in near real time. Sometimes the relayed commands are stored in a computer onboard the spacecraft for later execution. These are referred to as delayed commands.

8.2 Spacecraft Telemetry

Janet Niblack

Telemetry is a sequence of measurements being transmitted from one location to another.* The data are usually a continuous stream of binary digits (or pulses representing them). A single stream of digits is normally used for the transmission of many different measurements. One way of doing this is to sequentially sample various data sources in a repetitive manner. This process is called commutation, and the device which accomplishes the sequential switching is a commutator. The commutator may be either a mechanical or electronic device or a program in an onboard computer.

A minor frame of telemetry data contains measurements resulting from one complete cycle of the main commutator. Each frame consists of a fixed number of bit segments called telemetry words. Each word in a frame is a commutator channel. If the telemetry word contained in a main commutator channel is supplied by another commutator (called a subcommutator), data appearing in that channel are said to be subcommutated. If a single data source is sampled more than once within a minor frame, the data item is said to be supercommutated. The level of commutation for a particular data item determines the relative frequency at which it is transmitted. Whether a data item should be commutated, subcommutated, or supercommutated depends on how the measurement will be used and at what rate the value will change.

A major frame (sometimes called a master frame) contains the minimum number of minor frames required to obtain one complete cycle of all subcom-mutators, or an integral multiple of this number. (Bccause not all spacecraft telemetry systems use subcommutators, the major frame concept is not always relevant.) A minor frame counter or minor frame ID is often telemetered to identify the position of a minor frame within a major frame. This counter is particularly useful when minor frames are lost in transmission, since minor frame location determines what type of data a subcommutator channel will contain. Figure 8-11 shows a simple eight-channel main commutator with two subcommutators. Table 8-4 gives the sequence of telemetry words which would be generated by this commutator for one major frame. Note that the relative frequency at which a subcommutated data item appears depends on the number of channels in the subcommutator.

Because commutation involves time-dependent functions, some method of establishing and maintaining exact sychronization of data sampling is necessary. Spacecraft clocks provide the signals for synchronization. A frame synchronization signal, described in Section 8.1, is a series of pulses which marks the start of a minor frame period. These pulses are transmitted as part of each main commutator cycle and are used in identifying individual frames when the data are received on the ground.

The assignment of specific data items to commutator and subcommutator channels defines the telemetry format. Commutator or subcommutator channels are allocated to experimental data, to attitude determination and control data, and to

* For an extended discussion of spacecraft telemetry, see Stiltz [1961],

Table 8-4. Contents of a Typical Major Frame of Telemetry Data. Major frame words 2 and 7 are subcommutated. Words 4 and 8 are supercommutated.


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