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Tracking and Data Relay Satellite System<

When fully operational, the TDRSS will provide continuous global coverage of Earth-orbiting satellites at altitudes from 750 miles to about 3,100 miles. At lower altitudes, there will be brief periods when satellites or spacecraft over the Indian Ocean near the equator are out of view. The TDRSS will be able to handle up to 300 million bits of information per second. Because eight bits of information make one word, this capability is equivalent to processing 300 14-volume sets of encyclopedias every minute.

The fully operational TDRSS network will consist of three satellites in geosynchronous orbits. The first, positioned at 41 degrees west longitude, is TDRS-East (TDRS-A). The next satellite, TDRS-West, will be carried into Earth orbit aboard the space shuttle and deployed and positioned at 171 degrees west longitude. The remaining TDRS will be positioned above a central station just west of South America at 62 degrees west longitude as a backup.

The satellites are positioned in geosynchronous orbits above the equator at an altitude of 22,300 statute miles. At this altitude, because the speed of the satellite is the same as the rotational speed of Earth, it remains fixed in orbit over one location. The eventual positioning of two TDRSs will be 130 degrees apart instead of the usual 180-degree spacing. This 130-degree spacing will reduce the ground station requirements to one station instead of the two stations required for 180-degree spacing.

The TDRS system serves as a radio data relay, carrying voice, television, and analog and digital data signals. It offers three frequency band services: S-band, C-band and high-capacity Ku-band. The C-band transponders operate at 4 to 6 GHz and the Ku-band transponders operate at 12 to 14 GHz.

The highly automated TDRSS network ground station, located at the White Sands Ground Terminal, is owned and managed by Contel.

TDRSS also provides communication and tracking services for low Earth-orbiting satellites. It measures two-way range and Doppler for up to nine user satellites and one-way and Doppler for up to 10 user satellites simultaneously. These measurements are relayed to the Flight Dynamics Facility at GSFC from the WSGT.

Six TDRSs will be built by TRW's Defense and Space Systems Group, Redondo Beach, Calif. Contel owns and operates the satellites and the White Sands Ground Terminal, which was built jointly by the team of TRW, Harris Corporation and Spacecom. Electronic hardware was jointly supplied by TRW and Harris's Government Communications Division, Melbourne, Fla. TRW integrated and tested the ground station, developed software for the TDRS system and integrated the hardware with the ground station and satellites.

The ground station is located at a longitude with a clear line of sight to the TDRSs and very little rain, because rain can interfere with the Ku-band uplink and downlink channels. It is one of the largest and most complex communication terminals ever built.

The most prominent features of the ground station are three 60-foot Ku-band dish antennas used to transmit and receive user traffic. Several other antennas are used for S-band and Ku-band communications. NASA developed sophisticated operational control facilities at GSFC and next to the WSGT to schedule TDRSS support of each user and to distribute the user's data from White Sands to the user.

Automatic data processing equipment at the WSGT aids in satellite tracking measurements, control and communications. Equipment in the TDRS and the ground station collects system status data for transmission, along with user spacecraft data, to NASA. The ground station software and computer component, with more than 900,000 machine language instructions, will eventually control three geosynchronous TDRSs and the 300 racks of ground station electronic equipment.

Many command and control functions ordinarily found in the space segment of a system are performed by the ground station, such as the formation and control of the receive beam of the TDRS multiple-access phased-array antenna and the control and tracking functions of the TDRS single-access antennas.

Data acquired by the satellites are relayed to the ground terminal facilities at White Sands. White Sands sends the raw data directly by domestic communications satellite to NASA control centers at JSC (for space shuttle operations) and GSFC, which schedules TDRSS operations and controls a large number of satellites. To increase system reliability and availability, no signal processing is done aboard the TDRSs; instead, they act as repeaters, relaying signals to and from the ground station or to and from satellites or spacecraft. No user signal processing is done aboard the TDRSs.

A second TDRS ground terminal is being built at White Sands approximately 3 miles north of the initial ground station. The $18.5-million facility will back up the existing facility and meet the growing communication needs of the 1990s.

When the TDRSS is fully operational, ground stations of the worldwide STDN will be closed or consolidated, resulting in savings in personnel and operating and maintenance costs. However, the Merritt Island, Fla.; Ponce de Leon, Fla.; and Bermuda ground stations will remain open to support the launch of the space transportation system and the landing of the space shuttle at the Kennedy Space Center in Florida.

Deep-space probes and Earth-orbiting satellites above approximately 3,100 miles will use the three ground stations of the deep-space network, operated for NASA by the Jet Propulsion Laboratory, Pasadena, Calif. The deep-space network stations are in Goldstone, Calif.; Madrid, Spain; and Canberra, Australia.

During the lift-off and ascent phase of a space shuttle mission launched from the Kennedy Space Center. the space shuttle S-band system is used in a high-data-rate mode to transmit and receive through the Merritt Island, Ponce de Leon and Bermuda STDN tracking stations. When the shuttle leaves the line-of-sight tracking station at Bermuda, its S-band system transmits and receives through the TDRSS. (There are two communication systems used in communicating between the space shuttle and the ground. One is referred to as the S-band system; the other, the Ku-band, or K-band, system.)

To date, the TDRSs are the largest privately owned telecommunication satellites ever built. Each satellite weighs nearly 5,000 pounds in orbit. The TDRSs will be deployed from the space shuttle at an altitude of approximately 160 nautical miles, and inertial upper stage boosters will propel them to geosynchronous orbit.

The TDRS single-access parabolic antennas deploy after the satellite separates from the IUS. After the TDRS acquires the sun and Earth, its sensors provide attitude and velocity control to achieve the final geostationary position.

Three-axis stabilization aboard the TDRS maintains attitude control. Body-fixed momentum wheels in a vee configuration combine with body-fixed antennas pointing constantly at Earth, while the satellite's solar arrays track the sun. Monopropellant hydrazine thrusters are used for TDRS positioning and north-south, east-west stationkeeping.

The antenna module houses four antennas. For single-access services, each TDRS has two dual-feed S-band / Ku-band deployable parabolic antennas. They are 16 feet in diameter, unfurl like a giant umbrella when deployed, and are attached on two axes that can move horizontally or vertically (gimbal) to focus the beam on satellites or spacecraft below. Their primary function is to relay communications to and from user satellites or spacecraft. The high-bit-rate service made possible by these antennas is available to users on a time-shared basis. Each antenna simultaneously supports two user satellites or spacecraft (one on S-band and one on Ku-band) if both users are within the antenna's bandwidth.

The antenna's primary reflector surface is a gold-clad molybdenum wire mesh, woven like cloth on the same type of machine used to make material for women's hosiery. When deployed, the antenna's 203 square feet of mesh are stretched tautly on 16 sup porting tubular ribs by fine threadlike quartz cords. The antenna looks like a glittering metallic spiderweb. The entire antenna structure, including the ribs, reflector surface, a dual-frequency antenna feed and the deployment mechanisms needed to fold and unfold the structure like a parasol, weighs approximately 50 pounds.

For multiple-access service, the multielement S-band phased array of 30 helix antennas on each satellite is mounted on the satellite's body. The multiple-access forward link (between the TDRS and the user satellite or spacecraft) transmits command data to the user satellite or spacecraft, and the return link sends the signal outputs separately from the array elements to the WSGT's parallel processors. Signals from each helix antenna are received at the same frequency, frequency-division-multiplexed into a single composite signal and transmitted to the ground. In the ground equipment, the signal is demultiplexed and distributed to 20 sets of beam-forming equipment that discriminates among the 30 signals to select the signals of individual users. The multiple-access system uses 12 of the 30 helix antennas on each TDRS to form a transmit beam.

A 6.6-foot parabolic reflector is the space-to-ground-link antenna that communicates all data and tracking information to and from the ground terminal on Ku-band. The omni telemetry, tracking and communication antenna is used to control TDRS while it is in transfer orbit to geosynchronous altitude.

The solar arrays on each satellite, when deployed, span more than 57 feet from tip to tip. The two single-access, high-gain parabolic antennas, when deployed, measure 16 feet in diameter and span 42 feet from tip to tip.

Each TDRS is composed of three distinct modules: the equipment module, the communication payload module and the antenna module. The modular structure reduces the cost of individual design and construction.

The equipment module housing the subsystems that operate the satellite and the communication service is located in the lower hexagon of the satellite. The attitude control subsystem stabilizes the satellite so that the antennas are properly oriented toward the Earth and the solar panels are facing toward the sun. The electrical power subsystem consists of two solar panels that provide approximately 1,850 watts of power for 10 years. Nickel-cadmium rechargeable batteries supply full power when the satellite is in the shadow of the Earth. The thermal control subsystem consists of surface coatings and controlled electric heaters. The solar sail compensates for the effects of solar winds against the asymmetrical body of the TDRS.

The communication payload module on each satellite contains electronic equipment and associated antennas required for linking the user spacecraft or satellite with the ground terminal. The receivers and transmitters are mounted in compartments on the back of the single-access antennas to reduce complexity and possible circuit losses.

TDRS-A and its IUS were carried aboard the space shuttle Challenger on the April 1983 STS-6 mission. After it was deployed on April 4, 1983, and first-stage boost of the IUS solid rocket motor was completed, the second-stage IUS motor malfunctioned and TDRS-A was left in an egg-shaped orbit 13,579 by 21,980 statute miles-far short of the planned 22,300-mile geosynchronous altitude. Also, TDRS-A was spinning out of control at a rate of 30 revolutions per minute until the Contel/TRW flight control team recovered control and stabilized it.

Later Contel, TRW and NASA TDRS program officials devised a procedure for using the small (1-pound) hydrazine-fueled reaction control system thrusters on TDRS-A to raise its orbit. The thrusting, which began on June 6, 1983, required 39 maneuvers to raise TDRS-A to geosynchronous orbit. The maneuvers consumed approximately 900 pounds of the satellite's propellant, leaving approximately 500 pounds of hydrazine for the 10-year on-orbit operations.

During the maneuvers, overheating caused the loss of one of the redundant banks of 12 thrusters and one thruster in the other bank. The flight control team developed procedures to control TDRS-A properly in spite of the thruster failures.

TDRS-A was turned on for testing on July 6, 1983. Tests proceeded without incident until October 1983, when one of the Ku-band single-access-link diplexers failed. Shortly afterward, one of the Ku-band traveling-wave-tube amplifiers on the same single-access antenna failed, and the forward link service was lost. On November 19, 1983, one of the Ku-band TWT amplifiers serving the other single-access antenna failed. TDRS-A testing was completed in December 1984. Although the satellite can provide only one Ku-band single-access forward link, it is still functioning.

TDRS-B, C and D are identical to TDRS-A except for modifications to correct the malfunctions that occurred in TDRS-A and a modification of the C-band antenna feeds. The C-band minor modification was made to improve coverage for providing government point-to-point communications. TDRS-B was lost on the 51-L mission.

The mission plan for TDRS-C is similar to that originally planned for TDRS-A. Backup project operations control centers have been added at TRW and at the TDRS Launch/Deployment Control Center in White Sands. These facilities will improve the reliability of control operations and the simultaneous control of TDRS-A in mission support and of TDRS-C during launch and deployment operations.

TDRS-C and its IUS are to be deployed from the space shuttle orbiter. Approximately 60 minutes later, the IUS first-stage solid rocket motor is scheduled to ignite. This will be followed by five maneuvers to allow monitoring of TDRS-C telemetry.

After the IUS second-stage thrusting is completed, the TDRSS mission team at White Sands will command deployment of the TDRS-C solar arrays, the space-ground link antenna and the C-band antenna while the TDRS is still attached to the IUS. Upon separation of the IUS from TDRS-C, the 16-foot-diameter single-access antennas will be deployed, unfurled and oriented toward Earth. Nominal deployment will place TDRS-C at 178 degrees west longitude.

Testing of TDRS-C will be initiated; and after initial checkout, TDRS-C will drift westward to its operational location at 171 degrees west longitude, southwest of Hawaii, where it will be referred to as TDRS-West. Operational testing will continue to verify the full-system capability with two operating satellites. On completion of this testing, about three to five months after the launch of TDRS-C, the TDRSS, for the first time, will provide its full-coverage capability in support of NASA space missions.

TDRS-D, identical to TDRS-C, will take the place of TDRS-A at 41 degrees west longitude above the equator, over the northeast corner of Brazil, and will be referred to as TDRS-East. TDRS-A will then be relocated, probably 79 degrees west longitude above the equator, over central South America, and will be maintained as an on-orbit spare.

These three satellites will make up the space segment of the TDRS system. The on-orbit spare, available for use if one of the operational satellites malfunctions, will augment system capabilities during peak periods. The two remaining satellites will be available as flight-ready spares.

The failure of TDRS-A's Ku-band forward link prohibits the operation of the text and graphics system that it is desired be placed on board all space shuttle orbiters. TAGS is a high-resolution facsimile system that scans text or graphic material and converts the analog scan data into serial digital data. It provides on-orbit capability to transmit text material, maps, schematics and photographs to the spacecraft through a two-way Ku-band link through the TDRSS. This is basically a hard-copy machine that operates by telemetry.

Until there is a dual TDRS capability, a teleprinter must be used on orbit to receive and reproduce text only (such as procedures, weather data and crew activity plan updates or changes) from the Mission Control Center. The teleprinter uses S-band and is not dependent on the TDRSS Ku-band.

When the space shuttle orbiter is on orbit and its payload bay doors are opened, the space shuttle orbiter Ku-band antenna, stowed on the right side of the forward portion of the payload bay, is deployed. One drawback of the Ku-band system is its narrow pencil beam, which makes it difficult for the TDRS antennas to lock on to the signal. Because the S-band system has a larger beamwidth, the orbiter uses it first to lock the Ku-band antenna into position. Once this has occurred, the Ku-band signal is turned on.

The Ku-band system provides a much higher gain signal with a smaller antenna than the S-band system. The orbiter's Ku-band antenna is gimbaled so that it can acquire the TDRS. Upon communication acquisition, if the TDRS is not detected within the first 8 degrees of spiral conical scan, the search is automatically expanded to 20 degrees. The entire TDRS search requires approximately three minutes. The scanning stops when an increase in the received signal is sensed. The orbiter Ku-band system and antenna then transmits and receives through the TDRS in view.

At times, the orbiter may block its Ku-band antenna's view to the TDRS because of attitude requirements or certain payloads that cannot withstand Ku-band radiation from the main beam of the orbiter's antenna. The main beam of the Ku-band antenna produces 340 volts per meter, which decreases in distance from the antenna-e.g., 200 volts per meter 65 feet away from the antenna. A program can be instituted in the orbiter's Ku-band antenna control system to limit the azimuth and elevation angle, which inhibits direction of the beam toward areas of certain onboard payloads. This area is referred to as an obscuration zone. In other cases, such as deployment of a satellite from the orbiter payload bay, the Ku-band system is turned off temporarily.

When the orbital mission is completed, the orbiter's payload bay doors must be closed for entry; therefore, its Ku-band antenna must be stowed. If the antenna cannot be stowed, provisions are incorporated to jettison the assembly from the spacecraft so that the payload bay doors can be closed for entry. The orbiter can then transmit and receive through the S-band system, the TDRS in view and the TDRS system. After the communications blackout during entry, the space shuttle again operates in S-band through the TDRS system in the low- or high-data-rate mode as long as it can view the TDRS until it reaches the S-band landing site ground station.

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