Space


Tracking and Data Relay Satellite System (TDRSS)

In addition to the previously discussed systems, which are all for communications between terrestrial forces, satellite systems also support near-real time communications between low altitude intelligence satellites and ground control stations. NASA's Tracking and Data Relay Satellite System (TDRSS), which plays a major though generally unappreciated role in supporting near-real time data transmission from low altitude reconnaissance satellites such as the Lacrosse.(1) It is possible that the KH-12 are also using TDRSS, but there's no public indication to this effect, whereas there are published statements to the effect that Lacrosse is using TDRSS.

With NASA's Tracking and Data Relay Satellite System (TDRSS) fully operational, virtually all communications between U.S. scientists and technicians and orbiting American spacecraft pass through the White Sands space network complex located in Southwest New Mexico.(2)

The White Sands space network complex consists of the White Sands Ground Terminal (WSGT), the NASA Ground Terminal (NGT) and the Second TDRSS Ground Terminal (STGT) all within the White Sands Missile Range very near the Organ mountains. Situated just east of Las Cruces and surrounded by greasewood and mesquite, it has been in operation since 1983, when NASA launched its first Tracking and Data Relay Satellite (TDRS). The ground stations, operated by Contel Federal Systems and Bendix Field Engineering Corporation, under contract to NASA, send data to and receive data from TDRSs, which are in geosynchronous orbit 22,300 miles (35,880 kilometers) over the Equator. In turn, TDRSs relay instructions to and receive data from spacecraft orbiting at lower altitudes.

The operational TDRS system permits communications with spacecraft during at least 85 percent of their orbit. Without TDRSS acting as a space-based relay, communications with satellites can be accomplished only when the spacecraft are in sight of a ground station, which seldom is more than 15 percent of an orbit.

In addition, the TDRSS can relay information faster, being capable of sending the equivalent of the text of a 20-volume encyclopedia of data in one second.

The space network is currently composed of three satellites: TDRS-1, TDRS-3 and TDRS-4. TDRS-E, which will be redesignated TDRS-5 after launch, is scheduled for launch from Kennedy Space Center, FL, on Space Shuttle Atlantis (STS-43) in mid-summer of 1991 to provide greater flexibility to the TDRS Constellation. TDRS-F is scheduled for launch in 1993.

Tracking and Data Relay Satellite System officials at NASA's Goddard Space Flight Center, Greenbelt, MD, indicate that the satellite system requires three fully capable satellites in orbit. Currently in orbit, one satellite is stationed at 41 degrees West Longitude (East), over the Atlantic just east of Brazil; and two partially disabled satellites are at 171 and 174 degrees West Longitude (West) over the Pacific east of the Gilbert Islands. After the TDRS-E launch, one of the partially disabled satellites will be centrally located and serve as a spare.

The existing White Sands Ground Terminal (WSGT) has three K- Band 60-foot (18 meter) antennas, and two S-Band Telemetry and Command antennas.

A new ground terminal known as the Second Tracking and Data Relay Satellite Ground Terminal (STGT) was dedicated in 1990. The STGT is expected to be operational in 1993 and will house over $400 million in sophisticated computer and communications equipment. The facility is necessary as a backup to the WSGT and is needed to meet increased mission support requirements in the Nation's space program in the 1990s.

So vast is the capacity of the new system that it can transmit the contents of an average library from an orbiting spacecraft down to the Earth in a few minutes. At its highest transmission rate, the new system can transfer in a single second the contents of a 20-volume encyclopedia with 1,200 pages in each volume and 2,000 words on each page.

TDRSS has four arms or paddles. Two opposing paddles are flat, square solar panels, measuring 151 inches on each side. The two other paddles look like upside-down umbrellas. They are parabolic dish antennas with diameters of 16.3 feet.

Holding these paddles in place are booms of extruded beryllium, which form the arms and legs of the X. At the center of the X is a box to which other antennas of various shapes and sizes are attached. Inside the box are the subsystems that control communications, electric power, satellite position, and other essential functions.

Weighing 2.5 tons and stretching to 57 feet between its most distance rims, it is the largest and heaviest satellite ever launched into a geosynchronous orbit.

The system allows nearly continuous command and telemetry communications between ground control centers and unmanned, automated research and applications spacecraft orbiting up to several thousand miles above the Earth.

Though the TDRSS satellites pass commands to spacecraft to adjust their positions by firing a thruster, to turn a camera or heater or other on-board equipment on or off, to start or stop observations, and to begin or stop transmissions to the Earth. Also passing through the uplinks are instructions or data for storage in a spacecraft's memory. Later the spacecraft can draw on this information for guidance in automated operations.

Current Status and History

Tracking and Data Relay Satellite (TDRS)-E is the fifth in a series of communications spacecraft planned for the Tracking and Data Relay Satellite System (TDRSS).(3) TDRS-A, now is orbit and known as TDRS-1, was deployed from the Space Shuttle Challenger on April 5, 1983 on Space Transportation System (STS)-6. TDRS-B was destroyed during the Challenger accident in January 1986. TDRS-C, known as TDRS 3 in orbit, was launched from Discovery on September 29, 1988 on STS-26. TDRS-D, known as TDRS-4 in orbit, was launched from Discovery on March 13, 1990 on STS-29.

Currently (June 7, 1991), TDRS-4 is located at 41 degrees West longitude, over the Atlantic Ocean off Brazil, TDRS-3 is located at 174 degrees West longitude, and TDRS-1 is located at 171 degrees West longitude. Both TDRS-3 and TDRS-1 are over the Pacific, East of the Gilbert Islands and South of Hawaii. TDRS-4 is also known as TDRS-East and the combination of TDRS-1 and TDRS-4 provide the TDRS Western satellite capability.

The satellite communications system was initiated following studies in the early 1970s which showed that a system of telecommunication satellites operated from a single ground station could better support the space shuttle and scientific application mission requirements planned for the Nation's space program. In addition, the system was seen as a means of halting the spiralling costs of upgrading and operating a network of tracking and communications ground stations located around the world.

The TDRS is three-axis stabilized with the multiple access body fixed antennas pointing constantly at the Earth while the solar arrays track the Sun.

Communication System

The TDRSs do no processing of user traffic in either direction. Rather, they operate as "bent pipe" repeaters, relaying signals and data between the user spacecraft and the ground terminal and vice versa without processing.

The TDRSS is equipped to support up to 26 user spacecraft, including the space shuttle, simultaneously. It will provide two types of service: (1) multiple access which can relay data from as many as 20 low data rate (100 bits per second to 50 kilobits per second) user satellites simultaneously and (2) single access which will provide two high data rate channels (to 300 megabits per second) from both the East and West locations.

The TDRSS ground terminal is located at White Sands, NM. It provides a location with a clear line-of-sight to the TDRSs and a place where rain conditions have limited interference with the availability of the Ku-band uplink and downlink channels. The White Sands Ground Terminal (WSGT) is operated for NASA by Contel Federal Systems under a contract that expires in 1995.

Co-located at White Sands is the NASA Ground Terminal (NGT), which is operated for NASA by Bendix Field Engineering and provides the interface between WSGT and other primary network elements located at NASA's Goddard Space Flight Center, Greenbelt, MD.

Those facilities at Goddard include the Network Control Center (NCC), which provides system scheduling and is the focal point for NASA communications and the WSGT and TDRSS users; the Flight Dynamics Facility (FDF), which provides the network with antenna pointing information for user spacecraft and the TDRSs; and the NASA Communications Network (NASCOM), which provides ground to ground communications through Earth terminals at Goddard, White Sands and the Johnson Space Center in Houston, TX.

The Network Control Center console operators monitor the network performances, schedule emergency interfaces, isolate faults in the system, account for system use, test the system and conduct simulations. The user services available from the Space Network, which includes TDRSS and its supporting Goddard Space Flight Center elements, are provided through NASCOM, a global system which provides operational communications support to all NASA projects. NASCOM offers voice, data and teletype links with the Space Network, the Ground Spaceflight Tracking and Data Network (GSTDN), and the user spacecraft control centers.

TDRS Components

The TDRSs are composed of three distinct modules: an equipment module, a communications payload module and an antenna module. The modular design reduces the cost of individual design and construction efforts that, in turn, lower the cost of each satellite.

The equipment module housing the subsystems that operate the satellite is located in the lower hexagon of the spacecraft. The attitude control subsystem stabilizes the satellite to provide accurate antenna pointing and proper orientation of the solar panels to the Sun. The electrical power subsystems consists of two solar panels that provide a 10-year power supply of approximately 1,700 watts. The thermal control subsystem consists of surface coatings and controlled electric heaters.

The payload module is composed of the electronic equipment required to provide communications between the user spacecraft and the ground. The receivers and transmitters for single access services are mounted in compartments on the back of the single-access antennas.

The antenna module is composed of seven antenna systems: two single-access, the multiple access array, and space-to-ground link and the S-band omni for satellite heath and housekeeping. Commercial K-band and C-band antennas round out the complement.

For single-access service, the TDRSs have dual-feed S-band, Ku-band parabolic (umbrella-like) antennas. These antennas are free to be positioned in two axes directing the radio beam to orbiting user spacecraft below. These antennas are used primarily to relay communications to and from user spacecraft. The high data rates provided by these antennas is available to users on a time-shared basis. Each antenna is capable of supporting two user spacecraft services simultaneously -- one at S-band and one at Ku- band -- provided both users are within the beam width of the antenna.

The multiple access antenna array is hard-mounted in one position on the surface of the antenna module facing the Earth.

Another antenna, a 6.5 foot (two meter) parabolic reflector, provides the prime link for relaying transmissions to and from the ground terminal at Ku-band.

Project Support and Staff

TRW Space and Technology Group in Redondo Beach, CA, is the prime spacecraft contractor. Ground operations at the White Sands complex are conducted by Contel Federal Systems and Bendix Field Engineering.

Other Facts

The White Sands Ground Terminal (WSGT) is one of the largest and most complex satellite communications terminals ever built.

The Second Tracking and Data Relay Ground Terminal (STGT) is nearly the size of three football fields.

Although weighing 5,000 lbs., the TDRS is moved by 12 one-pound thrusters -- each no bigger than a small human finger.

At its highest capacity, TDRSS can transfer in a single second -- the equivalent of a 20-volume encyclopedia containing over 34-million words.

The TDRS umbrella-like antennas are plated in 24K gold.

The TDRS solar panels generate more than 17 hundred watts of power. That is approximately what the average house in the Southwest U.S. uses daily.

The TDRS solar panel span is equivalent to the height of a five story building.

TDRS was the first communications satellite to offer Ku, S, and C band all in one spacecraft.

The TDRS system supports 800 to 1,000 shared user communications events per week with greater than 99.5 percent availability.

The TDRS system can provide service to up to 24 separate spacecraft simultaneously.

There are more than 1.2 million lines of software codes used in the WSGT computer system.




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