The Era of the Tracking and Data Relay Satellite System

NASA's space tracking system has significantly enhanced data communications capability. The system's implementation in the late 1980s is a constellation of satellites and a ground terminal, called the Tracking and Data Relay Satellite System or TDRSS. This system achieves the highest amount of coverage ever for Earth-orbiting satellites. With a growing number of satellites placed in orbit for voice, video, and digital uses and with increasing data rates, the need for a more advanced system of communicating with manned and unmanned satellites was recognized by engineers at the National Aeronautics and Space Administration (NASA). Researchers determined that a series of geostationary satellites for those in fixed positions above the Earth) could provide better tracking of satellites than the existing ground stations, could cover almost the entire orbital period of a satellite, and could support several space vehicles simultaneously.

The tracking network used by NASA since 1958 included a series of powerful antennas at ground stations located around the world. The main disadvantage of this system was a limited field of view for antennas that could only "see" the spacecraft when it was above the local horizon.

The TDRSS provides satellite coverage from 85 to 100 percent of an orbit, depending on the user satellite's orbital altitude. The TDRS satellites, built by TRW, Redondo Beach, California, the largest and most sophisticated communications spacecraft ever developed, are currently positioned 22,300 miles (35,887 kilometers) over the Equator, about 130Ż apart. TDRS-East for TDRS-4) is currently stationed over the northeast corner of Brazil at 41Ż West Longitude, along with a backup at 62Ż West Longitude; the TDRS-West position, southwest of Hawaii, has two satellites located at 171Ż and 174Ż West Longitude. With the launch of the fifth satellite, TDRS-F (to be renamed TDRS-6 after launch), the constellation was reconfigured.

How TDRSS Works

Each Tracking and Data Relay Satellite and its two-stage rocket, the Air Force-developed, Boeing-built Inertial Upper Stage (IUS), is carried into space onboard a space shuttle launched from Kennedy Space Center in Florida. When the Shuttle reaches the position for TDRS deployment, the TDRS/IUS combination is elevated in the orbiter's cargo bay, given a final check, and released. The orbiter then moves away and, at the proper moment, the IUS engine is fired to boost the TDRS into geostationary orbit. The orbit is then circularized using the second stage of the IUS.

Each TDRS weighs 4,600 pounds (2,086 kilograms) and measures 57 feet (17 meters) across the solar arrays and 44 feet (13 meters) across the antennas. The satellites have two wing-like solar panels that provide 1,700 watts of power and two 50-pound (23-kilogram), 16-foot (4.9-meter) diameter high-gain parabolic antennas that look like giant umbrellas. Operating in S-band and Ku-band frequencies, the TDRS can handle up to 300 million bits of information each second from a user satellite, the equivalent of the material in a 20-volume encyclopedia.

Control from the Ground

Two ground terminals are responsible for communication with the Tracking and Data Relay Satellites. The terminals are located at White Sands, New Mexico. One of the terminals began operations in 1983. Ground was broken for the second terminal, three miles north of the existing site, in 1987. It is scheduled to become fully operational in 1994. The New Mexico location was chosen for its low geographic latitude within the United States, affording a clear view of the satellites, and because service disruptions caused by weather are rare in that area. The terminals are responsible for maintaining such functions as transmitting commands to the spacecraft, receiving the user data returned through each TDRS, and keeping track of system status. Also, a remote ground terminal is being established in Tidbinbilla, Australia.

Network Control Center

The Network Control Center, located at NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the total Space Network. The control center schedules and configures the TDRSS and monitors the status data sent back for ongoing scheduled services. Control center operators schedule emergency services, isolate any problems in the system, and restore faulty user services. The control center has the ability to communicate with other ground tracking stations through the NASA Communications (NASCOM) network.

NASA Communications Network

The NASCOM network is a global system that provides communications support to all NASA projects. Voice, data, and teletype links are available through the network for connecting the TDRSS with user spacecraft control centers. Land lines, submarine cables, and microwave/satellite links make up this communications system.

NASA Ground Terminal

The NASA Ground Terminal interfaces with the TDRSS and NASCOM network for information to be transferred between the White Sands Ground Terminal, the relay satellites, and the users. The terminal completes formatting of information for transmission to TDRSS facilities at the Goddard Space Flight Center, monitors the quality of input and output, and reports on the status of the system to the Network Control Center.

White Sands Ground Terminal

This facility, operated by GTE Government Systems Corp., Needham Heights, Massachusetts, under contract to NASA, is a part of the White Sands complex. It is responsible for monitoring the health of the TDRS satellites, transmitting the commands to equipment onboard the satellites, maintaining attitude control and station- keeping functions, transmitting user satellite command data forwarded from the user's Payload Operations Control Center (POCC), and receiving user satellite data and forwarding it to the user.

Payload Operations Control Center (POCC)

All data from the TDRSS goes directly to Payload Operations Control Centers (POCCs). A control center can provide support to one or many spacecraft. Some are located at the Goddard Space Flight Center, and some are located at the site of the investigator. Equipment located in control centers handles all data, generates commands, and interfaces the communications. A control center also processes experiment status, command, and telemetry: handles attitude data for orienting equipment; controls payload operations and instrument sensors; and plans and analyzes the mission. The POCCs also interface with other components in the TDRSS network for scheduling and planning.

Into the Future of Communications

The Tracking and Data Relay Satellite System moves large volumes of data to and from satellites with great speed. NASA uses this advanced system to track Earth-observing satellites, space shuttle missions, and special spacecraft like NASA's Hubble Space Telescope, placed in orbit to study the universe. The TDRSS is an important instrument in a new era of space exploration, utilization, and research and will play a vital role in the support of all near-Earth orbiting scientific and applications spacecraft.