The Tracking and Data-Relay Satellite System replaced NASA’s ground-tracking stations, making possible nearly continuous communication with most U.S. spacecraft in Earth orbit.
The National Aeronautics and Space Administration (NASA) became seriously interested in placing communications satellites in geostationary orbit as a solution to the problem of frequent tracking and communication lapses inherent in Earth-based tracking systems. In the mid-1960’s, NASA began research and development toward what became known as the Tracking and Data-Relay Satellite System (TDRSS) in the early 1970’s. Roy Browning was appointed program manager by NASA for the TDRSS project.
Up to that time, spacecraft in low Earth orbit were able to communicate with ground control centers only if they were above the horizon over strategically placed antennas in NASA’s Ground Spaceflight Tracking and Data Network
In December of 1976, NASA signed a joint-endeavor agreement with the Space Communications Company (Spacecom)
Three TDRS spacecraft were to operate in geostationary orbit over the equator. At an altitude of 35,680 kilometers (about 22,170 miles), they would orbit eastward at a speed matching exactly the rotation of Earth on its axis, so that each would remain over the same point on the planet’s surface indefinitely. From a vantage point 150 times higher than the spacecraft it served, a single TDRS could maintain communications with a low-orbiting satellite as the latter traveled halfway around the world. Two predetermined orbital “duty stations” (TDRS-East and TDRS-West), spaced 130 degrees of longitude apart, would let the satellites observe a minimum of 80 percent of the orbit of any spacecraft dependent on them. The choice of orbital location allowed both duty stations to “see” the ground terminal at White Sands. A third position, midway between the other two, would be used for a “standby” TDRS.
The Tracking and Data-Relay Satellites were the largest and heaviest spacecraft designed up to that time for geostationary orbit and the largest privately owned communications satellites ever built. Each weighs 2,120 kilograms (about 4,674 pounds) and measures 17.4 meters (about 57 feet) across. TDRS’s were the first communications satellites able to handle all three main frequencies employed by scientific and defense spacecraft, manned spacecraft, and commercial satellites: the S, Ku, and C bands.
A single TDRS spacecraft’s capacity as a conduit for communications traffic is enormous. Digital and analog data, along with audio and video signals, flow simultaneously at transmission rates so great that the entire contents of a twenty-volume encyclopedia could be transmitted in one second. Like water moving through a bent pipe, the communications signals enter TDRS from one direction and leave in another. Signals originating on Earth involve an “uplink” to a TDRS and a “forward link” to the user spacecraft. Signals originating on the user spacecraft involve a “return link” to a TDRS and a “downlink” to the receiving antennas on Earth.
Two large dish antennas, each 4.8 meters (15.7 feet) in diameter, aboard the TDRS are used for high-data-rate Ku- and S-band transmissions to and from other spacecraft. Fabricated of molybdenum mesh and plated with 24-karat gold, each can be used by only one spacecraft at a time but can receive up to 300 megabits (300 million electrical impulses) per second on the return link. The forward-link transmission capability of a TDRS is 25 megabits per second. Both antennas are independently steerable by ground command, so that each follows the spacecraft it is communicating with as it passes below. A phased array antenna consisting of thirty elements is used for low-data-rate S-band transmissions and is available for two simultaneous forward links and ten simultaneous return links per TDRS. All communications between TDRS and Earth (both uplink and downlink) pass between the satellite’s 2-meter (6.6-foot) dish antenna called the Space Ground Link
A Tracking and Data-Relay Satellite right before it was released from the cargo bay of shuttle orbiter Endeavour during a 1993 mission.
The spacecraft requires 1.7 kilowatts of electrical power, which is obtained from two solar cell arrays extended at right angles to the large dish antennas. Batteries store excess electrical power so that the satellite can continue to operate during the hours-long periods when it, like the ground point below, experiences night. The batteries are contained in a hexagonally shaped core module, along with more than forty transponders and the spacecraft’s attitude-control system and hydrazine propellant.
Compactly folded up to fit into the payload bay of a space shuttle, a TDRS and its two-stage Inertial Upper Stage (IUS) booster occupy most of the total space available and weigh 16,783 kilograms (about 37,000 pounds). In addition, a large piece of payload support equipment called a tilt table is required to deploy the TDRS/IUS once the shuttle is in orbit. The tilt table supports the weight of the payload during ground handling and launch. Once in space, it elevates the forward end of the TDRS/IUS at a steep angle to the body of the shuttle to facilitate deploying the payload by a spring-activated mechanism in the tilt table.
TDRS-1 was carried into orbit on April 4, 1983, as the major payload of the sixth space shuttle mission, which was also the maiden flight of the Challenger orbiter. Liftoff from Cape Canaveral occurred at 1:30
The IUS second stage was to have circularized that orbit at geostationary altitude, but problems developed soon after its ignition at 5:45
On May 2, the spacecraft was commanded to begin firing a pair of small attitude-control thrusters for periods of up to three hours per orbit to maneuver the satellite slowly into its intended orbit. The process required weeks because the thrusters could generate less than a kilogram of thrust apiece. TDRS-1 finally reached its 35,680-kilometer orbital altitude on June 29, 1983, and was subsequently moved to the TDRS-East duty station, located above the equator at 41 degrees west longitude. It went into service on October 17, 1983.
Unfortunately, problems with the IUS booster, the TDRS spacecraft, and the shuttle caused serious delays in the launch of TDRS-2 and TDRS-3. Originally expected to go into service within months of its sister craft, TDRS-2 was finally manifested for flight in January of 1986 and was lost in the explosion of Challenger.
The communications revolution on Earth has linked individuals to information regardless of where the information might be located physically, thereby increasing the speed and accuracy with which sound decisions can be made. The Tracking and Data-Relay Satellite System is an analogous development in the space program of the United States. It has substantially enhanced the performance and utility of manned and unmanned spacecraft in low Earth orbit by maintaining almost constant contact with them and providing very high-capacity data throughput.
Many modern spacecraft could not operate if they were still bound by the limitations of the former GSTDN system. Landsat Earth resources satellites utilize an instrument called a thematic mapper, which collects so many data about each ground site imaged that the former means of communication could never have kept up with the information coming from only one such satellite. The Hubble Space Telescope
The space shuttle is a major user of TDRSS services. The operation and safety of such a complex spacecraft and the variety of routine tasks carried out simultaneously by crews of up to seven individuals require many channels of information to be provided almost around the clock. Routine communications demands are escalated significantly by certain payloads and activities. One example is the European Space Agency’s
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Covault, Craig. “Loss of TDRS-A Averted by Joint Action.” Aviation Week and Space Technology 118 (April 11, 1983): 19-21. Presents a detailed account of the events that nearly resulted in the loss of the first TDRS spacecraft, including a breakdown of the period immediately following second-stage ignition of the faulty Inertial Upper Stage booster. Reveals comments and decisions of some of the key flight controllers. (TDRS-A was the designation of the satellite that became TDRS-1 when it successfully went into operation.) -
Faget, Max. “Tracking and Communications.” In Manned Space Flight. New York: Holt, Rinehart and Winston, 1965. Describes the general functions of tracking and communications and the specific problems and solutions developed for Projects Mercury and Apollo. Figure 6.3, which includes a map of the location of the Manned Space Flight Network tracking centers around the world and a listing of their capabilities, reveals many of the serious limitations of an Earth-based tracking network. -
“First Flight of SS Challenger.” Space World 234-235 (June/July, 1983): 4-6. Staff report on the STS-6 mission, which carried the first TDRS into orbit. Presents a brief day-by-day summary of mission activities and a discussion of the problems that developed because of the malfunction of the satellite’s IUS booster. -
Froelich, Walter. The New Space Network: The Tracking and Data Relay Satellite System. NASA EP-251. Washington, D.C.: U.S. Government Printing Office, 1986. Twenty-eight-page booklet is one of the best single sources of information for laypersons about the overall system and its various components. Written in nontechnical language and illustrated with numerous color photographs and clear diagrams. -
Smith, Bruce A. “TDRS Thrusters Readied for Geostationary Shift.” Aviation Week and Space Technology 118 (May 9, 1983): 16-17. Details week-by-week efforts to raise TDRS-1 from the useless orbit into which it was placed by the malfunction of its IUS booster. Explains general engineering considerations related to raising the satellite to a geostationary orbit and reveals the profound impact of the problems on NASA’s future plans. Suitable for readers with some technical background. -
Thomas, Shirley. Satellite Tracking Facilities: Their History and Operation. New York: Holt, Rinehart and Winston, 1963. Useful for readers who want a fuller understanding of the problems of providing guidance and communications to spacecraft in low Earth orbit from Earth-based tracking stations. Describes the two networks originally used by NASA, the Manned Space Flight Network and the Spaceflight Tracking and Data Network, later combined into the GSTDN. Suitable for a general readership. Includes bibliographic notes and index.
Launch of the First Earth Resources Technology Satellite
European Space Agency Is Formed
Global Positioning System Becomes Operational
Columbia’s Second Flight Proves the Practicality of the Space Shuttle
Two Women Walk in Space
Challenger Accident
First Permanently Manned Space Station Is Launched
Endeavour Maps Earth from Space
International Space Station Is Manned