TRACKING AND DATA RELAY SATELLITES NASA is building a new Earth-to orbit and orbit-to-Earth communications link named "TDRSS." The letters stand for Tracking and Data Relay Satellite System. When completed, the system, together with its various NASA support elements, will be known simply as the "Space Network." It will substantially increase information exchanges between low-orbiting spacecraft and the ground. So far only two of the system's three planned communications satellites have been placed in orbit. Nevertheless, TDRSS has been keeping the electronic uplink and downlink channels between the Earth and orbiting spacecraft bristling with voices, video, and data. When the system's other satellite is launched, the completed Space Network will represent one of the biggest advances in space communications technology in the 1980's. 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. The large-capacity, near-continuous exchanges achievable with the TDRSS are essential for the expanded scientific research and the burgeoning commercial and industrial operations envisioned for space in the late 1980's and early 1990's. Facilities for carrying out these modern research and commercial ventures are already in use, or will be shortly. Sophisticated instruments carried aboard the bus-size cargo bay of a Shuttle generate huge volumes of data. The data flow becomes even more abundant when the Shuttle carries Spacelab. In this compact $1 billion research laboratory built by the European Space Agency (ESA), scientists and technicians work in a shirt-sleeve environment almost as if they were in their laboratories on Earth. If this research could not be promptly transmitted to Earth through TDRSS, the Shuttle and many of the automated spacecraft of today and tomorrow would need to carry additional, often bulky, data storage equipment. This would take away precious space, weight, and power from research and operational payloads. High-rate data flows are generated nowadays not only by the Shuttle, but also by automated orbiting craft such as modern environmental and Earth resources observation satellites and by the soon-to-be launched large new Hubble Space Telescope that will conduct unprecedented astronomical research. If the TDRSS could be seen from the Earth's surface, it might look like a giant X far up in the sky. Closer up, it would resemble a windmill. Like a windmill, it 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. They are adding new dimensions to space communications. 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. The first satellite has been in orbit since April 1983. It is currently in operation at an altitude of 22,300 miles above the equator. It is too far away to be visible from the Earth's surface. The TDRSS travels in a "geostationary orbit" (also called geosynchronous orbit), meaning its movements correspond with the Earth's rotation. Thus, if it could be watched from the ground, the satellite would appear to be hanging in almost motion-free suspension above the Earth. 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 satellite differs from conventional communications satellites in a big way: Our conventional communications satellites connect points on Earth. They transmit communications between cities, countries, and continents. The TDRSS satellite connects the Earth with low-orbiting spacecraft. When the TDRSS is completed with the two other satellites, almost uninterrupted voice and data exchanges will be routinely possible between the Earth and orbiting U.S. Shuttles - all with only one Earth-based communications station. The system will also allow 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. Since the new system's first satellite has been in orbit, it alone (without the help of the remaining Earth stations) has stretched communications between the Earth and the Shuttle from about 15 percent of the time during each orbit to about 50 percent. Each TDRSS satellite is launched from the Shuttle. The satellite's solar panels and antennas are compactly folded as it is stowed in the Shuttle's cargo bay. After the Shuttle attains orbit at an altitude of about 175 miles, its cargo bay doors open and the satellite is ejected. At launch, each satellite weighs about 5,000 pounds. After the Shuttle moves a safe distance away, the satellite's attached booster rocket, known as an "Inertial Upper Stage" (IUS), ignites and lifts the satellite to its geosynchronous orbit. There, the satellite detaches itself from its booster and then the solar panels and antennas unfold. When unfolded, the satellite measures 57 feet from the outer edge of one solar panel to the outer edge of the other, and 46 feet from the outermost edge of one dish antenna to the outermost edge of the other. The satellite is allowed to drift - assisted by its attitude control thrusters - to the position assigned to it in orbit. The Space Networks's link with the Earth is the TDRSS White Sands Ground Terminal in New Mexico. Three giant 60-foot dish antennas reach skyward above a desert plain surrounded by mountains. Several smaller antennas are nearby. So are office and equipment buildings. The White Sands antennas connect the Earth with the TDRSS satellites and through them with the growing community of low-orbiting spacecraft. The White Sands Ground Terminal acts like the neck of an hourglass or the tube of a funnel. All transmissions from Earth to the TDRSS satellites, or from them to Earth, pass through this station. Since the Space Network will eventually serve nearly all low-orbiting U.S. spacecraft, virtually all U.S. communications traffic between the Earth and nearby space - uplink and downlink - will ultimately pass through the White Sands facility. The large dishes, designated the North, South, and Central antennas, dispatch transmissions to, and receive transmissions from, the satellites. These dishes are the links between the TDRSS satellites and the Earth. Smaller antennas are used for related functions such as testing spacecraft for their compatibility with the Network before their launch. These antennas can simulate transmissions to and from a spacecraft while it is still on the ground. 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. The White Sands Ground Terminal is only a part of the Network's space services. The Network Control Center is at the Goddard Space Flight Center in Greenbelt, Maryland, a few miles from Washington, D.C. Here, at the main control room, technicians work at 19 dual monitor consoles 24 hours a day, seven days a week. Ten other consoles are nearby for supervisory personnel, for service scheduling, and for emergency backup. The Center monitors, manages, controls, and coordinates the Network. Typically, telemetry from a low-orbiting spacecraft may follow a zigzag route to its ultimate destination into the hands of researchers. Transmissions from a low-orbiting spacecraft are first directed upward to a TDRSS satellite, which instantaneously relays them down to the White Sands Ground Terminal. From there the transmissions may be sent up to a commercial communications satellite, which relays them to the Goddard Space Flight Center. There data is processed into forms useful for research and applications. The demand for Earth-to-orbit and orbit-to-Earth communications is multiplying rapidly. High-volume, continuous communications channels are needed by Shuttle crews, research scientists, users of weather and Earth resources spacecraft, and even by entrepreneurs looking into investment opportunities for commercial and industrial high-technology ventures in space. The new TDRSS is designed to fill these growing communications needs and wants. The single TDRSS satellite in orbit has already proven it can provide the stringent communications needs of space recovery operations. The satellite's ability to communicate with the Shuttle for almost half of each orbit greatly aided Mission Control in monitoring the successful in-orbit repairs by astronauts of the malfunctioning Solar Maximum Mission scientific spacecraft in 1984. Similarly, the TDRSS played a crucial role in the retrieval of two communications satellites in improper orbits in that same year, and in the repair of the failing Syncom IV-3 in 1985. Experience over a quarter of a century of manned space operations has shown that lengthy dialogue between Mission Control and spacecraft crews can become essential for survival in emergencies. Life-threatening damage to the Apollo 13 craft from a on-board explosion during a flight to the Moon in 1970 was overcome mainly because experts on the ground were able to discuss at length with the crew how best to compensate for the impaired equipment. One of the chief science users of the new Space Network will be the Hubble Space Telescope. The new telescope's optics will see objects 50 times fainter than today's best instruments can discern. It will look back in time, viewing radiations from the edge of the universe that have been traveling at the speed of light for billions of years. From these observations researchers expect to learn much about still mysterious celestial entities and processes taking place at nearly incredible distances and about the evolution and possible future of the universe. About 30 minutes of extensive transmissions through the TDRSS satellites are expected to be needed on each of the telescope's instruments, for tracking the craft, and for relaying to Earth its large quantities of measurements and images gathered on each orbit. These transmissions will be channeled to the Space Telescope Science Institute Facility at the Johns Hopkins University, Baltimore, Maryland. There, all of the telescope's observations will be analyzed and archived for continuing studies by scientists. Although the contributions of TDRSS are less obviously visible than those of many other space events, TDRSS constitutes a valuable national resource. The advantages emerging from it and the benefits that will flow from it are bound to add up in the years ahead to a giant leap for humankind. TDRS: NASA's Tracking and Data Relay Satellite The Tracking and Data Relay Satellite (TDRS) system rep- resents a new way of tracking Earth-orbiting spacecraft, in- cluding the Shuttle, and transmitting their data back to Earth. The TDRS concept was conceived following early 1970s studies which showed that a system of orbiting telecommunications satellites, operated from a single ground terminal link, could more effectively support Space Shuttle, scientific and other NASA mission requirements than the nearly 25-year-old tracking and communications network of ground stations located worldwide. The TDRS network will be able to provide almost full-time coverage not only for the Shuttle, but also for up to 25 other orbiting spacecraft simultaneously. The TDRS satellites will orbit geosynchronously at 22,250 miles above the Earth, and look down on an orbiting Shuttle. This means any given orbiter will remain in sight of one or the other of the satellites for most of its circuit around the Earth. In the past, spacecraft could communicate with Earth only when they were in sight of a ground tracking station, typically less than one fifth of the time. The full TDRS constellation will enable spacecraft to communicate with Earth for about 85 to 100 percent of the orbit, depending on their altitude. Ground stations of the existing Spaceflight Track- ing and Data Network (STDN), partially obsolete and costly to operate, can then be consolidated or closed. The fully operational TDRS constellation will comprise three on-orbit satellites positioned over the equator, with two stationed 130 degrees apart and a third centrally located between them and designated as an on-orbit spare. NASA also plans three launch-ready ground spares to ensure continuous operation. The TDRS satellites are the largest, most advanced, privately developed communications satellites. TDRS can provide continuous global coverage of Earth-orbiting spacecraft above 750 miles to an altitude of about 3,100 miles. At lower altitudes there will be brief periods when satellites over the Indian Ocean near the equator will be out of view. This area is called the geometric zone of exclusion. Deep space probes and Earth-orbiting spacecraft above about 3,100 miles will use the three ground stations of the Deep Space Network (DSN), operated for NASA by the Jet Propulsion Laboratory, Pasadena, Calif. Each TDRS is a three-axis stabilized satellite weighing almost 5,000 pounds (about two and a half tons) and measuring 57 feet across the fully deployed solar panels. Spacecraft design employs a modular concept aimed at reducing the cost of individual design and construction efforts that in turn lowers the cost of each satellite. Each TDRS comprises three modules. The equipment module houses the subsystems that operate the satellite. The attitude control system stabilizes the satellite to enable the antennas to have proper orientation toward the Earth and the solar panels toward the Sun. The solar panel arrays will generate more than 1,700 watts of electrical power for ten years. When the TDRS is in the shadow of the Earth, nickel cadmium batteries supply power. The thermal control subsystem consists of surface coatings and controlled electric heaters. The communications payload module is composed of the electronic equipment required for linking the user spacecraft with the ground terminal. The receivers and transmitters are mounted in compartments on the back of the single-access antennas to reduce the complexity and possible circuit losses. The fully operational TDRS will provide tracking and communications services for up to 26 users simultaneously, with coverage extending from 85 to 100 percent of the user satellite's orbit. An individual TDRS spacecraft can relay signals to up to 22 users at the same time. TDRS is the first telecommunications satellite with simultaneous three-band frequency service capability: S-band, C-band and high data rate Ku-band. The telecommunications payload relays signals to and from the ground station or to and from user satellites. No user signal processing is done onboard. As many functions as possible have been removed from the satellite for performance by the ground station to ensure longer life and allow for more onboard spacecraft communications channels. The antenna module is composed of four antennas. For single access services, the TDRS satellites have two dualfeed S-band/Ku-band deployable parabolic (umbrella-like) antennas. The antennas are attached on two axes that can move horizontally or vertically to focus the beam on orbiting spacecraft below. The primary reflector surface of each antenna is a gold-clad molybdenum wire mesh. When deployed, 203 square feet of mesh are stretched between 16 supporting tubular ribs. The fully deployed antennas span 44 feet from side to side and 16.3 feet from edge to edge and are used primarily to relay communications to and from user spacecraft. The entire antenna structure, including the ribs, reflector surface, dual-frequency antenna feed and deployment mechanisms that fold and unfold the structure, weighs a mere 53.5 pounds. 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 spacecraft services (one at S-band and one at Ku-band). For multiple-access service, the multi-element S-band phased array of helical (spiral-like) radiators is mounted on the satellite body. The multiple-access forward link (between TDRS and the user spacecraft) transmits command data to the user spacecraft. In the return link, the signal outputs from the array elements are sent to the ground ter- minal where they are separated. Launch and Launch History The TDRS spacecraft is shipped to Kennedy Space Center, Fla., where it undergoes final assembly, checkout and mating with the two-stage Inertial Upper Stage (IUS) which will boost it into geosynchronous orbit. Tests are conducted using the Cargo Integrated Test Equipment (CITE) to make sure the two elements are correctly mated and that they will function as an integrated unit following deployment from the Shuttle. The MILA (Merritt Island Launch Area) tracking station at KSC has a TDRS ground terminal which can relay test data between the spacecraft being checked out on the ground to Project Control Centers at other locations via the on-orbit TDRS East and the White Sands, N.M., facility. TDRS spacecraft are launched from the Shuttle and boosted into low-Earth orbit of 150 nautical miles by the IUS. The spacecraft and IUS are deployed from the Shuttle about six hours after launch. The first burn from the IUS will take place about an hour later, and a second and final burn to circularize the orbit will be made about 12 1/2 hours into the mission. The booster and communications satellite will separate at about 13 hours after launch. The appendages-- solar panels, C-band antenna, space ground link antenna and the single access parabolic antennas--are then deployed, and the spacecraft will be ready for controllers to begin checkout about 24 hours after launch. The first TDRS was launched from Kennedy Space Center on April 4, 1983. It was the sixth Shuttle flight, the first mission for the orbiter Challenger and the first flight of the IUS aboard the Shuttle. A failure in the IUS second stage initially placed the spacecraft in an improper but stable orbit. After 58 days of delicate maneuvers using tiny one-pound boosters on the spacecraft, a NASA-industry team succeeded in placing the satellite into its correct geosynchronous orbit. TDRS-1 has been supporting users since the STS-9 Spacelab mission in November 1983. The benefits of even one on-orbit TDRS were clearly demonstrated during STS-9. In that ten-day mission, more data were retrieved through space-to-ground communications than on all of the 39 previous U.S. manned spaceflights. NASA maintained more than 50 minutes of continuous communications between the Shuttle and its Spacelab payload during each 90 minute orbit, compared to about 14 minutes of total orbit coverage previously available through the ground tracking network. A joint Air Force-NASA Investigation Board was convened three days after the IUS failure to determine the cause of the anomaly and recommend corrective actions. Analysis of flight data and extensive tests indicated the booster failure was most likely caused by the collapse of a nozzle gimbal mechanism and design changes were made to rectify the problem. TDRS-1 is on station above the equator near the city of Fortaleza on the northeast coast of Brazil (41 degrees west longitude). The launch of TDRS-B was scheduled for March 1985. When a timing circuit problem developed on the orbiting TDRS-1, the launch was delayed to modify TDRS-B. It was lost in the Challenger explosion on January 26, 1986. Launch processing of the TDRS-C spacecraft began at the Kennedy Space Center after its arrival on May 16, 1988. TDRS-C and its IUS booster were launched on STS-26, Discovery, on September 29, 1988. TDRS-C was designated TDRS-3, or TDRS-West, upon achieving geosynchronous orbit. It operates from 171 degrees west longitude. The time planned for the checkout of TDRS-C in orbit was 54 days, followed by 31 days for the spacecraft to be integrated with the Space Flight Tracking and Data Network. It was scheduled to be fully operational 85 days after launch. After STS-26, the next launch of a TDRS spacecraft (TDRS-D) is slated for the first quarter of 1989 on STS-29, again using Discovery. TDRS-D is undergoing final building at the manufacturer's facilities and is slated for arrival at KSC in November 1988. Launch of TDRS-D (TDRS-4 once in orbit) will complete the three-satellite constellation. TDRS-4 will operate from 41 degrees west longitude and TDRS-1 will be moved to 79 degrees west longitude where it will become the spare. The three TDRS spacecraft which will serve as launchready ground spares are in various stages of development. TDRS-E has completed environmental testing. The spacecraft will remain in storage and undergo final buildup in time to meet a July 1990 launch date if needed. TDRS-F is undergoing initial integration, test and partial build-up. Environmental testing and build-up will be completed to the call-up level by late 1989 and then the spacecraft will be placed in storage. TDRS-G, the replacement for the TDRS lost with Challenger, will be available for a May 1992 launch. It is in the early stages of design and manufacturing. The manufacturer is operating under a letter contract while contract definitization is completed. TDRS Designation Designation of a TDRS satellite depends on whether it is on-orbit or still on the ground. On the ground TDRS spacecraft receive a letter designation: TDRS-A, TDRS-B, etc. Once on-orbit, this designation changes to a numeral: TDRS-A to TDRS-1, TDRS-B to TDRS-2, TDRS-C to TDRS-3, etc. Even though the TDRS-B spacecraft launched on Challenger never achieved operational orbit, the TDRS-2 designation it was to have received will not be re-assigned. The concept of a three-satellite constellation has engen- dered another way of referring to the TDRS spacecraft. TDRS-1, positioned at 41 degrees west longitude above the northeast coast of Brazil, is also referred to as TDRS-East. TDRS-3, when it is fully operational and positioned at 171 degrees west longitude--above the equator north of the Phoenix Islands in the mid-Pacific Ocean--also will be known as TDRS-West. After TDRS-4 becomes operational following launch in 1989, it will replace TDRS-1 as TDRS-East. TDRS-1 will be repositioned as the on-orbit spare at 79 degrees west longitude. TDRS Tracking and Communications All satellite telemetry data relayed by TDRS is channeled through a highly automated ground station located at White Sands, N.M. NASA then routes the data to Project Control Centers. The White Sands location is at a longitude with a clear line-of-sight to the TDRS satellites. The White Sands ground station, one of the largest and most complex tracking facilities ever built, performs many command and control functions ordinarily found in the space segment of a system, such as tracking the deployable spacecraft antennas from the ground and transmitting their positioning commands to the spacecraft. The station includes the electronic equipment, three 60- foot dish antennas for Ku-band, one 20-foot dish antenna for the S-band, a number of small antennas, and a multiprocessor computer network. Automated data processing makes user satellite tracking measurements, selects and controls all communications equipment in the satellite and the ground station, and collects system status data for transmission along with user spacecraft data to NASA. The interface between the White Sands ground terminal and the other TDRS network elements is called the NASA Ground Terminal (NGT). TDRS primary tracking and communications facilities are located at Goddard Space Flight Center, Greenbelt, Md. Also located at Goddard are the Network Control Center, which provides system scheduling and is the focal point for NASA communications with the TDRS spacecraft and other TDRS network elements, the Flight Dynamics Support Facility, which provides the network with orbital predictions and definitive orbit calculations for user spacecraft and the TDRS, and the NASA Communications Network (NASCOM), which provides the common carrier interface at network locations and consists of domestic satellites and their interface through terminals at Goddard, White Sands, and the Johnson Space Center, Houston, Tex. The Network Control Center at Goddard monitors TDRS data flow, isolating system faults, accounting for the system and testing it, and simulating user spacecraft. The user services available from TDRS are sent through NASCOM. Voice, data and teletype links with the TDRS network, the Ground Spaceflight Tracking and Data Network and the user spacecraft control centers are available. NASCOM'S circuits are provided and operated by commercial carriers under contract to NASCOM. NASCOM sends the TDRS user data to the Flight Dynamics Support Facility and to the Sensor Data Processing Facility, also at Goddard, where the data is processed and distributed. All telemetry data is routed directly to a user's Payload Operations Control Center (POCC), which interfaces directly with the scientific investigators to plan payload experiment operations and to determine support requirements. Each payload center is tailored to a specific space mission, providing support to one spacecraft or to a series of spacecraft in a project. Eventually the White Sands computers will control the three on-orbit spacecraft, 300 racks of ground station electronic equipment and seven ground antennas. Control functions will be performed automatically in response to requests for user services which will reach the ground station from NASA via a computer-to-computer link. A second TDRS ground terminal (STGT) is planned at White Sands which will serve not only as a backup to the existing terminal, but will also provide additional capabilities to meet projected mid-to-late 1990s user requirements. The STGT will contain two identical, autonomous space ground link terminals capable of providing additional Earth-to-space and space-to-Earth single access links for user satellites, plus tracking, telemetry and control functions for TDRS satellites. A contractor to equip the STGT, scheduled to become operational in early 1993, is expected to be named in the fall of 1988. Construction of the building itself is already underway. TDRS Lead Contractors Prime TDRS is owned and operated by the Technical Services Div. of Contel Federal Systems, Fairfax, Va., with its services being leased to NASA for a ten-year period. Contract was awarded in December 1976, with initial funding arranged through the Federal Financing Bank in Washington, and subse- quent funding to be appropriated annually by Congress. Period of performance extends through December 1993. Spacecraft and Ground Station TRW Space and Technology Group, Redondo Beach, Calif., is responsible for design, fabrication, test and launch of the spacecraft as well as ground and spacecraft systems integra- tion. TRW also provides ground terminal software and software maintenance support, ground terminal hardware, integration and test. Harris Corp.'s Government Systems Sector, Melbourne, Fla., provides the TDRS 16.3-foot deployable antennas under a subcontract to TRW. Harris also designed and built the three White Sands communications and tracking antennas, several smaller command and control antennas, and more than 130 racks of communications, telemetry and control equipment. Harris provides ground terminal integration and test support, and is leading a team which includes TRW, Motorola and Allied Signal's Bendix Engineering Div. to compete for the STGT contract. Upper Stage The Inertial Upper Stage (IUS) used to boost the satellite into its final orbit is made by the Boeing Aerospace Co., Seattle, Wa., under contract to the Air Force, which in turn supplies the stage to NASA. Cost NASA estimates total cost of the TDRS program to be about $3.1 billion. This includes not only the spacecraft but the ground station, interest charges and estimated cost to complete the program through 1993. Contel estimates its total contract value to be about $2.6 billion. Approximate value of a single TDRS spacecraft is $100 million and of an IUS, about $45 million.