The Tracking and Data Relay Satellite System (TDRSS) is a space-based network that provides communications, tracking, telemetry, data acquisition and command services essential to the Space Shuttle and low-Earth orbital spacecraft missions. All Shuttle missions and nearly all NASA spacecraft in Earth orbit require TDRSS's support capabilities for mission success.
The TDRSS was initiated following studies in the early 1970s which showed that a system of telecommunications satellites, operated from a single ground station, could better meet the requirements of NASA missions. In addition, the system was seen as a means of halting the spiraling costs of upgrading and operating a network of tracking and communications ground stations located around the world.
The TDRSS has enabled NASA to cut telecommunications costs by as much as 60 percent while increased data acquisition and communications with Earth-orbital spacecraft from 15 to 85 percent -- and in some cases to 100 percent -- depending on a spacecraft's orbital position.
In addition to the Shuttle, TDRSS customers include the Compton Gamma Ray Observatory, Upper Atmosphere Research Satellite, Hubble Space Telescope, Cosmic Background Explorer, Extreme Ultraviolet Explorer, TOPEX-Poseidon, both Landsat spacecrafts and other non-NASA missions. Among future TDRSS-dependent missions are Space Station Freedom (SSF) and the Earth Observation System (EOS). It is estimated that over $70 billion in space missions through the end of this decade are TDRSS-dependent.
The TDRSS consists of two major elements: A constellation of three geosynchronous satellites -- two operational and one in ready reserve -- and a ground terminal located at White Sands, NM. A second TDRSS ground terminal is under development to eliminate a critical single point of failure.
To meet the growing demand for communications capabilities for future missions, such as SSF and the EOS, increased TDRSS capacity will be required to meet these additional mission requirements.
Current Status
The Tracking and Data Relay Satellite (TDRS-6) is the sixth in a series of communications spacecraft planned for the TDRSS.
TDRS-l has exceeded its design life of 7 years and is continuing to provide limited services. TDRS-2 was lost in the Challenger accident. TDRSs 3-5 are operating, but only two are fully functional. In the event of malfunction of one of these fully operational TDRSs, the absence of a third fully operational satellite in ready reserve would severely impact orbiting customers for nearly a year before an emergency replenishment launch could be conducted.
The successful launch and checkout of TDRS-6 will give NASA the essential requirement of having two fully operational satellites and a fully operational ready reserve capability. This will assure that NASA communications, telemetry and data acquisition capabilities required by space missions will not be jeopardized.
Following the successful launch and checkout of TDRS-6, the TDRSS constellation will be reconfigured. Because of the flexible capability of the TDRSS, one TDRS spacecraft will provide service to the Compton Gamma Ray Observatory (GRO), including real-time transmission of scientific data. This is required because of a problem with the GRO's tape recorders. To accommodate this activity, NASA will operate TDRS-1 through an existing station at Tidbinbilla, Australia, moving TDRS-1 from 171 degrees west longitude to 85 degrees east longitude (over the Indian Ocean, south of Ceylon).
Data from GRO will be relayed to the ground terminal at White Sands via an Intelsat satellite. From White Sands, the data will be sent to the Goddard Space Flight Center, Greenbelt, Md. Control of the TDRS spacecraft will remain at White Sands.
TDRS SPACECRAFT LAUNCH AND OPERATIONAL STATUS
Spacecraft Mission Status
TDRS- 1 STS-6 April 5, 1983 Partially functional
TDRS-2 STS-51L January 1986
TDRS-3 STS-26 September 29, 1988 Partially functional
TDRS-4 STS-29 March 13, 1989 Fully functional
TDRS-5 STS-43 August 2, 1991 Fully functional
Current Position
TDRS-l 171 degrees west (East of Gilbert Islands and South of Hawaii)
TDRS-3 62 degrees west
TDRS-4 41 degrees west (over the Atlantic Ocean off Brazil)
TDRS-5 174 degrees west (East of Gilbert Islands and South of Hawaii)
Reconfigured Position after TDRS-F (6 on orbit)
TDRS-l 85 degrees east
TDRS-3 171 degrees west
TDRS-4 41 degrees west
TDRS-5 174 degrees west
TDRS-6 62 degrees west
Deployment Sequence
TDRS-6 will be deployed from Endeavour cargo bay approximately 6 hours after launch on orbit 5 over the Pacific Ocean north of Hawaii. Injection burn to geo-stationary orbit will be initiated at 77 degrees east longitude (Indian Ocean, south of India), placing the satellite in orbit at 178 degrees west longitude (over the Pacific near the Gilbert Islands).
The STS-54 crew elevates the Inertial Upper Stage/TDRS (IUS/TDRS) to 29 degrees in the payload bay for preliminary tests and then raises it to 58 degrees for deployment. A spring-loaded ejection system is used for deploying the IUS/TDRS.
The first burn of the IUS booster will take place 1 hour after deployment or about 7 hours after STS-54 launch. The IUS second and final burn, to circularize the orbit, will take place 5.5 hours after the first burn, approximately 12.5 hours into the mission. Separation of the booster and satellite will occur at 13 hours after launch.
1 - Deploy solar arrays.
2 - Deploy space-ground link boom.
3 - Deploy C-band boom.
4 - Separation of IUS and TDRS.
5 - Release single access booms.
6 - Position single access antennas.
7 - Open single access antennas.
During steps 5, 6 and 7, Earth acquisition is taking place concurrently.
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
TDRS satellites do not process customer 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.
Nominally, the TDRSS is intended to meet the requirements of up to 24 customer spacecraft, including the Space Shuttle, simultaneously. It provides two types of service: multiple access which can relay data from as many as 20 low data rate (100 bits per second to 50 kilobits per second) customer satellites simultaneously and single access antennas which provide two high data rate channels to 300 megabits per second from both the east and west locations.
The White Sands Ground Terminal (WSGT) 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 WSGT is operated for NASA by GTE Government Systems Corp., Needham Heights, Mass.
Co-located at White Sands is the NASA Ground Terminal operated by Bendix Field Engineering Corp., Columbia. Md. This terminal provides the interface between WSGT and other primary network elements located at NASA's Goddard Space Flight Center, Md.
Facilities at GSFC 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 which provides the network with antenna pointing information for user spacecraft and the TDRSs; and the NASA Communications Network (NASCOM) which provides the common carrier interface through Earth terminals at Goddard, White Sands and the Johnson Space Center, Houston.
The NCC console operators monitor 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 are provided through NASCOM, a global system providing 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 and the user spacecraft control centers.
NASA's Office of Space Communications, Washington, D.C., has overall management responsibility of these tracking, data acquisition and communications facilities.
TDRS Components
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, space-to-ground link and the S-band omni for satellite health 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 primarily relay communications to and from user spacecraft. The high data rates provided by these antennas are 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 (2-meter) parabolic reflector, provides the prime link for relaying transmissions to and from the ground terminal at Ku-band.
Project Support
TRW Space & Electronics Group, Redondo Beach, Calif., is the prime spacecraft contractor. Ground operations at the White Sands complex are conducted by GTE Government Systems Corp., Needham Heights, Mass., and Bendix Field Engineering Corp., Columbia, Md.
INERTIAL UPPER STAGE (IUS)
The Inertial Upper Stage (IUS) will be used with the Space Shuttle to transport NASA's sixth Tracking and Data Relay Satellite (TDRS-F) to geosynchronous orbit, some 22,300 statute miles (35,880 km) from Earth.
Background
The IUS was originally designed as a temporary stand-in for a reusable space tug, and the IUS was named the Interim Upper Stage. The word "Inertial" (signifying the guidance technique) later replaced "Interim" when it was determined that the IUS would be needed through the l990s. In addition to the TDRS missions, the IUS was utilized for the Magellan, Galileo and Ulysses planetary missions.
Specifications
The IUS-13, to be used on mission STS-54, is a two-stage rocket. Each stage has a solid rocket motor, preferred over liquid-fueled engines for their relative simplicity, high reliability, low cost and safety.
The IUS is 17 feet (5.18 meters) long and 9.25 feet (2.8 m) in diameter. It consists of an aft skirt; an aft stage solid rocket motor containing 21,400 pounds (9,707 kg) of propellant generating approximately 42,000 pounds (188,496 newtons) of thrust; an interstage; a forward stage solid rocket motor with 6,000 pounds (2,722 kg) of propellant generating approximately 18,000 pounds (80,784 newtons) of thrust; and an equipment support section.
The equipment support section contains the avionics which provide guidance, navigation, control, telemetry, command and data management, reaction control and electrical power. All mission-critical components of the avionics system, along with thrust vector actuators, reaction control thrusters, motor igniter and pyrotechnic stage separation equipment are redundant to assure reliability of better than 98 percent.
Airborne Support Equipment
The IUS Airborne Support Equipment (ASE) is the mechanical, avionics, and structural equipment located in the orbiter. The ASE supports the IUS and the TDRS-F in the orbiter payload bay and elevates the IUS/TDRS for final checkout and deployment from the orbiter.
The IUS ASE consists of the structure, aft tilt frame actuator, batteries, electronics and cabling to support the IUS/TDRS combination. These ASE subsystems enable the deployment of the combined vehicle; provide, distribute and/or control electrical power to the IUS and satellite and serve as communication conduits between the IUS and/or satellite and the orbiter.
IUS Structure
The IUS structure is capable of supporting the loads generated internally and also by the cantilevered spacecraft during orbiter operations and the IUS free flight. In addition, the structure physically supports all the equipment and solid rocket motors within the IUS and provides the mechanisms for IUS stage separation. The major structural assemblies of the two-stage IUS are the equipment support section, interstage and aft skirt. It is made of aluminum skin-stringer construction with longerons and ring frames.
Equipment Support Section
The Equipment Support Section houses the majority of the IUS avionics. The top of the equipment support section contains the spacecraft interface mounting ring and electrical interface connector segment for mating and integrating the spacecraft with the IUS. Thermal isolation is provided by a multilayer insulation blanket across the interface between the IUS and TDRS.
IUS Avionics Subsystems
The avionics subsystems consist of the telemetry, tracking and command subsystems; guidance and navigation subsystem; data management; thrust vector control and electrical power subsystems. These subsystems include all electronic and electrical hardware used to perform all computations, signal conditioning, data processing and formatting associated with navigation, guidance, control, data and redundancy management. The IUS avionics subsystems also provide the equipment for communications with the orbiter and ground stations as well as electrical power distribution.
Altitude control in response to guidance commands is provided by thrust vectoring during powered flight and by reaction control thrusters while coasting.
Altitude is compared with guidance commands to generate error signals. During solid motor firing, these commands gimbal the IUS's movable nozzle to provide the desired attitude pitch and yaw control. The IUS's roll axis thrusters maintain roll control. While coasting, the error signals are processed in the computer to generate thruster commands to maintain the vehicle's attitude or to maneuver the vehicle.
The IUS electrical power subsystem consists of avionics batteries, IUS power distribution units, power transfer unit, utility batteries, pyrotechnic switching unit, IUS wiring harness and umbilical, and staging connectors. The IUS avionics system distributes electrical power to the IUS/TDRS interface connector for all mission phases from pre-launch to spacecraft separation.
IUS Solid Rocket Motors
The IUS uses a large and a small solid rocket motor employing movable nozzles for thrust vector control. The nozzles provide up to 4 degrees of steering on the large motor and 7 degrees on the small motor. The large motor is the longest thrusting duration solid rocket motor ever developed for space, with the capability to thrust as long as 150 seconds. Mission requirements and constraints (such as weight) can be met by tailoring the amount of solid propellant carried.
Reaction Control System
The reaction control system controls the IUS/TDRS's attitude during coasting, roll control during SRM thrustings and velocity impulses for accurate orbit injection.
As a minimum, the IUS includes one reaction control fuel tank with a capacity of 120 pounds (54.4 kg) of hydrazine. Production options are available to add a second or third tank. IUS-13 will carry two tanks, each with 120 pounds (54.4 kg) of fuel.
To avoid spacecraft contamination, the IUS has no forward facing thrusters. The reaction control system also provides the velocities for spacing between several spacecraft deployments and for avoiding collision or contamination after the spacecraft separates.
IUS-to-Spacecraft Interfaces
The TDRS spacecraft is physically attached to the IUS at eight attachment points, providing substantial load-carrying capability while minimizing the transfer of heat across the connecting points. Power, command and data transmission between the two are provided by several IUS interface connectors.
In addition, the IUS provides an insulation blanket of multiple layers of double-aluminized Kapton and polyester net spacers across the IUS/TDRS interface. The outer layer of the blanket, facing the TDRS spacecraft, is a special Teflon-coated fabric called Beta cloth. The blankets are vented toward and into the IUS cavity, which in turn is vented to the orbiter payload bay. There is no gas flow between the spacecraft and the IUS. The thermal blankets are grounded to the IUS structure to prevent electrostatic charge buildup.
Flight Sequence
After the orbiter payload bay doors are opened in orbit, the orbiter will maintain a preselected attitude to keep the payload within thermal requirements and constraints.
On-orbit predeployment checkout begins, followed by an IUS command link check and spacecraft communications command check. Orbiter trim maneuvers normally are performed at this time.
Forward payload restraints will be released and the aft frame of the airborne support equipment will tilt the IUS/TDRS to 29 degrees. This will extend the TDRS into space just outside the orbiter payload bay, allowing direct communication with Earth during systems checkout. The orbiter will then be maneuvered to the deployment attitude. If a problem has developed within the spacecraft or IUS, the IUS and its payload can be restowed.
Prior to deployment, the spacecraft electrical power source will be switched from orbiter power to IUS internal power by the orbiter flight crew. After verifying that the spacecraft is on IUS internal power and that all IUS/TDRS predeployment operations have been successfully completed, a GO/NO-GO decision for IUS/TDRS deployment will be sent to the crew.
When the orbiter flight crew is given a GO decision, they will activate the pyrotechnics that separates the IUS/TDRS umbilical cables. The crew will then command the electromechanical tilt actuator to raise the tilt table to a 58-degree deployment position.
The orbiter's RCS thrusters will be inhibited and a pyrotechnic separation device initiated to physically separate the IUS/spacecraft combination from the tilt table. Compressed springs provide the force to jettison the IUS/TDRS from the orbiter payload bay at approximately 0.10 meters (4.2 inches) per second. The deployment is normally performed in the shadow of the orbiter or in an Earth eclipse.
The tilt table will be lowered to minus 6 degrees after IUS and its spacecraft are deployed. Approximately 19 minutes after IUS/TDRS deployment, the orbiter's engines will be ignited to move the orbiter away from the IUS/TDRS.
At this point, the IUS/TDRS is controlled by the IUS onboard computers. Approximately 10 minutes after the IUS/TDRS is ejected from the orbiter, the IUS onboard computer will send out signals used by the IUS and/or TDRS to begin mission sequence events. This signal also will enable the reaction control system. All subsequent operations will be sequenced by the IUS computer, from transfer orbit injection through spacecraft separation and IUS deactivation.
After the RCS has been activated, the IUS will maneuver to the required thermal attitude and perform any required spacecraft thermal control maneuvers.
At approximately 45 minutes after ejection from the orbiter, the pyrotechnic inhibits for the first solid rocket motor will be removed. The belly of the orbiter has been oriented towards the IUS/TDRS combination to protect the orbiter windows from the IUS's plume. The IUS will recompute the first ignition time and maneuvers necessary to attain the proper attitude for the first thrusting period.
When the proper transfer orbit opportunity is reached, the IUS computer will send the signal to ignite the first stage motor. This is expected at approximately 60 minutes after deployment (L+7 hours, 13 minutes). After firing approximately 146 seconds and prior to reaching the apogee point of its trajectory, the IUS first stage will expend its fuel. While coasting, the IUS will perform any maneuvers needed by TDRS for thermal protection or communications. When this is completed, the IUS first stage and interstage will be separated from the IUS second stage.
Approximately 6 hours, 12 minutes after deployment at approximately L + 12:30, the second stage motor will be ignited, thrusting for about 108 seconds. After burn is complete, the IUS stabilizes the TDRS while the solar arrays and two antennas are deployed. The IUS second stage will separate and perform a final collision/contamination avoidance maneuver before deactivating.
