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Tracking and Data Relay Satellite

Release No. 91-41
June 7, 1991

Current Status and History

        The 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).  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, 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.

Deployment Sequence
        TDRS-E will be deployed from Atlantis approximately six
hours after launch on orbit five (over the Pacific south of
Hawaii).  Injection burn to geosynchronous 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-43 crew elevates the Inertial Upper State (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 an hour
after deployment, or about seven hours after launch.  The second
and final burn (to circularize the orbit) will take place five and
one half hours after the first burn, approximately 12 and one half
hours into the mission.  Separation of the booster and satellite
will occur at 13 hours after launch.

        Upon reaching geosynchronous orbit, the deployment of antennas
and appendages is started.  The deployment sequence is:

        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 five, six and seven, Earth acquisition is taking
place concurrently.

        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.

        The Project Manager for the Advanced Tracking and Data Relay
Satellite Project is Charles Vanek; Nicholas G. Chrissotimos is
TDRS Manager for the spacecraft project; Daniel A. Spintman is
Chief, Networks Division; Wesley J. Bodin is Associate Chief for
Ground Network; Phillip Liebrecht is Assistant Chief for TDRSS and
Gary A. Morse is Goddard Network Director.

Comments and questions: Jennifer Green
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