and Data Relay Satellite System<
operational, the TDRSS will provide continuous global coverage of
Earth-orbiting satellites at altitudes from 750 miles to about 3,100
miles. At lower altitudes, there will be brief periods when satellites
or spacecraft over the Indian Ocean near the equator are out of
view. The TDRSS will be able to handle up to 300 million bits of
information per second. Because eight bits of information make one
word, this capability is equivalent to processing 300 14-volume
sets of encyclopedias every minute.
The fully operational
TDRSS network will consist of three satellites in geosynchronous
orbits. The first, positioned at 41 degrees west longitude, is TDRS-East
(TDRS-A). The next satellite, TDRS-West, will be carried into Earth
orbit aboard the space shuttle and deployed and positioned at 171
degrees west longitude. The remaining TDRS will be positioned above
a central station just west of South America at 62 degrees west
longitude as a backup.
are positioned in geosynchronous orbits above the equator at an
altitude of 22,300 statute miles. At this altitude, because the
speed of the satellite is the same as the rotational speed of Earth,
it remains fixed in orbit over one location. The eventual positioning
of two TDRSs will be 130 degrees apart instead of the usual 180-degree
spacing. This 130-degree spacing will reduce the ground station
requirements to one station instead of the two stations required
for 180-degree spacing.
The TDRS system
serves as a radio data relay, carrying voice, television, and analog
and digital data signals. It offers three frequency band services:
S-band, C-band and high-capacity Ku-band. The C-band transponders
operate at 4 to 6 GHz and the Ku-band transponders operate at 12
to 14 GHz.
automated TDRSS network ground station, located at the White Sands
Ground Terminal, is owned and managed by Contel.
provides communication and tracking services for low Earth-orbiting
satellites. It measures two-way range and Doppler for up to nine
user satellites and one-way and Doppler for up to 10 user satellites
simultaneously. These measurements are relayed to the Flight Dynamics
Facility at GSFC from the WSGT.
Six TDRSs will
be built by TRW's Defense and Space Systems Group, Redondo Beach,
Calif. Contel owns and operates the satellites and the White Sands
Ground Terminal, which was built jointly by the team of TRW, Harris
Corporation and Spacecom. Electronic hardware was jointly supplied
by TRW and Harris's Government Communications Division, Melbourne,
Fla. TRW integrated and tested the ground station, developed software
for the TDRS system and integrated the hardware with the ground
station and satellites.
station is located at a longitude with a clear line of sight to
the TDRSs and very little rain, because rain can interfere with
the Ku-band uplink and downlink channels. It is one of the largest
and most complex communication terminals ever built.
The most prominent
features of the ground station are three 60-foot Ku-band dish antennas
used to transmit and receive user traffic. Several other antennas
are used for S-band and Ku-band communications. NASA developed sophisticated
operational control facilities at GSFC and next to the WSGT to schedule
TDRSS support of each user and to distribute the user's data from
White Sands to the user.
processing equipment at the WSGT aids in satellite tracking measurements,
control and communications. Equipment in the TDRS and the ground
station collects system status data for transmission, along with
user spacecraft data, to NASA. The ground station software and computer
component, with more than 900,000 machine language instructions,
will eventually control three geosynchronous TDRSs and the 300 racks
of ground station electronic equipment.
and control functions ordinarily found in the space segment of a
system are performed by the ground station, such as the formation
and control of the receive beam of the TDRS multiple-access phased-array
antenna and the control and tracking functions of the TDRS single-access
by the satellites are relayed to the ground terminal facilities
at White Sands. White Sands sends the raw data directly by domestic
communications satellite to NASA control centers at JSC (for space
shuttle operations) and GSFC, which schedules TDRSS operations and
controls a large number of satellites. To increase system reliability
and availability, no signal processing is done aboard the TDRSs;
instead, they act as repeaters, relaying signals to and from the
ground station or to and from satellites or spacecraft. No user
signal processing is done aboard the TDRSs.
A second TDRS
ground terminal is being built at White Sands approximately 3 miles
north of the initial ground station. The $18.5-million facility
will back up the existing facility and meet the growing communication
needs of the 1990s.
When the TDRSS
is fully operational, ground stations of the worldwide STDN will
be closed or consolidated, resulting in savings in personnel and
operating and maintenance costs. However, the Merritt Island, Fla.;
Ponce de Leon, Fla.; and Bermuda ground stations will remain open
to support the launch of the space transportation system and the
landing of the space shuttle at the Kennedy Space Center in Florida.
probes and Earth-orbiting satellites above approximately 3,100 miles
will use the three ground stations of the deep-space network, operated
for NASA by the Jet Propulsion Laboratory, Pasadena, Calif. The
deep-space network stations are in Goldstone, Calif.; Madrid, Spain;
and Canberra, Australia.
lift-off and ascent phase of a space shuttle mission launched from
the Kennedy Space Center. the space shuttle S-band system is used
in a high-data-rate mode to transmit and receive through the Merritt
Island, Ponce de Leon and Bermuda STDN tracking stations. When the
shuttle leaves the line-of-sight tracking station at Bermuda, its
S-band system transmits and receives through the TDRSS. (There are
two communication systems used in communicating between the space
shuttle and the ground. One is referred to as the S-band system;
the other, the Ku-band, or K-band, system.)
To date, the
TDRSs are the largest privately owned telecommunication satellites
ever built. Each satellite weighs nearly 5,000 pounds in orbit.
The TDRSs will be deployed from the space shuttle at an altitude
of approximately 160 nautical miles, and inertial upper stage boosters
will propel them to geosynchronous orbit.
The TDRS single-access
parabolic antennas deploy after the satellite separates from the
IUS. After the TDRS acquires the sun and Earth, its sensors provide
attitude and velocity control to achieve the final geostationary
stabilization aboard the TDRS maintains attitude control. Body-fixed
momentum wheels in a vee configuration combine with body-fixed antennas
pointing constantly at Earth, while the satellite's solar arrays
track the sun. Monopropellant hydrazine thrusters are used for TDRS
positioning and north-south, east-west stationkeeping.
module houses four antennas. For single-access services, each TDRS
has two dual-feed S-band / Ku-band deployable parabolic antennas.
They are 16 feet in diameter, unfurl like a giant umbrella when
deployed, and are attached on two axes that can move horizontally
or vertically (gimbal) to focus the beam on satellites or spacecraft
below. Their primary function is to relay communications to and
from user satellites or spacecraft. 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 satellites
or spacecraft (one on S-band and one on Ku-band) if both users are
within the antenna's bandwidth.
primary reflector surface is a gold-clad molybdenum wire mesh, woven
like cloth on the same type of machine used to make material for
women's hosiery. When deployed, the antenna's 203 square feet of
mesh are stretched tautly on 16 sup porting tubular ribs by fine
threadlike quartz cords. The antenna looks like a glittering metallic
spiderweb. The entire antenna structure, including the ribs, reflector
surface, a dual-frequency antenna feed and the deployment mechanisms
needed to fold and unfold the structure like a parasol, weighs approximately
service, the multielement S-band phased array of 30 helix antennas
on each satellite is mounted on the satellite's body. The multiple-access
forward link (between the TDRS and the user satellite or spacecraft)
transmits command data to the user satellite or spacecraft, and
the return link sends the signal outputs separately from the array
elements to the WSGT's parallel processors. Signals from each helix
antenna are received at the same frequency, frequency-division-multiplexed
into a single composite signal and transmitted to the ground. In
the ground equipment, the signal is demultiplexed and distributed
to 20 sets of beam-forming equipment that discriminates among the
30 signals to select the signals of individual users. The multiple-access
system uses 12 of the 30 helix antennas on each TDRS to form a transmit
parabolic reflector is the space-to-ground-link antenna that communicates
all data and tracking information to and from the ground terminal
on Ku-band. The omni telemetry, tracking and communication antenna
is used to control TDRS while it is in transfer orbit to geosynchronous
The solar arrays
on each satellite, when deployed, span more than 57 feet from tip
to tip. The two single-access, high-gain parabolic antennas, when
deployed, measure 16 feet in diameter and span 42 feet from tip
Each TDRS is
composed of three distinct modules: the equipment module, the communication
payload module and the antenna module. The modular structure reduces
the cost of individual design and construction.
module housing the subsystems that operate the satellite and the
communication service is located in the lower hexagon of the satellite.
The attitude control subsystem stabilizes the satellite so that
the antennas are properly oriented toward the Earth and the solar
panels are facing toward the sun. The electrical power subsystem
consists of two solar panels that provide approximately 1,850 watts
of power for 10 years. Nickel-cadmium rechargeable batteries supply
full power when the satellite is in the shadow of the Earth. The
thermal control subsystem consists of surface coatings and controlled
electric heaters. The solar sail compensates for the effects of
solar winds against the asymmetrical body of the TDRS.
payload module on each satellite contains electronic equipment and
associated antennas required for linking the user spacecraft or
satellite with the ground terminal. The receivers and transmitters
are mounted in compartments on the back of the single-access antennas
to reduce complexity and possible circuit losses.
its IUS were carried aboard the space shuttle Challenger on the
April 1983 STS-6 mission. After it was deployed on April 4, 1983,
and first-stage boost of the IUS solid rocket motor was completed,
the second-stage IUS motor malfunctioned and TDRS-A was left in
an egg-shaped orbit 13,579 by 21,980 statute miles-far short of
the planned 22,300-mile geosynchronous altitude. Also, TDRS-A was
spinning out of control at a rate of 30 revolutions per minute until
the Contel/TRW flight control team recovered control and stabilized
TRW and NASA TDRS program officials devised a procedure for using
the small (1-pound) hydrazine-fueled reaction control system thrusters
on TDRS-A to raise its orbit. The thrusting, which began on June
6, 1983, required 39 maneuvers to raise TDRS-A to geosynchronous
orbit. The maneuvers consumed approximately 900 pounds of the satellite's
propellant, leaving approximately 500 pounds of hydrazine for the
10-year on-orbit operations.
maneuvers, overheating caused the loss of one of the redundant banks
of 12 thrusters and one thruster in the other bank. The flight control
team developed procedures to control TDRS-A properly in spite of
the thruster failures.
turned on for testing on July 6, 1983. Tests proceeded without incident
until October 1983, when one of the Ku-band single-access-link diplexers
failed. Shortly afterward, one of the Ku-band traveling-wave-tube
amplifiers on the same single-access antenna failed, and the forward
link service was lost. On November 19, 1983, one of the Ku-band
TWT amplifiers serving the other single-access antenna failed. TDRS-A
testing was completed in December 1984. Although the satellite can
provide only one Ku-band single-access forward link, it is still
TDRS-B, C and
D are identical to TDRS-A except for modifications to correct the
malfunctions that occurred in TDRS-A and a modification of the C-band
antenna feeds. The C-band minor modification was made to improve
coverage for providing government point-to-point communications.
TDRS-B was lost on the 51-L mission.
plan for TDRS-C is similar to that originally planned for TDRS-A.
Backup project operations control centers have been added at TRW
and at the TDRS Launch/Deployment Control Center in White Sands.
These facilities will improve the reliability of control operations
and the simultaneous control of TDRS-A in mission support and of
TDRS-C during launch and deployment operations.
its IUS are to be deployed from the space shuttle orbiter. Approximately
60 minutes later, the IUS first-stage solid rocket motor is scheduled
to ignite. This will be followed by five maneuvers to allow monitoring
of TDRS-C telemetry.
After the IUS
second-stage thrusting is completed, the TDRSS mission team at White
Sands will command deployment of the TDRS-C solar arrays, the space-ground
link antenna and the C-band antenna while the TDRS is still attached
to the IUS. Upon separation of the IUS from TDRS-C, the 16-foot-diameter
single-access antennas will be deployed, unfurled and oriented toward
Earth. Nominal deployment will place TDRS-C at 178 degrees west
TDRS-C will be initiated; and after initial checkout, TDRS-C will
drift westward to its operational location at 171 degrees west longitude,
southwest of Hawaii, where it will be referred to as TDRS-West.
Operational testing will continue to verify the full-system capability
with two operating satellites. On completion of this testing, about
three to five months after the launch of TDRS-C, the TDRSS, for
the first time, will provide its full-coverage capability in support
of NASA space missions.
to TDRS-C, will take the place of TDRS-A at 41 degrees west longitude
above the equator, over the northeast corner of Brazil, and will
be referred to as TDRS-East. TDRS-A will then be relocated, probably
79 degrees west longitude above the equator, over central South
America, and will be maintained as an on-orbit spare.
satellites will make up the space segment of the TDRS system. The
on-orbit spare, available for use if one of the operational satellites
malfunctions, will augment system capabilities during peak periods.
The two remaining satellites will be available as flight-ready spares.
of TDRS-A's Ku-band forward link prohibits the operation of the
text and graphics system that it is desired be placed on board all
space shuttle orbiters. TAGS is a high-resolution facsimile system
that scans text or graphic material and converts the analog scan
data into serial digital data. It provides on-orbit capability to
transmit text material, maps, schematics and photographs to the
spacecraft through a two-way Ku-band link through the TDRSS. This
is basically a hard-copy machine that operates by telemetry.
is a dual TDRS capability, a teleprinter must be used on orbit to
receive and reproduce text only (such as procedures, weather data
and crew activity plan updates or changes) from the Mission Control
Center. The teleprinter uses S-band and is not dependent on the
When the space
shuttle orbiter is on orbit and its payload bay doors are opened,
the space shuttle orbiter Ku-band antenna, stowed on the right side
of the forward portion of the payload bay, is deployed. One drawback
of the Ku-band system is its narrow pencil beam, which makes it
difficult for the TDRS antennas to lock on to the signal. Because
the S-band system has a larger beamwidth, the orbiter uses it first
to lock the Ku-band antenna into position. Once this has occurred,
the Ku-band signal is turned on.
system provides a much higher gain signal with a smaller antenna
than the S-band system. The orbiter's Ku-band antenna is gimbaled
so that it can acquire the TDRS. Upon communication acquisition,
if the TDRS is not detected within the first 8 degrees of spiral
conical scan, the search is automatically expanded to 20 degrees.
The entire TDRS search requires approximately three minutes. The
scanning stops when an increase in the received signal is sensed.
The orbiter Ku-band system and antenna then transmits and receives
through the TDRS in view.
At times, the
orbiter may block its Ku-band antenna's view to the TDRS because
of attitude requirements or certain payloads that cannot withstand
Ku-band radiation from the main beam of the orbiter's antenna. The
main beam of the Ku-band antenna produces 340 volts per meter, which
decreases in distance from the antenna-e.g., 200 volts per meter
65 feet away from the antenna. A program can be instituted in the
orbiter's Ku-band antenna control system to limit the azimuth and
elevation angle, which inhibits direction of the beam toward areas
of certain onboard payloads. This area is referred to as an obscuration
zone. In other cases, such as deployment of a satellite from the
orbiter payload bay, the Ku-band system is turned off temporarily.
When the orbital
mission is completed, the orbiter's payload bay doors must be closed
for entry; therefore, its Ku-band antenna must be stowed. If the
antenna cannot be stowed, provisions are incorporated to jettison
the assembly from the spacecraft so that the payload bay doors can
be closed for entry. The orbiter can then transmit and receive through
the S-band system, the TDRS in view and the TDRS system. After the
communications blackout during entry, the space shuttle again operates
in S-band through the TDRS system in the low- or high-data-rate
mode as long as it can view the TDRS until it reaches the S-band
landing site ground station.