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.