NASA’s Tra,cking and Data

Relay Satellite System

W. M. Holmes, Jr.

Every year NASA launches scientific and experimen- tal satellites. These satellites generate large volumes of valuable data. Today this data is stored in the satellite, usually in a tape recorder, .and “dumped” at high speed when the satellite is in view of one of NASA’s Satellite Tracking and Data Network (STDN) stations. Collection of satellite data is an expensive business since a large number of stations are required, and many of these must be located in foreign countries where staffing and‘oper-- ating costs are high.

In 1980 the Tracking and Data Relay Satellite System (TDRSS) will begin operation.’ When TDRSS starts operating, NASA’s scientific and experimental satellites will send their data “up” to Tracking and Data Relay Sat- ellites (TDRS) instead of “down” to an earth station. TDRSS will increase the percentage of timethat satel- lites can transmit data to the ground, thus reducing the requirements for storing data .on-board satellites. This will also allow missions that generate volumes of data that are beyond the capacities of current tape recorders.

Redondo Beach, CA 80278. The author is with the TRW Defense and Space Systems Group.

‘For.-a more detailed description of t.he various technical features of TDRSS, see the papers of Sessions 9 and 19 in the NTC‘77 Conference Record, IEEE Publication No. 77CH1292-2 CSCB.

TDRS will be operated ‘at “synchronous altitude”’ (about 35 800 km) where they circle the earth once every 24 hours. Because their orbit period is synchronous with the.earth’s rotation, the TDRS remain over fixed loca- tions on the earth’s surface.

Most of NASA’s satellites operate at much lower orbi- -tal altitudes than TDRS. Satellites looking at the earth operate at altitudes from 200 to about 1000 kmsince they can “see”more detail by be’ing close. Satellites that look away from the earth, for instance astronomical satellites like space telescopes, operate at low altitudes since a launch vehicle (rocket or Shuttle) can carrya muchlarger payload to low orbit-, High orbits are expensive to reach, and are only used when they ire needed to provide im- portant earth coverage characteristics.

Because the TDRS are.so high, they look down toward the earth and the satellites that use TDRSS (user satel- lites). One TDRS can “see” more than half of all the user satellite orbit locations. Two TDRS could see all of NASA’s low and medium orbit satellites, all the time, if the TDRS locations were not constrained by the need for both TDR‘S to see a common ground station. Because of this constraint, we use the orbit configuration shownin Fig. 1.

With 130° longitudinal spacing between two TDRS,

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TDRS SPARE

SA EAST A N STEERING L

SA WEST ANTENNA

TDRS EAST M 41 % LONGITUDE, SA WEST

rm ANTENNA

SA EAST

STEERING LIMIT If\\ ANTENNA

. \

TDRS WEST 171%

SA EAST ANTENNA

///

ANTENNA STEERlNG.LlMlT

STEERING ANGLE f225’ = 45O \

\ \ \

NOTE: TDRSS E A S T M S T \ (MINIMUM REOUIR SA ANTENNA POINTING

TDRSS NORTHISOUTH \ POINTING LIMITS ARE *SOo

LIMITS ARE SHOWN, \ / FOR BOTH MA AND SA USERS ’ FOR 100% COVERAGE)

/ \ /

I . \ / V

‘ED

Fig. 1. T D R S system configuration. Each satellite will have a single access (SA) link with a high data rate capability and up to 20 lower rate multipleaccess (MA) links. The single earth station tor TDRSS is the White Sands Test Facility (WSTF).

there is a small “shadow zone” where neither TDRS can see a’user satellite. The area that is not covered gets smaller with greater user satellite altitude, and for user satellites above 1200 km, TDRSS provides 100 percent coverage. The areas not covered are plotted in Fig. 2 for several user satellite altitudes.

TDRSS and STDN coverage of user satellites at vari- ous altitudes is compared in Fig. 3. The comparison shows that TDRSS is far superior for coverage of low and medium altitude user satellites, while earth terminals provide better coverage for very high altitude satellites and deep-space missions. ,The important difference is that TDRSS provides 85 percent coverage for low alti- tude user satellites, while STDN can provide only 15 per- cent. This is especially vital since most of NASA’s sci- entific and experimental satellites are in low and medium altitude orbits.

The most important factor is that TDRSS can provide superior coverage at much lower cost than just maintain- ing the current STDN. TDRSS will more than pay for

itself by allowing NASA to close the STDN stations . . that . are not required for high altitude satellite and deep-space mission coverage.

SYSTEM DESCRIPTION

TDRSS is being built now, and the first satellite will be launched in 1980 by the Space Transportation‘System (STS or Space Shuttle). There are two interesting fea- tures of TDRSS that affect the way the system is de- signed. First, TDRSS will be owned and operated by Western Union, and the services to NASA user satellites will be leased to NASA for a ten-year period after the system becomes operational. Second, the system is shared with commercial advanced WESTAR use which supplements Western Union’s existing WESTAR com- munications services. Both of these features are incor- porated to provide the lowest possible cost to NASA for the TDRSS services.

TDRSS will consist of four satellites in orbit (Fig. 4).

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b

C

WEST EAST

WEST EAST

Fig. 2. Coverage for (a) 200, (b) 400, (c) 1000 km user satellite altitude 7 O TORS inclined orbits, with 180" phasing.

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f r

Percentage Coverage at:

System

One TDRS TDRSS (Two TDRS) One Earth Station Current STDN Post-TDRSS STDN

36 000 km .Very High 200 km (Planetary,etc.) (Synchronous) 1200 km

55.0%

50 42 8 1.5 10 38 100 85.0 5 Yo 19% 70%

1 .50 80 100 100 7.5 100 100 40

Fig. 3. Coverage of user satellites for different user satellite altitudes.

Two of these are designated active TDRS. TDRS East A third satellite is designated Advanced WESTAR. and TDRS West are in synchronous orbit at locations This satellite provides domestic United States commer- separated.by 130' of longitude as described. Their loca- cial communications for Western Union as part of their tions are 65O to the east and west of White Sands, NM, nationwide communications system. The fourth satellite where the TDRSS ground station is located. The ground is designated the on-orbit spare and provides rapid .re- station longitude is 106O W and the two TDRS stations placement of services if one of the other three satellites are at 41°W and, 171OW longitudes. fails. There will be a fifth satellite in storage which can

Fig. 4.' TDRSS/Advanced WESTAR shared space segment configuration. The Advanced WESTAR Satellite, whose communication links are not shown here, will use the TDRSS ground terminal for tracking telemetry and command (TT&C).

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be launched in a relatively short time to replace the spare, should this ever be necessary. Oth,er satellites will be built as required to continue providing five satellites to support system operations.

All of the satellites are identical. Although the TDRSS and Advanced WESTAR missions are quite different, the satellite communications’ payload can be reconfigured by commands from the ground to support either mission. Furthermore, the requirements for numbers and types of transmitters and receivers, and the satellite power re- quirements, are similar enough to allow efficient utiliza- tion of the satellite hardware in both missions.

Designing a shared system with identical satellites has several advantages:

1) Design costs are reduced, since only one satellite must be designed rather than two. 2). Only one in-orbit spare is required to support both

missions. Two spares would be required without sharing.

3) Hardware failures in the satellite may affect only one. mission, Satellites may be interchanged between TDRSS and Advanced WESTAR missions to avoid the need for additional replacement launches.

White Sands was chosen as the optimum location for the TDRSS ground segment;Fig. 5. After a detailed study of potential sites; Some of the advantages of the White Sands site are: 1) Low geographic latitude: low latitude increases the

angle between the two TDRS in orbit while maintaining line-of-sight communications with the ground station.

2) Provides .down-range coverage of Cape Kennedy launches: locating the station any further west would move the TDRSS “blind spot” into a criticalregi.on where second-stage launch vehicle operations occur on many missions.

3) Very little rain: the radio frequencies used by TDRSS for communication between TDRS and.ground could be disrupted by heavy rainfall. This disruption will occur very seldom at. White Sands.

‘TDRSS is a communication service, and as such, does not need to know the content of the data being transmit- ted. The interface point between TDRSS and the NASA ground communication system, NASCOM, is at the White Sands station. Fig. 6 is a simplified ground station‘ block diagram which shows the TDRSS/NASCOM inter- faces. Four types of operational information flow across this interface. These are:

1) data from NASCOM to be transmitted to user satellites

2) data from user satellites to be transmitted by NASCOM to user control centers

3) schedules and control orders from the NASA, TDRSS control center telling TDRSS where to point antennas .and when to provide certain types of service, and

4) TDRSS reports and status data to notify NASA of the current condition of TDRSS

Two other types of data across the.NASCOM/TDRSS interface provide for data flow to and from simulated user satellites. TDRSS has the capability to generate sig- nals simulating almost any user satellite with which the

system is capable of operating. TDRSS will accept simu- lated user data, transmit it to TDRS using a simulated user signal, and return it to NASA through the regular TDRSSINASCOM interfaces. The ground station will also receive signals from the TDRS using simulated user .receivers and return this data to NASA for comparison with the transmitted data. The simulation capability will aid .NASA in the development of user satellite commur nication hardware. As an example, simulated data from actual user satellite hardware being developed could be .sent to White Sands over telephone lines. There the data could be modulated on a simulated user signal with the same characteristics planned for the future mission. The data would flow through the entire TDRSS and back to NASA over the standard TDRSS/NASCOM interfaces. This type of test could reveal problems, if any, with pro- posed satellite .systems early enough to allow changes without program disruption,

TDRSS generates modulated radio signals at the White Sands station for transmission through a TDRS to user

Fig. 5. TDRSS ground regment.at Whlte Sands, NM.

satellites, and demodulates radio signals. which have passed through TDRS from user satellites. The TDRS does no processing of user satellite traffic, in either direction, other than reception,, frequency translation, amplification, and radiation through the appropriate antennas. Thus, the TDRS operates as a “bent pipe” ’repeater, and all of the signal processing equipment is in the ground station.

In addition to modulating anddemodulating user satel- lite signals, the TDRSS ground station uses the signal modulation to measure the range to the user satellite, and the range rate [or first derivative of range]. This data is used with computer programs at NASA-Goddard to de- termine the position of user satellites. User satellite positions will be determined to the same accuracy cur- rently provided by the NASA STDN network.

The TDRSS tracking parameters, as illustrated in Fig. 7, are the range and range rate from the White Sands station through TDRS to the user satellite. Range and range rate are found by measuring the time required for signals to propagate through the closed.loop from White Sands to the user satellite and back to White Sands.

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4.5 METER 3,METER

SIMULATION SIMULATION K-BAND S- BAN D

ANTENNA ANTENNA

SHUTTLE SHUTTLE’ S-BAND S-BAND

A N D RETURN SIMULATORS

NASCOM SIMULATION TRAFFIC INTERFACE

3 METEK 6 METER

ANTENNA ANTENNA 18 METER K-BAND TDRSS ANTENNAS AW K-BAND S-BAND

AW K-BAND TTBC

TDRS K-BAND TThC I 1

DATA SUBSYSTEM RF AND SIGNAL EQUIPMENT PROCESSING A N D TOCC EQUIPMENT

t NASCOM SYSTEM CONTROL INTERFACE

t NASCOM USER TRAFFIC INTERFACE

Fig. 6. TDRSS White Sands ground terminal.

TDRSS can also measure range and range rate in the loop from White Sands through one TDRS to the user satellite, and back through a different TDRS to White Sands, This may provide better position accuracy for those user sat- ellites whose antennas allow this operating mode. For any tracking mode, NASA does the computation required to convert range and range rate data to user satellite position.

THE SPACE SEGMENT The Tracking and Data Relay Satellite, Fig. 8, is large

orbit satellites, it is huge. A TDRS will weigh 5000 lbs in synchronous orbit at the beginning of life. When TDRS unfolds its antennas and solar panels, it will measure over 57 ft along the solar panel axis, and over42 ft across the large antennas.

TDRS is carried into low orbit by the Space Shuttle. An Interim Upper Stage (IUS] rocket takes TDRS to synch‘ronous orbit and places it “on-station.’’ Each sat- ellite is tested for proper operation at a longitude close to the middle of the United States before moving to its operational location as TDRS East, TDRS West, Ad-

by any standard. In comparison to previous synchronous vanced WESTAR, or spare.

/USER SATELLITE

> GROUND USER SATELLITE

USER SATELLITE

I TDRS EAST

Fig. 7. TDRSS tracking parameters.

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Three different types of communication are provided by a TDRS. Two of these use the 16-ft diameter deploy- able dish antennas. These are located on booms on each side of the satellite body." These antennas must be mechanically pointed at user satellites. Because of this, they can normally provide communication with only one user satellite at a time, and they are called "Single- Access" antennas. The communications systems are called SSA, for S-band single-access,' and KSA, for K-band single-access. The terms S-band and K-band refer to frequency ranges of 2-2.3 GHz and 13.7-15.2 GHz, respectively.

The SSA system transmits data at rates from 100,

Some very small satellites may not have any control of their operation. These will use omnidirectional antennas which have very low gain, but work equally well in any direction.

There are two SA antennas per TDRS, and threeTDRS in the system, so six user satellites can normally be ser- viced at one time by the SSAand KSA systems combined. If a SSA user satellite is located close to a KSA user, both can be serviced by a single SA antenna. This type of oper- ation will occur after the Space Shuttle releases an inde- pendent payload, for example, and during rendezvous operations. Under this special condition, as many as twelve SA users could be simultaneously serviced.

4

Fig. 8. The tracking and data relay satellite.

bitslsecond to 300 kbitslsecond to user satellites, and receives' data at rates from 100 bits/second to 1 2 Mbitslseoond from user satellites. The KSA system transmits data at from 1000 bitslsecond to 25 Mbits/ second to user satellites and receives data at from 1000 bitslsecond to 300 Mbits/second from user satellites. The KSA system provides higher data rates because the higher frequency provides better antenna performance, and wider allocations of frequency spectrum are'avail- able. The SSA system is less expensive for user satellites with low gain antennas. Low gain antennas are espe- cially useful for small satellites which do not have the capability to point an antenna at the TDRS accurately.

The Multiple Access [MA] system uses 30 helix anten- nas, mounted on the TDRS front platform, as a phased array antenna to form up to 20 independent antenna beams directed towards user satellites. The phased array system is ground implemented, so the beams are actually formed in the White Sands station. The number of beams is limited only by the amount of 'ground hardware. Twenty users may be tracked through one TDRS or through any combination of the three TDRS in orbit.

The multiple access system can transmit from 100 bits/second to 10 kbits/second to user satellites and receive from 100 bitsisecond to 50 kbitslsecondfrom user ,

satellites. The lower data rate capability is the price paid

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for the large number of channels. The availability of 20 channels allows almost full-time coverage of many users.

The round solid dish antenna deployed on one side of the satellite is the Space Ground Link [SGL) antenna. All signals to and from user satellites go through this an- tenna to or from the White Sands ground station.

There are two antennas on a boom on the opposite side of the TDRS. A small conical S-band antenna is used for command and housekeeping telemetry. The “D” shaped solid dish antenna is part of the commercial Advanced WESTAR system. In addition to the antennas, there is also a small rectangular solar sail mounted on the same boom. This balances the torques caused by the pressure of sunlight on the satellite. Over the ten years of planned satellite operation, even a very small, unbalanced torque requires a large amount of hydrazene fuel to maintain satellite orientation.

There is one other antenna! mounted on the TDRS front panel. This is a 1.1 m Advanced WESTAR K-band dish antenna, used for commercial communication.

The large solar panels rotate every24 hours to track the sun, and produce a minimum of 1.8 kW of power at the end of the 10-year satelli$e life. The TDRS has batteries large enough to provide continuous operation through the eclipses that occur during the spring and,fall, when the earth’s shadow falls across the satellite as it revolves around the earth.

There are small rockets which control the satellite orbit and correct for any perturbations that occur over the satellite lifetime. Most of the correction will be to cancel NorthiSouth perturbations caused by the Sunand Moon gravity effects. This will require rhost of the 1500 lbs of hydrazene monopropellant fuel ca xied by the sat- ellite. These rockets also allow satellit i , s to be moved East or West to different locations. East and West move- ments will usually be at a rate of about 1-3O of longitude

Each TDRSS satellite, after being transported by the Space Shuttle to an orbiting altitude of 150 mi, will be released and will then be boosted up into its 22 300 mi orbit by its attached Interim Upper Stage. The satellite is shown folded with its solar panels visible.

change per day. These movements will occur .if a spare is called on to replace a failed TDR,S [or Advanced WESTAR).

The TDRS system is an efficient, flexible solution to the problems of getting data from low altitude satellites to the ultimate data users. The efficiency of experimental satellite utilization will take a quantum jump forward with the introduction of this service in 1980. O

W. M. Holmes, Jr. is Assistant Proj- ect Manager for TDRSS, Redondo Beach, CA, where his responsibili- ties include system and satellite requirements definition, launch re- quirements, system verification, and orbital evaluation. Earlier he served as Manager of the Signal Transmission Department in the System Engineering,Laboratory.

Mr. Holmes has 11 years of experience at TRW in the field of communications and communications system design, with emphasis on satellite communications, command, telemetry, and navigation systems. Before joining TRW in 1967, he was employed for four years at the Jet Propulsion Laboratory, Pasa- dena, CA, where he participated in the design and flight qualifi- cation of the radio subsystem for Mariner C and was responsible for the development of high-efficiency S-band traveling wave tube amplifiers for advanced spacecraft applications.

Mr. Holmes received the B.S.E.E. degree from the Georgia Institute of Technology, and the MS. and M.E. degrees in engi- neering and engineering management from the University of California at Los Angeles.

TDRSS Launching Delayed A design snag will delay the TDRSS launching,

originally scheduled for 1980, according to a re- port in the New York Times, July 3,1978. Appar- ently, routine Soviet radar activities in Eastern Europe will cause excessive radio interference with the giant satellite.

NASA is negotiating with Western Union re- garding the extent of time the launching will be ,

delayed and the potential cost increase. Western Union was originally awarded $786 million to build and operate the TDRSS.

According to the Washington Post, part of the reason for the belated discovery of the problem was that the Pentagon and the CIA had not alerted NASA to the size and scope of the radio interfer- ence. The Times quotes NASA sources as saying that NASA officials were informed of the poten- tial problem, but that the officials had not fully understood its seriousness. 0

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