The space segment of the SN consists of up to six operational TDRS spacecraft in geostationary orbits at allocated longitudes.  The spacecraft are deployed in geostationary orbits to provide the broadest possible coverage to customers.  Each TDRS provides a two?way data communications relay between customer spacecraft and the WSC/GRGT for data transfer and tracking.  The TDRS spacecraft are extremely sophisticated and are able to track and communicate with up to 21 separate LEO customer satellites from their positions 22,300 miles above the Earth. 

The TDRS fleet is composed of two generations of spacecraft: 

• F1-F7 are first generation spacecraft.
• F8-F10 are second generation spacecraft.

Currently a third generation spacecraft, TDRS K, L, and M is in development. The contract to build these additional TDRS spacecraft was awarded to Boeing Satellite Systems in December 2007. TDRS K and L are scheduled for launch in 2012 and 2013, respectively, and TDRS M has a launch readiness date of 2015.  The contract also has options for an additional spacecraft, TDRS N. To read more about the development of TDRS K, L and M, click here.

The table below shows a comparison of the service capabilities for the first and second generations of spacecraft.

Baseline Service Comparison

Service	F1-F7	F83-F10
SSAF	300 kbps	300 kbps
SSAR	6 Mbps	6 Mbps
KuSAF	25 Mbps	25 Mbps1
KuSAR	300 Mbps	300 Mbps
KaSAF (22.55?23.5 GHz)	N/A	25 Mbps1
KaSAR (25.25?27.5 GHz)	N/A	800 Mbps2
SA Links per Spacecraft (forward and return)	2 SSA 2 KuSA	2 SSA 2 KuSA 2 KaSA
MAF Links per Spacecraft	1 at 300 kbps	1 at 300 kbps
MAR Links per Spacecraft	5 at 300 kbps	5 at 3 Mbps
Customer Tracking Accuracy	150 meters 3 sigma	150 meters 3 sigma
Notes: 1.  There is a 7 MB limit on the Forward service due to a constraint with the WSC /GRGT Data Interface System (DIS). 2.  Data rates above 300 Mbps requires ground station modifications. 3.  The F8 spacecraft is unavailable for normal customer scheduling. 4.  F8-F10 spacecraft are equipped for second MAF service capability, however only one MAF service per F8-F10 is currently supporting users due to ground system limitations.

 

First Generation TDRS

The first generation satellites include the original spacecraft designs, F1-F6, and the replacement TDRS, F7.  These spacecraft carry functionally identical payloads. 

The first generation spacecraft are three axis stabilized, momentum biased spacecraft with sun-oriented solar panels.  Antenna pointing is accomplished by mechanically steering the SA and SGL antennas, or by electronically steering the MA phased array.

The spacecraft design can be traced back to technologies developed for the Fleet Satellite Communications (FLTSATCOM) program.  The dominant mechanical design considerations included satisfying the field of view and orientation requirements of the solar arrays, antennas, and sensors, as well as developing a stowed arrangement compatible with the Space Shuttle cargo bay.

Solar arrays provide all system power by recharging nickel-cadmium (NiCad) batteries inside the spacecraft structure.  The arrays are positioned to preclude excessive shadowing by the 4.9-meter antenna reflectors.

The fully deployed satellite is 57 feet from solar panel to panel, and 46 feet from dish to dish.  The total weight of the satellite, which varied with design changes between F1-F7, was approximately 5,000 pounds, which includes approximately 1,200 pounds of hydrazine fuel and approximately 300 pounds for the Inertial Upper Stage (IUS) adapter ring.  Earth and sun sensors are used to determine spacecraft orientation.  A set of two motor driven flywheels and a set of 12 fully redundant hydrazine thrusters are combined to maintain the desired spacecraft orientation with respect to the Earth throughout the spacecraft life.  A spacecraft thermal control subsystem maintains satellite component temperatures within acceptable limits.  The physical characteristics of the first generation spacecraft can be seen in the below table.

Physical Characteristics of F1-F7

Contractor	TRW (which was bought by Northrop Grumman)
Lifetime	10 years
Dimensions	17.4 x 14 m
Mass	2270 kg (at launch)

 

Second Generation TDRS

The three Second Generation TDRS satellites, delivered by Boeing, were designed, built, and tested to provide service for 11 years in Geostationary Earth Orbit (GEO) and maintain health for four additional years of on-orbit storage.  The nominal satellite separation weight is approximately 3,200 kg (7,000 lb).  The total required satellite power is about 2.3 kW. 

The second generation spacecraft maintain user service compatibility with the first generation spacecraft.  Fully deployed, they measure approximately 69 feet long (21 meters) by 43 feet wide (13 meters).  The silicon celled solar arrays generate 2,300 watts of on-orbit power and Nickel-hydrogen (Ni?H) batteries supply power during solar eclipses. 

The spacecraft are based on the Boeing 601 bus.  The electrical power, attitude determination and control, and tracking, telemetry and control units are mounted on the bus structure, as are the solar array wings. 

The propulsion and reaction control system uses a bi-propellant system.  The system uses a 110-pound-force (lbf) liquid apogee motor for orbit insertion and four 2 lb thrusters and eight 5 lb thrusters for on-orbit operations and attitude control during orbit insertion. 

The attitude control system is a momentum bias design, using a gimbaled momentum wheel for active three-axis torquing and momentum storage.  Continuously operating gyros, updated by earth and sun sensors, provide accurate three axis attitude sensing and are used both to maintain an earth pointed spacecraft attitude and to provide antenna pointing control compensation. 

The attitude control subsystem has an on-board processor, providing some measure of autonomy over the first generation fleet.  It also provides tighter pointing capability, which is required for the narrow beam used for the Ka-band service.  Additional features include fault protection and the use of a hemispherical resonating gyro rather than one of mechanical design with moving parts that can wear out. 

A system of heat pipes, multi-layer insulation, radiators and thermostatic heater control, provide autonomous thermal control for all deployed operations.  Two wings covered with silicon solar cells provide a 15-year end of life power of approximately 2,300 watts.  Ni-H batteries, which supply power during eclipses, have autonomous battery charge maintenance.  They do not require the seasonal reconditioning of the commonly used Ni-Cad batteries.