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The Geopotential Research Mission (GRM) consists of a pair of spacecraft in identical polar orbits at 160 km altitude, but with a nominal 300-km separation from each other. The objective of the GRM is to determine the earth's gravity and magnetic fields in order to provide accurate mathematical models for studies of the structure, composition and movement of the solid earth and oceans; resource exploration; orbit determination; and navigation. The spacecraft are flown in a "drag-free" orbit obtained by providing thrust to counter the atmospheric drag forces. A disturbance compensation system senses the drag forces and actuates the thrusters. Accurate measurement of the gravity field is obtained by the sensitive spacecraft-to-spacecraft velocity measurement system. The precise orbital position is measured using the ground-based Doppler tracking stations operated by the Defense Mapping Agency (DMA). The two spacecraft are alike with respect to the gravity field detection system, but this particular spacecraft, A1, also carries scalar and vector magnetometers, with four star cameras to provide accurate orientation information for the vector magnetometer. Command, telemetry, and tracking use the TDRSS Single Access (SA) link. In order to operate with the TDRSS, the conformal array antennas are in two parts to allow communications whether approaching or receding from a particular TDRS. Redundant data storage devices are used to record the data during the TDRSS Zone of Exclusion (ZOE), during switchover from one TDRS to the other, and at other times as required. Recorder playback at a rate of 34 kbps for 12 min is required for one orbit of data. The disturbance compensation system contains a 14-cm diameter ball housed in a 16-cm diameter spherical cavity in which the position of the ball is electrically sensed. When in orbit, the ball responds only to the gravity fields as the spacecraft shields the ball from all other forces. When the position of the ball in the cavity changes, the sensor commands the propulsion system to "fly" the spacecraft to re-center the ball in the cavity. The propulsion system is able to move the spacecraft linearly and angularly with six degrees of freedom, so that the ball remains at the center of the cavity. Since the ball is attracted by the mass of the spacecraft and the propulsion fuel, the fuel must be balanced between the front and rear tanks to null out the gravity fields generated by the mass of fuel in each tank. NASA standard reaction wheels are used to provide the torque to control the spacecraft. An onboard computer provides for autonomous control of the spacecraft, independent of ground command control. To eliminate perturbations that could be induced by rotating solar panels, the panels are rigidly attached. The solar array can support an orbital average load of 400 W. The structure of the spacecraft consists of an axial cruciform aluminum basic frame which supports all of the subsystems. Strong rings at each end support the 1-m diameter propellant tanks. The outer monocoque shell is a secondary structural element and serves primarily to support the thermal heat pipes and the solar array mounted on the upper half of the cylindrical surface. A 4-m boom separates the magnetometers from the main body of the spacecraft. Because of the need for stability of the thermal rate of change of spacecraft dimensions, the thermal design concept uses the lower half of the spacecraft as a radiator for internal power and isolates the upper body and solar array from the lower body and from each other. Heat pipes are used to distribute heat uniformly over the spacecraft. Expected mission lifetime is 7 months, with 6 months of scientific data.

Alternate Names

    Facts in Brief

    Launch Date: 1989-01-01
    Launch Vehicle: Shuttle
    Launch Site: Vandenberg AFB, United States
    Mass: 2800 kg

    Funding Agency

    • NASA-Office of Space Science Applications (United States)


    • Earth Science

    Additional Information

    Questions and comments about this spacecraft can be directed to: Coordinated Request and User Support Office



    NameRoleOriginal AffiliationE-mail
    Mr. Thomas L. FischettiProgram ManagerNASA Headquarters
    Dr. Edward A. Flinn, IIIMission ScientistNASA Headquarters
    Dr. David E. SmithProject ScientistNASA Goddard Space Flight
    Dr. Robert A. Langel, IIIProject ScientistNASA Goddard Space Flight Center
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