GRACE Mission Systems
Satellite System (SAT)
The spacecraft's exterior shape is shown here. The science instruments are mounted on a CFRP bench in the interior, as are the fuel tanks and the batteries and other satellite sub-systems.
The TT&C activities are carried out using a pyro-deployed S-Band receive and transmit antenna, mounted on a nadir-facing deployable boom. Two back-up zenith antennae, one each for transmitting and receiving, along with the appropriate RF electronics assembly, complete the telemetry and telecommand sub-system.
The CM Trim Assembly consists of six Mass Trim Mechanisms, associated electronics, and the power and signal harness. Each MTM consists of a trim mass driven on a nut rotor with a stepper motor. The CMT Assembly is used to center the CG of the satellite at the center of the proof-mass of the accelerometer, during CG calibration maneuvers.
The Thermal Control Sub-System consists of 64 independent heater circuits, 45 resistors and 30 thermistors for in-flight temperature housekeeping, monitoring and heater control, as well as for on-ground verification testing.
The Power System is responsible for generation, storage, conditioning and distribution of electrical power in accordance with instrument and satellite bus users needs. Electrical energy is generated using a solar array of Globalstar silicon cells, placed on the top and side exterior surface of the satellites. Excess energy is stored in a battery of NiH2 common pressure vessel cells. The power bus delivers unregulated power to all users at the respective user interface.
The Attitude and Orbit Control System (AOCS) consists of sensors, actuators and software. The sensors include the DSS Coarse Earth Sun Sensor (CESS) for omni-directional, coarse attitude measurement in the initial acquisition, survival and stand-by modes of the satellite. The high precision sensors include the SCA and GPS receiver. An Inertial Reference Unit (IRU), used in survival modes, provides 3-axis rate information. A Förster Magnetometer is mounted on the deployable boom, to provide additional rate information. The actuators for the AOCS include Cold Gas Assembly and a Magnetorquer. The GN2 reaction control system includes two pressure vessels, valves, regulators and filters, along with 12 attitude control thrusters and two orbit control thrusters. Three magnetorquers with linear dipole moments of 30 Am2 complete the set of AOCS actuators.
The On-Board Data Handling (OBDH) System provides processor and software resources, as well as necessary I/O capabilities for AOCS, Power and Thermal Systems operations, including necessary fault detection, isolation and recovery operations.