The two GRACE satellites were developed by Astrium GmbH and Space Systems/Loral (SSL) . Astrium GmbH has build - under contract by JPL - major elements of the two flight satellites using heritage from the CHAMP mission. SSL provides the attitude control system, microwave instrument electronics and system and environmental testing.
Both satellites are completely identical and differ only in the S-band radio frequencies used for communication with the ground and in the K-band frequencies used for the inter-satellite link. The satellite design features a simple and robust structure design, mainly based on Carbon Fiber Reinforced Plastic (CFRP) sandwich panels with aluminum core and edge profiles for low thermal distortion. The outer shape of the satellite is optimized with respect to its aerodynamic behavior. This requires a symmetrical form with the center of pressure always in a single plane. Because the spacecraft center of mass is on the same level as the center of pressure, the disturbances due to air drag and solar pressure are minimized.
All electrical units, the harness, the GN2 tanks, as well as the piping to the thrusters are located on both sides of the central equipment panel. The two main solar arrays are symmetrically canted to the equipment panel with two additional solar array panels on the satellite roof. The satellite body is closed at its front and aft side by sandwich panels of the same composite as the solar array panels. The aft panel carries the occultation GPS antenna, while the front panel has the cut-out for the Ku/Ka-band horn. The nadir and zenith S-band antennae are mounted on brackets to minimize gain disturbance.
The following figure describes the physical layout of and provides a view inside the GRACE satellites.
After successful instrument integration, which has been finished in March 2000, both satellites were shipped to IABG for environmental testing.
The Attitude and Orbit Control (AOCS) System is composed of the necessary sensors, actuators, electronics, and software to provide adequate knowledge of spacecraft attitude during all phases of the mission, to generate on-board signals to accurately maintain spacecraft attitude, and to provide necessary orbital control to satisfy the GRACE mission requirements. The AOCS is comprised of the following elements:
- a cold gas propulsion system for attitude control and orbit change maneuvers,
- a set of three magnetic torque rods for attitude control in support of the cold gas system,
- interfaces to star sensors providing the inertial attitude,
- interfaces to a GPS receiver that will provide on-board orbital position,
- a course Earth -sun sensor to provide attitude measurements with respect to Earth and sun,
- a three-axis Inertial Reference Unit used to measure angular rates,
- a three-axis magnetometer mounted in the S-band antenna boom and
- the AOCS flight software.