How it Works

The satellites’ “attitude,” or orientation and orbit control are controlled by a system consisting of sensors, actuators and software. The Attitude and Orbit Control System provides three-axis stabilized Earth-pointing attitude control during all mission modes and measures spacecraft rates and orbital position. It features numerous improvements from the GRACE design. The system consists of a GPS receiver, Star Tracker Assembly, high-performance gyro package, coarse Earth and Sun sensor, fluxgate magnetometer, inertial measurement unit, magnetic torquers and a cold gas propulsion system.

The GPS receivers are used as references to determine the precise location of the two satellites in orbit. The receivers measure changes in the distance of the GRACE-FO satellites to the constellation of GPS satellites circling Earth. Each spacecraft has three antennas. One antenna is used to collect navigation data, one collects the mission’s atmospheric occultation data, and the other is used for backup navigation. It also provides digital signal processing functions for the Microwave Instrument. The GPS receivers were manufactured by JPL.

The Star Tracker Assembly enables fine attitude and orbit control of the satellites and precise transformation of science data into inertial references. It precisely determines each satellite’s orientation by tracking their relative position in reference to the stars. It consists of three redundant star tracker heads. The GRACE mission used two star tracker camera heads. The GRACE-FO design increases attitude data availability and accuracy about all spacecraft axes.

The gyro provides attitude rates during spacecraft emergency modes.

The coarse Earth/Sun sensor provides coarse attitude determination during all mission phases.

The magnetometer provides coarse attitude based on the satellite’s position as determined by onboard GPS position and a model of Earth’s magnetic field.