During mission operations, the GRACE science data will be made available to the scientific community in an EOS compatible format on a timely basis,shortly after calibration and validation. The Level-1 data products will include the line-of-sight range change and its derivatives, the non-gravitational accelerations, and other tracking and ancillary data. The Level-2 data products will include the coefficients of the geopotential field, the satellite position and velocity, and related geophysical products.
The primary science goal of the GRACE mission will be achieved by collecting and processing the following data types:
Accurate measurements of the inter-satellite range change between two co-planar, low altitude polar orbiting satellites, using a microwave tracking system - Since the two satellites are separated in orbit by only a few hundred kilometers along track, the errors due to media effects are minimized as compared to space-based or ground-based tracking. This also ensures the homogeneity of data quality over the mission lifetime of GRACE. By choosing to fly in nearly polar orbits, the coverage of the globe is also maximized.
High accuracy accelerometer measurements from each satellite - The presence of high accuracy accelerometers on board each satellite ensures that the differential range changes due to the non-gravitational accelerations can be accurately modeled and removed in the processing of the GRACE inter-satellite tracking data.
Precise carrier phase and pseudo-range measurements of GPS satellites - Each satellite carries a geodetic quality GPS receiver to ensure that the orbits for the satellites can be continuously and accurately determined and that the gravity field estimates can be correctly registered in a terrestrial reference frame.
Highly accurate attitude quaternions Each satellite is equipped with two Star Cameras capable of providing accurate attitude information. CCD technology and a high precision star catalog enable pointing accuracy at the milliradian level by the cold gas thruster (aided by magnetic torque rods) Attitude Control System.
A dedicated Laser Retro Reflector (LRR) on each satellite allows an external calibration of the onboard GPS system for orbit determination. Laser ranging from the ground can also be used in conjunction with the GPS to support precise orbit determination and gravity field recovery.