Page - 24 - in Contributions to GRACE Gravity Field Recovery - Improvements in Dynamic Orbit Integration, Stochastic Modelling of the Antenna Offset Correction, and Co-Estimation of Satellite Orientations

Star camera heads Accelerometer K-Band horn GPS antenna To o the r sa tell ite zSRF xSRF ySRF Figure 4.1: Interior view of GRACE with most important instruments. Adapted from NASA (2002). K/Ka Band Ranging Instrument Mounted on the front panel of the satellite, the K/Ka band ranging instrument (KBR) horn allows for continuous instantaneous observations of the separation of the satellites up to some unknown integer multiple-of-wavelength ambiguity, based on the interferometric principle. Each satellite transmits two carrier signals at specific frequencies in the K and Ka-Band, which are independently tracked by the other satellite. This is referred to as the dual one-way ranging observation, a realization of the low-low satellite-to- satellite tracking (ll-SST) principle. These observations are then combined and corrected to yield observations of the satellites’ separation. Global Positioning System Receiver Realizing the principle of high-low satellite-to- satellite tracking (hl-SST), GRACE is equipped with two primary global position- ing system (GPS) antennae, one on the top panel for positioning of the satellite, and one on the rear panel for radio occultation observations. Laser Retro-reflector Each satellite is equipped with a laser retro-reflector (LRR) for tracking through ground-to-space satellite laser ranging (SLR) observations. 4.2 Reference Frames During acquisition and processing of GRACE data, several different reference frames are employed. The following paragraphs will give a brief description of the frames that are of particular relevance in this document, a complete and authoritative list can be found in (Bettadpur, 2012). Chapter4 The GRACE Satellites24