Page - 25 - 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

Science Reference Frame The origin of the science reference frame (SRF) is defined to coincide with the satellite center of mass. The axes of the SRF are parallel to the measurement axes of the accelerometer. The axes are enumerated in such a way that the x-axis of the SRF is the roll axis of the satellite. It is positive in the in-flight direction for the trailing satellite, and negative in the in-flight direction for the leading satellite. During flight, the z-axis (yaw rotation) points roughly nadir. The y-axis (pitch rotation) completes a right-handed coordinate system. The GRACE science data products are released in the SRF. Star Camera Frame One star camera frame (SCF) is defined for each star camera head on the satellite. The x-axes are parallel to the x-axis of the SRF. The z-axis of each SCF is the optical axis of the respective star camera head. The y-axis completes a right-handed triad. K-Frame The origin of the K-Frame (KF) coincides with the origin of the SRF. In difference to the SRF, the x-axis of the KF points towards the calibrated antenna phase centre (APC) of the K-band horn, as depicted in fig. 4.2. As such the KF can change with in-orbit K-band calibration manoeuvres. The z-axis of the KF is orthogonal to the x-axis of the KF and the y-axis of the SRF. The y-axis completes the right-handed triad. Line Of Sight Frame The origin of the line of sight frame (LOSF) coincides with the satellites’ SRF origin. The x-axis points towards the other satellite’s center of mass. The y-axis is orthogonal to the position vector of the satellite in the celestial reference frame and the x-axis, the z-axis completes a right-handed triad. Celestial Reference Frame The celestial reference frame (CRF) is an earth-centred, space-fixed reference frame that serves as a quasi-inertial frame for GRACE pro- cessing. The chosen realisation of the celestial reference frame is the international celestial reference frame (ICRF) as defined in the IERS Conventions (2010) (Petit and Luzum, 2010). Terrestrial Reference Frame The terrestrial reference frame (TRF) is an earth-centred, earth-fixed reference frame. The chosen realisation of the terrestrial reference frame is the international terrestrial reference frame (ITRF) as defined in the IERS Conventions (2010) (Petit and Luzum, 2010). zSRF xSRF COM APC xKF GRACE Figure 4.2: Exaggerated depiction of the misalignment of the calibrated KBR antenna phase centre with the SRF x-axis (side view). 4.2 Reference Frames 25