Re-analysing the data processing of the K-band ranging system on GRACE Follow-On

Abstract The K-Band Ranging System (KBR) is the key payload for measuring inter-satellite distance variations with micrometer accuracy on the GRACE and GRACE Follow-On (GRACE-FO) missions. Alongside KBR, GRACE-FO includes a novel and more precise Laser Ranging Interferometer (LRI). The KBR observations not only support the gravity field recovery, but also play an irreplaceable role in estimating the LRI scale factor. Although recent analyses suggest that the KBR-LRI residuals at low frequencies are mainly limited by time-tag errors, this study investigates also smaller contributions in detail. We reprocess the KBR data from Level-1A to Level-1B in alternative ways, e.g. to study different approaches for gap-filling of missing phase observations and the impacts of clock offsets. We find that low-pass filtering the clock offset improves data quality, and additional smoothing at day boundaries reduces some jumps at these day-to-day transitions. In addition, carrier Frequency Variation Correction is applied in the phase-to-range conversion with amplitudes at a level close to KBR noise requirements. Other enhancements include the refinement of the Light Time Correction and the consideration of satellite center-of-mass motion in the Antenna Offset Correction. These updates have been incorporated into our new KBR1B v50 dataset, which is publicly available for the GRACE-FO period (2018–2024). While these changes have little effect on the Level-2 gravity field maps at the current level of precision, they do improve the Level-1B KBR data for GRACE missions and are partially applicable to LRI processing, which has potential applications for future more accurate missions.