Scale Factor Determination for the GRACE-FO LRI and Thermal Coupling in KBR-LRI Residuals

The GRACE-FO missions hosts two ranging instruments, namely the prime K-Band Ranging (KBR) and the Laser Ranging Interferometer (LRI) as a technology demonstrator.

The LRI scale factor and a time-tag bias are currently estimated on a daily basis in post-processing through cross-calibrating KBR and LRI range, assuming that the KBR range is accurate enough to estimate these parameters. However, this might not hold, and errors of the KBR might get imprinted onto the LRI measurement. Prominent errors in the inter-satellite ranging data are so-called tone errors, which have a sinusoidal form and appear mainly at 1/rev and 2/rev frequencies. These errors have a significant influence on the estimated scale and time shift.

In this talk, we present a model for the LRI scale factor when GF-1 satellite is in the reference role, i.e., a model for the resonance frequency of the optical reference cavity of GF-1. The model was derived based on four years of flight data. Moreover, we show that the seasonal or beta-angle-related variations in the daily scale and time-shift estimates can be described by a thermal coupling, which produces mainly ranging tone errors proportional to the temperature of specific thermistors. We considered two possible coupling mechanisms, temperature-induced changes to the phase and to the frequency. The dominant coupling term is proportional to the temperature at the zenith-pointing solar array and produces range variations of up to +/- 8 µm at 1/rev frequency in the KBR-LRI residuals. These results can be used to improve the LRI1B data product and have relevance for the development of future LRI-like instruments for upcoming missions.