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The GRACE Follow-On (GRACE-FO) mission features a unique setup with two independent inter-satellite ranging systems that operate simultaneously. In addition to the K-Band Ranging System (KBR), the GRACE-FO satellites are equipped with an experimental Laser Ranging Interferometer (LRI), offering significantly higher measurement precision than the KBR. The availability of these two concurrent ranging systems enables cross-calibration between the instruments and helps to partially distinguish ranging noise from other sources, such as noise from the accelerometer measurements.A stochastic model is presented here taking the cross-correlation of the two ranging observation types into account by using variance component estimation to determine the noise covariance function. With this method, the KBR, LRI, and accelerometer noise can be partially separated. This modeling approach results in formal errors of the spherical harmonic coefficients that align well with empirical estimates which is crucial for a combination with other data types and uncertainty propagation. Gravity field solutions from the combined least-squares adjustment using LRI and KBR will be compared to KBR-only solutions.Monthly gravity field solutions up degree and order 120 only benefit slightly from the incorporation of the LRI data, however when determining gravity field solutions to a higher degree (200) a significant improvement can be seen. This is especially beneficial for the upcoming combined gravity field model GOCO, which is planned to be published in 2025.
Öhlinger et al. (Mon,) studied this question.