What can we expect from angular velocity sensing for future gravity missions?

The GRACE-like gravity satellites are designed to provide accurate measurements of the global time-varying gravity field, allowing to monitor the changes in the distribution of the Earth's mass caused, for example, by climate change. The gravity information is encoded in the line-of-sight vector, which is given by the position difference between the satellites. The key observable of GRACE and GRACE follow-on is the change in length of the LOS vector, measured by dedicated ranging instruments (K-band ranging system (KBR) or laser ranging interferometer (LRI)). However, the orientation of the LOS vector is measured with some precision using GNSS/GPS, which limits the accuracy of the gravity field maps. To overcome this problem, we propose the use of a dedicated angular velocity sensing (AVS) system that tracks the changes in the orientation of the line of sight with respect to the inertial frame. The angular velocity vector of the LOS has two independent components, yaw and pitch, which we introduce as additional observations in the gravity field recovery process. We firstly show how the AVS and range observables can be used to reconstruct the 3-dimensional differential acceleration vector. Then we find that the inversion results with AVS and regular ranging can mitigate the effect of AOD errors and significantly improve the accuracy of the gravity fields if more accurate AVS observations are available than currently provided by GNSS/GPS orbits. One potential way to achieve more accurate AVS observations is to use differential wavefront sensing (DWS) to accurately measure the orientation of the accelerometer test masses with respect to the platform, giving lower noise than the conventional star cameras, and combine these measurements with the DWS measurements from the inter-satellite laser interferometer to obtain the orientation of the LOS with respect to the inertial frame. With a DWS noise level of 0.1 nrad/√Hz, a single pair of GRACE satellites can achieve the same accuracy as a four-satellite Bender configuration. This study provides a promising alternative to the development of multiple satellite pairs to improve the accuracy of gravity missions.