Next generation gravity missions: Near-real time gravity field retrieval strategy

The goal of next generation gravity missions (NGGM) is to improve the monitoring of mass transport in the Earth system by an increased space-time sampling capability as well as higher accuracies of a new generation of instrumentation. They should be able to fulfil the scientific and societal needs of providing high-resolution short-time gravity field solutions for geophysical applications like for for example service applications such as flood and drought monitoring and forecast or applications in water management. To facilitate this need a near-real time (NRT) processing scheme based on a coparametrization of low-resolution daily and longer-term gravity field solution, combined with a sliding window averaging, was set up. In contrast to other strategies that are usually based on Kalman filtering, the proposed NRT concept is independent of any prior information about the temporal gravity field, and does not require any regularization. The enhanced spatial-temporal resolution opens the possibility to self-dealias high-frequency atmospheric and oceanic signals, and additionally provides gravity field solutions on short timescales. In order to quantify the capabilities of the proposed NRT approach, a numerical closed-loop simulation of a low-low satellite-to-satellite tracking (ll-sst) mission for a two-pair Bender-type constellation with realistic noise assumptions was performed. While for the daily parametrization a spherical harmonics degree and order of 15 turns out to be a favourable choice, by applying the sliding window NRT approach stable daily gravity field estimates up to degree/order 50 with latencies of down to 1 d could be achieved.