Potential and limits of small satellite networks for temporal gravity field retrieval in the frame of the CubeGrav Project

In the frame of the CubeGrav project, funded by the German Research Foundation, Cube-satellite networks for geodetic Earth observation are investigated on the example of the monitoring of Earth’s gravity field. Satellite gravity missions are an important element of Earth observation from space, because geodynamic processes are frequently related to mass variations and mass transport in the Earth system. As changes in gravity are directly related to mass variability, satellite missions observing the Earth’s time-varying gravity field are a unique tool for observing mass redistribution among the Earth’s system components, including global changes in the water cycle, the cryosphere, and the oceans. The basis for next generation gravity missions (NGGMs) is based on the success of the single satellite missions CHAMP and GOCE as well as the dual-satellite missions GRACE and GRACE-FO launched so far, which are all conventional satellites.   
In particular, feasibility as well as economic efficiency play a significant role for future missions, with a focus on increasing spatio-temporal resolution while reducing error effects. The latter include the aliasing of the time-varying gravity fields due to the under-sampling of the geophysical signals and the uncertainties in geophysical background models. The most promising concept for a future gravity field mission from the studies investigated is a dual-pair mission consisting of a polar satellite pair and an inclined (approx. 70°) satellite pair. Since the costs for a realization of the Bender constellation are very high, this contribution presents results of the CubeGrav project and focuses on alternative concepts in the form of different constellations and formations of small satellites. The latter includes both satellite pairs and chains consisting of trailing satellites. The aim is to provide a cost-effective alternative to the previous gravity field satellites while simultaneously increasing the spatiotemporal resolution and minimizing the above-mentioned error effects.

In numerical closed-loop simulations, the impact of different satellite formations and constellations will be investigated for the retrieval of monthly gravity fields. The configurations differ in the orbital setup including the number of orbital planes and key orbit parameters like altitude and inclination. The ground track coverage of the selected orbits will be analysed since an improved spatial sampling with specific sub-cycles is beneficial for estimating short-temporal gravity fields which will be co-parametrized in the overall solution approach. Due to the large number of observations, it is possible to retrieve sub-daily gravity fields down to quarter-day resolution, which exceeds the capabilities of the existing gravity mission like GRACE or GRACE-FO by far. These (sub-)daily gravity field solutions can also improve the overall monthly gravity product, which will be proven for several satellite constellations and formations. All in all, the opportunities and limits of multiple satellites pairs and chains of trailing satellites for achieving the highest possible spatial and temporal resolution shall be analysed in detail.