Gravity field recovery of the MOCAST+ quantum mission proposal

The main scope of the MOCAST+ project was the investigation of the performance of a gravity field mission based on a constellation of spacecrafts, each having both an atomic clock and a single-axis cold atom gradiometer onboard. The proposed payload is based on the integration of two different technologies: atomic interferometry gravity sensors and optical clocks, the atomic species being strontium atoms. This study was focused on investigating whether this combination can give the possibility of improving the estimation of both temporal and static gravity field models. 

Several different mission scenarios e.g., by considering different atomic species (Rb and Sr), inter-satellite distances, noise power spectral densities, and observation sampling rates were considered. Moreover, the same scenarios were applied to different satellite configurations such as the Bender configuration with either two or three satellites along each orbit. For these simulations, the so-called space-wise approach was exploited. This approach consists of estimating the long wavelengths of the field from the potential differences and then using this estimation to reduce the already filtered gravity gradients. Later, these residuals are processed by a local collocation gridding procedure with the aim of improving the solution especially (but not only) for the shorter wavelengths. In order to obtain spherical harmonic coefficients, the conversion from gridded values is performed by discretized quadrate formula, and finally, the error budget is computed by Monte Carlo simulations. The processing method was validated by comparing its results with those obtained by a classical time-wise approach working in the frequency domain. 

The results of the end-to-end simulations performed during the MOCAST+ study showed that the Bender configuration with either two or three satellites along each orbit provides significantly better monthly gravity field solutions, as compared to a “nominal” configuration with two or three satellites in a “GRACE-like” formation. In this way, it is in fact possible to obtain better performances than GRACE at low harmonic degrees. For the static gravity field retrieval, periods longer than two months were considered. In this case, the results showed that thanks to the lower noise level and stability of the cold atom gradiometer, there will be the opportunity to improve the GOCE performances at high harmonic degrees.