# Gravity Field Recovery Using High-Precision, High-Low Inter-Satellite Links
## 5 Conclusions  
In this study, we investigated the gravity field retrieval performance of the novel and innovative MOBILE high-low satellite tracking concept and compared it with a low-low GRACE Follow-On-like configuration qualitatively and quantitatively. Based on full numerical simulations, gravity field parameters were estimated in terms of SH coefficients by solving a least-squares system by inverting full normal equations over a time span of 1 month. The most important error sources affecting the gravity field retrieval performance, key instruments on-board the satellites, as well as time varying mass flux signals, were included in order to assess their impact on gravity field retrieval for both mission concepts.  
The results regarding the instrumental impact on the gravity field solution show that the performance of the MOBILE configuration was mainly limited by the assumed micrometer accuracy of the laser interferometer, especially in the short wavelength spectrum, while the performance in the lower wavelengths of the gravity field benefited from the multi-directional observation geometry and optimized 3D accelerometer. In contrast, the gravity field retrieval of the low-low pair constellation was limited mainly by the accelerometer, which predominated the nanometer accuracy of the assumed laser interferometer. The multi-directional observations of MOBILE mentioned above included a strong radial component and led to an almost uniform (isotropic) error spectrum, while the low-low tracking concept showed the typical stripes caused by the north-south observation direction. However, the high accuracy of the low-low satellite pair's inter-satellite link led to an improved gravity field performance from SH degree 40 and higher compared to MOBILE, which performed better in the long wavelength spectrum where the largest amplitudes of time varying gravity field signals occurred.  
The benefit of MOBILE's multi-directional observation geometry arose when including tidal and non-tidal mass variation signals into the simulation process. The results revealed significantly reduced temporal aliasing errors in the recovered gravity field signal compared to the low-low tracking concept over the whole spectrum. In the case of the separate treatment of the HIS signal, the resulting gravity field error performance of MOBILE improved even by about 60%, and the application of an extended processing method to reduce temporal aliasing errors by co-estimation of daily gravity field parameters, led to a further reduction of retrieval errors of about 40%. Furthermore, the results show that the quality of recovered MOBILE gravity field solutions could make a treatment of such solutions using a posteriori filtering techniques obsolete.  
The gravity field solutions retrieved using MOBILE can contribute to an improved understanding of different components of the Earth system, such as the estimation of continental water storage and freshwater fluxes, the quantification of large-scale flood and drought events and their monitoring and forecasting, or understanding the mass balance of ice sheets and larger glacier systems, just to name a few. The application of the extended processing method implied the co-parameterized gravity field parameters (which, in our case, are daily gravity fields) as a side product. These daily solutions with low spatial resolution could aid in improving atmospheric models, and possibly be beneficial to the oceanography community as well, as many of these short-term signals have large spatial scales.