Optimal orbits for double pair low-low SST formations regarding temporal aliasing

The global monitoring of the temporal variations of the Earth's gravity field is very important. With current low-low Satellite-to-Satellite tracking (SST) satellite missions this can be done with a spatial resolution of a few hundred kilometers and a temporal resolution of several days. The largest restriction of current missions is temporal aliasing from both tidal and non-tidal mass variations. For future missions with enhanced sensor accuracies it will be even more important to reduce the effects of temporal aliasing. The goal of this work is to show the possibility of reducing temporal aliasing by an optimal choice of the orbit. Closed-loop simulations of single and double low-low SST pairs on circular orbits with different repeat ratios are performed based on full normal equations and including realistic error models. The observations are computed along the orbits from ocean tide models and from models of hydrological, oceanographic, ice and atmospheric mass variations. A well-known possibility to reduce temporal aliasing is by adding an inclined low-low SST pair to a polar one. The results of this study show a strong dependency of temporal aliasing on the repeat ratio and therefore on the orbit altitude. This dependency is linked to strong correlations of temporal aliasing effects with analytical derived spherical-harmonic (SH) resonance orders. The study supports to understand the properties of temporal aliasing for gravity satellite missions. The SH analysis of tidal and non-tidal signals on low-low SST single and double pairs is analyzed in detail focusing on dependencies on the repeat ratio and the altitude. Derived from the multitude of numerical simulations, this study targets on a systematic description of the impact of aliasing effects depending on the optimal orbit choice. For polar single low Earth orbiters optimal orbits regarding temporal aliasing are found around 301, 365, 421 and 487 km altitude. Furthermore it is analyzed if the combination of different repeat ratios within one double low-low SST pair can help reducing temporal aliasing. Planning a future gravity mission this study shows an effective method to reduce temporal aliasing by the choice of the orbit.