Assessment of a near-polar pair mission for detecting the Earth's temporal gravity field

Abstract

SUMMARYTo ensure the global observability of next generation gravimetric mission (NGGM), different agencies have to repeatedly launch satellites to about 89.0° orbit inclination. However, due to the poor isotropy of observation system, only minor improvement in terms of temporal gravity field estimation can be obtained via these repeatedly launched polar pair missions. To ensure the global observability as well as the isotropy of observation system, a near-polar pair mission rather than a polar mission is likely an optimal selection, especially considering that the Gravity Recovery and Climate Experiment (GRACE) Follow-On mission has been already in operation. In this study, for the upcoming NGGMs (for instance, the Chinese NGGM), we design a closed-loop simulation to assess the performance of a near-polar mission at a near-circular orbit with about 500 km altitude for detecting the Earth's temporal gravity field, and the main conclusions are summarized as follows. (1) Based on the statistic results, 85.0° is selected as the optimal orbit inclination for the near-polar mission, which provides 37 per cent noise reduction in terms of cumulative geoid height error in spectral domain, but also 31 per cent noise reduction in terms of mean oceanic root-mean-square (RMS) error in spatial domain when compared to the 89.0° polar mission (89-PM). (2) To figure out the reason of these noise reductions, we also compare the contribution of single error components [including instrument error, atmospheric and oceanic (AO) aliasing error, ocean tide error]. It indicates that the outperformance of the 85.0° near-polar mission (85-NPM) is mainly derived from the relatively smaller non-tidal atmospheric and oceanic mass variation error. (3) Although there are inevitable 5.0° polar gaps in the 85-NPM, the analysis result (including the comparison via in-orbit observations and simulated retrieved solutions) still demonstrates a comparable performance of the 85-NPM in tracking mass variations over the Antarctic, and even an outperformance with 12 per cent noise reduction over the Greenland when compared to the 89-PM. The result confirms the feasibility of implementing a near-polar mission as a stand-alone mission or a complementary observation system for the repeatedly launched polar missions, which offers an alternative option of launching the Chinese NGGM satellites to an 85.0° inclination orbit instead of 89.0°.