Global Gravity Field Recovery by Use of STEP Observations

Abstract

Over the past 20 years scientists in the fields of oceanography, solid-earth physics and geodesy joined efforts to establish a low-flying gravity field mission in order to obtain an important objective: the resolution of the spatial variations of the gravity field to 100 km half wavelenght. Nevertheless, though many concepts were studied (SLALOM, GRAVSAT, GRIM, ARISTOTELES,...), none of the proposed missions has been launched so far and the realisation of an improved gravity field remains an unfulfilled task. STEP is a mission in the field of fundamental physics: its parameters and payload were not determined for the purpose of a geodesy experiment. But despite this fact, STEP would fly the first gravity gradiometer in space and would produce an excellent high resolution gravity field model. Regarding the payload, STEP would accomodate on board a set of accelerometers in order to perform several experiments. As it is well known, differencing the readings of two accelerometers over the distance of their baseline, a gravity gradient component is derived. In the STEP situation, the G-experiment accelerometers could be used to obtain the T _yy “out-of-plane” component of the gravity tensor with a high accuracy (the intrinsic noise of the gradiometer being better than ${10^{ - 4}}E\,/\,\sqrt {Hz} \,$ for frequencies above $2 \cdot {10^{ - 4}}\,Hz\,$). The orbital altitude of STEP would be about 350 km: the low height is fundamental in a gradiometry mission, as the gravity signal attenuates exponentially. Other mission parameters include an inclination of 96°.85 (leaving unsurveyed areas at the poles) and a mission duration of 6 months.