Updated Status of the Planar Electrostatic Gradiometer GREMLIT for Airborne Geodesy

Abstract:

In spite of the important improvement provided by the recent space missions GRACE and GOCE to the accuracy of Earth’s gravity field models, many geodetic and geophysical issues require higher resolution models. To address spatial resolution in the range between 80 km and 10 km, the airborne gravity gradiometry is a convenient way to complete these global models at the short wavelengths in particular in land/sea transition areas where the gravity field is poorly covered by current measurements. Taking advantage of technologies, developed by ONERA for the accelerometers used in these space gravity missions, the new GREMLIT airborne gravity gradiometer is more particularly developed to determine the horizontal components of the gravity gradient tensor in the instrumental frame. GREMLIT is composed of a double deck of a compact planar assembly of 4 electrostatic accelerometers. The eight plane proof-masses are arranged in a cubic configuration and the whole instrument is mounted on a dedicated stabilized platform which is controlled by the common mode outputs of the instrument itself to achieve a sufficient rejection ratio of the perturbations induced by the airborne environment in the horizontal directions. In addition to be well suited to sustain the proof-mass levitation in the Earth’s gravity field, the planar configuration presents an intrinsic linearity of the control loops which minimizes the aliasing effects. Concerning the present development, in addition to the one axis prototype manufactured in order to verify the hardware performance of the concept, detailed numerical simulations were conducted over some realistic coastal test areas. Taking into account the data sheet performance of the associated commercial angular and data rate sensors and assuming actual airplane acceleration measurements, they lead to exhibit accuracy below 1E along the Txx and Tyy horizontal components. The characteristic of the controlled platform being one of the key point of the development, a specific study was conducted to conceive, specify and verify its performance through a complete simulation. Finally, the interest of a combined integration of this horizontal high resolution electrostatic instrument with a very stable and absolute vertical cold atoms interferometer will also be mentioned.