Satellite-to-satellite laser tracking mission for gravity field measurement

Abstract

A mission for monitoring the variations of Earth’s gravity field at high resolution (up to harmonic degree 200) over a long time period (>5 years) has been defined within the “Laser Doppler Interferometry Mission for Determination of the Earth’s Gravity Field” study for the European Space Agency. The mission exploits the use of a heterodyne laser interferometer for the high-resolution measurement of the displacement between two satellites flying at low altitude (325 km). Starting from the geophysical phenomena to be investigated, a detailed derivation of the mission requirements (orbit, satellites and measurement instruments: laser interferometer, accelerometer) was performed using analytical models and numerical simulations. A precision of about 1 nm rms is needed in the intersatellite distance measurement, while the non gravitational accelerations must be measured with a precision of about $10^{-10} m/s^2$ rms. The overall metrology system designed for this mission includes, beside the heterodyne laser interferometer for the measurement of the inter-satellite distance, devices for the measurement of the lateral displacement and of the relative orientation of the two satellites (also contributing to the mission performance). Laboratory prototypes of the novel lateral displacement and angle metrology have been submitted to first proof-of-concept tests, achieving promising results.