Which Orbit for the GRASP Mission ?

Abstract:

The Geodetic Reference Antenna in Space (GRASP) mission was first proposed in 2011 by JPL in response to the NASA NNH11ZDA012O call for Earth Venture-2 mission. Recently, considering the recommendation of the Prospective Scientific Seminar, CNES expresses its interest and the possibility to participate in a next new JPL proposal.

GRASP is a spacecraft system designed to build an enduring and stable Terrestrial Reference Frame (TRF) for accurately measuring and understanding changes in sea level, ice sheets and other elements of the dynamic Earth system. These objectives set the 1 mm accuracy and 0.1 mm/year stability (GGOS, Meeting the Requirements of a Global Society on a Changing Planet in 2020, Plag and Pearlman, 2009) as the goals for the TRF; goals which are an order of magnitude more accurate than the current performance of the TRF. For that, GRASP will carry very precise sensor systems for all the key geodetic techniques used to define and monitor the TRF: a Global Navigation Satellite Systems (GNSS) receiver, a Satellite Laser Ranging (SLR) retroreflector, a Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS) receiver, and a novel Very Large Baseline Interferometry (VLBI) beacon.

To reach mission goals, the first step is to determine the optimal orbit of this satellite. In this study, we present an original approach for determining such orbits, using evolutionary algorithms. The method allows us to optimize orbits according to specific criteria such as the visibility of the satellite from ground stations and satellites; especially the visibility of GRASP by at least two VLBI stations, with the longest possible baseline, and by GNSS satellites.