METRIC: A Mission Concept for a Spacecraft Aimed at Upper Atmosphere Mapping, Gravitational Physics, and Geodesy

We present a mission concept named Measurement of EnvironmenTal and Relativistic In-orbit pre-Cessions (METRIC) for a compact spacecraft to be placed in a low Earth orbit. Dedicated instrumentation to provide data useful for atmospheric science, fundamental physics, and geodesy will be placed on the platform to achieve the main scientific objectives of the mission: mapping the atmospheric density by in-situ acceleration measurements and by spacecraft tracking at altitudes of great interest for satellites deorbiting; fundamental physics tests and verification of the equation of motion for a well-characterized test mass; and providing an additional space-based node, to improve the tie among different space geodesy techniques. These three objectives need precise orbit determination and a well-calibrated space-based platform. GNSS, SLR instrumentation, and a VLBI beacon are planned to be installed on the METRIC spacecraft and metrologically referenced to one another. The potential contribution of METRIC to geodesy lies in the challenge of performing the co-location of the geodetic techniques in space to improve the International Terrestrial Reference Frame accuracy and stability. GNSS and SLR are space geodetic techniques that rely on satellite observations from the ground, but VLBI observations to satellites are not standard and are not performed routinely yet. In this work we introduce the METRIC concept as a whole and present some of the main requirements to observe the METRIC satellite by VLBI, regarding e.g. satellite visibility using legacy VLBI and VGOS networks and the characteristics of the signal that can be transmitted by the VLBI beacon.