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The Medium Earth Orbit (MEO) region, home of the operational Global Navigation Satellite Systems (GNSS) GPS and Glonass, is becoming more and more exploited with the advent of the European Galileo and the Chinese Beidou constellations, both in their build-up phase. The sensitive applications of the navigation satellites and the absence of any natural sink mechanism, such as the atmospheric drag, call for a careful debris prevention policy able to preserve the MEO environment, avoiding in the future the problems now already faced by the LEO and the Geostationary Orbit (GEO) environments.

In the context of Earth Observation missions the development of end-to-end simulators (E2ES) has proven to be a useful tool to assess the mission performance and support the consolidation of the technical requirements and conceptual design, as well as to allow end-users assessing the fulfillment of requirements by the mission. For Space Science missions the development of performance simulators is not as extended, and it is usually done externally to the Agency, directly by the scientific community, and mostly for independent instrument chains

All spacecraft in Earth orbit are exposed to the risk of impact with Micrometeoroids and Orbital Debris (M/OD). Because of the large collision velocities, Hypervelocity Impacts (HVI) with M/OD may cause significant damage to various subsystems and components up to mission failure.

The overall objective of this study is to assess constellations of two pairs of satellites for the retrieval of time-variable gravity field information for best quality scientific monitoring of mass distribution and transport in the Earth system. Before embarking on this study the geodetic community and space agencies have come to realize that weaknesses in current single-pair gravimetric satellite missions such as GRACE or GRACE-FO (to be launched in 2017) would significantly be alleviated by having a second GRACE-type pair in space simultaneously.