The objective of ESA’s Next Generation Gravity Mission is long-term monitoring of the temporal variations of Earth’s gravity at high time (3 days) and space (100 km) resolution. Such variations carry information about mass transport in the Earth system produced by the water cycle and the related mass exchange among atmosphere, oceans, cryosphere and land, and will complete our picture of Global Change with otherwise unavailable data. The basic datum is the distance variation between two satellites measured by a laser interferometer; as a necessary complement, accelerometers measure the non-gravitational accelerations, to be separated from the gravity signal in the data processing. The optimal satellite formation comprises two pairs of satellites, at 100 km mutual distance, on low (≈340 km) circular orbits with 89° and 70° inclination. The NGGM is a candidate Mission of Opportunity of ESA’s Earth Observation programme. Studies and technology development activities have advanced the maturity of the system concept and of the key subsystems (attitude and drag control, proportional thrusters, laser optics and electronics) for the mission to be proposed for adoption in 2022 and launch in the 2026-2028 time frame. The latest stand of the ESA studies is illustrated, concerning both the platform (featuring drag-free control, high-stability temperature control, drawing on the heritage of GOCE) and the laser interferometer instrument, for which two designs have been extensively studied, “Transponder” and “Retro-Reflector”, one of which will be selected for flight. A hybrid breadboard of the “off-axis” Retro-Reflector concept is being built and tested.
Measurement of the static and temporal variation of Earth’s gravity field yields important information on water storage, seasonal and sub-seasonal water cycles, their impact on water levels and delivers key data to Earth’s climate models. The satellite mission GOCE (ESA) and GRACE (US-GER) resulted in in a significant improvement on our understanding of the system Earth. On GRACE and GRACE Follow-On two satellites are following each other on the same orbit with approx. 200 km distance to each other. A microwave inter-satellite ranging system measures the variation of the intersatellite distance from which the gravity field is derived. In addition, on GRACE Follow-On, which has been launched May 22<sup>nd</sup> 2018, a laser interferometer is added as an experiment to demonstrate the capability of this system to improve the ranging accuracy by at least one order of magnitude. To significantly improve the gravity field measurement accuracy, ESA is investigating the concept of a ‘Next generation gravity mission’ (NGGM), consisting of two pairs of satellites and a laser interferometer as the sole inter-satellite ranging system. Based on the heritage of the development of the laser ranging interferometer for GRACE Follow-On and the former and ongoing studies for NGGM, several concepts for the laser metrology instrument (LMI) for NGGM, namely the on- and off-axis variants of the transponder and the retroreflector concept have been investigated in detail with respect to their application for an inter-satellite distance of approx. 100 km. This paper presents the results of the detailed tradeoff between different concepts, including laser link acquisition, ranging noise contributors, instrument performance analyses, technology readiness levels of the individual instrument units and an instrument reliability assessment.