Laser metrology concept consolidation for NGGM

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 22nd 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.