Next Generation Gravity Mission Elements of the Mass Change and Geoscience International Constellation: From Orbit Selection to Instrument and Mission Design

Luca Massotti,Pierluigi Silvestrin,Christian Siemes,Günther March,Roger Haagmans

doi:10.3390/rs13193935

Abstract

ESA’s Next Generation Gravity Mission (NGGM) is a candidate Mission of Opportunity for ESA–NASA cooperation in the frame of the Mass Change and Geosciences International Constellation (MAGIC). The mission aims at enabling long-term monitoring of the temporal variations of Earth’s gravity field at relatively high temporal (down to 3 days) and increased spatial resolutions (up to 100 km) at longer time intervals. This implies also that time series of GRACE and GRACE-FO can be extended towards a climate series. Such variations carry information about mass change induced by the water cycle and the related mass exchange among atmosphere, oceans, cryosphere, land and solid Earth and will complete our picture of global and climate change. The main observable is the variation of the distance between two satellites measured by a ranging instrument. This is complemented by accelerometers that measure the nongravitational accelerations, which need to be reduced from ranging measurements to obtain the gravity signal. The preferred satellite constellation comprises one satellite pair in a near-polar and another in an inclined circular orbit. The paper focuses on the orbit selection methods for optimizing the spatial sampling for multiple temporal resolutions and then on the methodology for deriving the engineering requirements for the space segment, together with a discussion on the main mission parameters.

Highlights

We introduce two graphs much less challenging to find a constellation that offers a sufficiently dense spatia for that purpose: the first graph will guide the selection of the altitudes of the individual pling forsatellite the retrieval monthly field models, capturing the time pairs, andof, the e.g., second is used forgravity fine-tuning of the selectedand altitudes, such that the interleaving the ground of the two satellite pairs remains fixed for one of themodels able signal within theof month at tracks shorter intervals will improve monthly temporal resolutions
ESA’s Mass Change and Geosciences International Constellation (MAGIC)/Next Generation Gravity Mission (NGGM) is a mission under definition to improve our knowledge and monitoring of geophysical phenomena revealed by Earth’s gravity field, in the wake of the GOCE, GRACE and GRACE-FO missions
More than 10 years of extensive preparation activities have advanced the maturity of the system, attitude and drag control, proportional thrusters, laser optics and electronics to a sufficient technological readiness level to propose the mission for adoption in 2022 and launch in 2028

Summary

Introduction

The GOCE satellite [2] was orbiting from 2009 to 2013 at a mean altitude of 255 km (nominal mission) and 225 km (extended mission) in drag-free mode. The scientific payload was a gravity gradiometer, which consisted of six ultraprecise accelerometers, and a dualfrequency GPS receiver. The measurements of these instruments were used to derive gravity gradients and precise orbits, which were transformed into a gravity map of Earth with a mean global accuracy of 2 cm in terms of geoid heights and 0.5 mGal for gravity anomalies, at 100 km spatial resolution [3]. The low controlled altitude, the drag compensation control (so-called “drag-free”) and the accurate angular accelerations measured as a by-product of the gradiometer payload were all instrumental in GOCE’s outstanding result

Objectives

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Discussion

Conclusion