Gravimetry

Satellite gravimetry measures Earth's gravity field and its temporal variations by detecting the effect of mass redistribution within the hydrosphere, cryosphere, oceans, and solid Earth on the orbits and inter-satellite ranges of precisely tracked spacecraft. It is the only space technique that directly measures mass change globally from orbit, independent of electromagnetic surface properties.

Two instrument approaches have been flown. In the GRACE approach, two co-orbiting satellites approximately 220 km apart measure their inter-satellite distance using a K/Ka-band microwave ranging system (24 GHz and 32 GHz dual-frequency, the HAIRS instrument), achieving range-rate accuracy of 0.1 micrometres per second. Monthly spherical harmonic solutions to degree and order 96 (approximately 300 km spatial resolution) yield global equivalent water height maps after atmospheric and ocean-tide correction, encoding terrestrial water storage, ice mass, and ocean bottom pressure. GRACE-FO (launched May 2018) continues this record with an added laser ranging interferometer demonstration. In the GOCE approach, a tri-axial electrostatic gradiometer with 50 cm arm length directly measures gravity gradient tensor components, resolving static gravity field structures at approximately 100 km half-wavelength from its 255 km orbit.

Representative missions include GRACE (NASA/DLR, April 2002 - October 2017), GRACE-FO (NASA/DLR, 2018 onward), GOCE (ESA, electrostatic gravitational gradiometer, 2009-2013), and CHAMP (DLR, high-low satellite-to-satellite tracking and accelerometry, 2000-2010).

Headline applications include ice sheet mass balance - Greenland lost approximately 4,000 Gt (11 mm sea level equivalent) and Antarctica approximately 2,500 Gt (7 mm SLE) between 2003 and 2020 - terrestrial water storage change and groundwater depletion monitoring, global ocean mass and barystatic sea level rise (approximately 1.88-2.15 mm per year, 2003-2019), earthquake-induced crustal mass redistribution, and geoid and crustal density structure determination for geodesy and ocean circulation modelling.

Principal limitations are coarse spatial resolution (approximately 300 km), monthly temporal cadence that averages short-duration events, signal leakage between land and ocean at coastlines, and post-processing requirements including glacial isostatic adjustment correction, destriping, and Gaussian smoothing. A one-year data gap occurred between GRACE decommissioning (November 2017) and GRACE-FO activation (May 2018).