Measuring Ice Loss at the Wrong Resolution

GRACE (2002-2017) and GRACE-FO (2018-present) revolutionized our understanding of ice sheet mass loss, groundwater depletion, and sea level change by measuring Earth's time-variable gravity field from orbit. However, their spatial resolution is fundamentally limited to ~300-500 km — the signal from two closely spaced ice streams draining into the same ocean sector is averaged into a single mass change estimate. For Greenland, where outlet glaciers 5-20 km wide are accelerating at different rates, GRACE/GRACE-FO cannot distinguish which glaciers are losing ice fastest or why. For groundwater, the ~300 km resolution means an entire multi-state aquifer system appears as one number, when water managers need basin- or county-level information. The Earth Science decadal survey identified improved mass change resolution as a highest-priority designated observable, but achieving 10-50 km spatial resolution in gravity from orbit requires either new measurement technology (laser ranging between multiple satellite pairs in different orbital planes) or fundamentally different mission architectures (satellite gradiometry at lower altitude), neither of which has been demonstrated.

Greenland and Antarctica are losing ice at accelerating rates — combined loss exceeded 400 billion tonnes/year in the 2010s, contributing ~1.1 mm/year to global sea level rise. But adaptation planning for coastal communities requires knowing not just the total ice loss but where and how fast: an outlet glacier that accelerates can raise local sea levels through gravitational effects and ocean circulation changes differently than a distributed surface melt signal. Groundwater depletion (the Ogallala Aquifer, Central Valley, North China Plain) is a slow-onset water security crisis affecting billions of people, but GRACE-FO resolution cannot distinguish sustainable from unsustainable withdrawal at the management-relevant scale. The gap between the 300 km resolution we have and the 10-50 km resolution we need is the difference between knowing "this region is losing water" and knowing "this specific basin is at risk of depletion within a decade."

GRACE/GRACE-FO use microwave (K-band) or laser (LRI on GRACE-FO) ranging between twin co-orbiting satellites to measure gravity field variations as the inter-satellite distance changes. The spatial resolution is limited by satellite altitude (~500 km) and the single-pair configuration, which samples the gravity field along one ground track per orbital pass. Post-processing techniques (mascons, Bayesian inversions) can sharpen the effective resolution to ~200 km but introduce model-dependent assumptions. Proposed next-generation concepts include: double-pair missions (two satellite pairs in different orbital planes, providing better spatial sampling), which are technically feasible but double the mission cost; quantum gravimeters (cold atom interferometers) in orbit, which offer higher intrinsic sensitivity but have not been demonstrated in space; and lower-orbit missions using drag compensation to fly at ~300 km altitude (vs. ~500 km), which improves resolution but shortens mission lifetime due to atmospheric drag. ESA's MAGIC (Mass-change And Geoscience International Constellation) concept study examined double-pair architectures but estimated costs exceeding $2 billion.

Demonstration of laser interferometric ranging at sub-nanometer precision between satellites in different orbital planes — extending the GRACE-FO laser ranging instrument to a constellation architecture. Space-qualified cold atom interferometer gravimeters with sensitivity sufficient to detect gravity variations at sub-monthly temporal resolution from orbit. Lower-cost satellite platforms (SmallSat-class or hosted payloads) that could make multi-pair or constellation architectures economically feasible. Improved data fusion methods that combine satellite gravity with complementary data (altimetry, InSAR surface deformation, seismology) to achieve higher effective resolution than gravity alone.

A student team could develop and test a data fusion algorithm that combines GRACE-FO gravity data with ICESat-2 altimetry and Sentinel-1 InSAR velocity maps to estimate ice sheet mass change at higher spatial resolution than any single data source, validating against GPS uplift measurements. Alternatively, a team could simulate a multi-pair gravity mission configuration and quantify the spatial resolution improvement over single-pair GRACE-FO as a function of orbital parameters and measurement noise. Relevant disciplines: geodesy, geophysics, signal processing, aerospace engineering, data science.