GRACE-FO (Gravity Recovery and Climate Experiment Follow-On)

A twin-satellite formation that measures Earth’s time-varying gravity field by precisely tracking the distance between two spacecraft (originally ~10 µm precision; LRI achieves ~1 nm). Detects changes in total mass at ~300 km resolution — including water, ice, and even atmospheric mass redistribution.

What it sees

• Not imagery — gravity field variations. The signal is interpreted as “equivalent water thickness” change at 300 km spatial scale.
• Twin satellites in formation; KBR microwave ranging measures the distance change between them to micrometer precision; LRI demonstrated ~1 nm precision.
• Monthly + 10-daily gravity solutions available.

Why it matters

• The only direct measurement of total water storage anomaly — soil moisture, groundwater, surface water, ice, all added together. Critical for groundwater monitoring where in-situ gauges don’t exist (California Central Valley, Indo-Gangetic Plain, Saudi Arabia).
• Ice-sheet mass balance (Greenland, Antarctica, glaciers) — fully complementary to ICESat-2 (lidar = volume change; GRACE-FO = mass change). Gives independent constraint on ice contribution to sea-level rise.
• Ocean bottom pressure for circulation + climate studies.
• The continuity from the original GRACE (2002-2017) + GRACE-FO (2018+) = 20+ years of mass-change observations.

Where to get the data

• earthaccess Python: short names with GRACEFO_* prefix
• PO.DAAC: podaac.jpl.nasa.gov/GRACE-FO
• Three official processing centers (CSR Austin, GFZ Potsdam, JPL): all provide L2 spherical harmonics; choose by community convention or use the JPL mascon solution for direct mass-anomaly maps.
• JPL Mascon visualization: grace.jpl.nasa.gov (web app)

What it enables

• Groundwater depletion monitoring (California, India, MENA region drought indices)
• Ice-sheet + glacier mass balance (cross-validation with ICESat-2)
• Drought severity indices (combined with soil moisture from SMAP for shallow + deep partitioning)
• Sea-level closure budget (steric + mass-based contributions)
• Earth’s rotation + axis-of-rotation studies
• Earthquake co-seismic gravity changes (rare events but detectable)

Gotchas

• 300 km native spatial resolution is the real limit. Downscaling techniques exist (mass-conservation + ancillary land-surface model) but always add assumption-bias. Don’t claim point-scale measurements from GRACE.
• Monthly cadence misses sub-monthly events. Flooding, sudden snowfall, fast aquifer pumping cycles are blurred or missed.
• Mascons vs spherical harmonics: mascons are gridded mass concentrations (~3°×3° equivalent), spherical harmonics are basis-function coefficients. They convey the same information but mascons are easier to use directly; harmonics give more flexibility for advanced filtering.
• GRACE → GRACE-FO has a one-year gap (June 2017 – June 2018). Don’t concatenate naïvely.
• Twin-satellite formation can fail. GRACE-FO’s accelerometer on GF-2 had issues 2018-19; degraded period. Always check QA flags.

Related missions

• Original GRACE (2002-2017): predecessor; 15 years of overlap-stitched record.
• GRACE-C (planned launch ~2028): direct successor; LRI primary instrument.
• ICESat-2 (s10 →): lidar ice-sheet altimetry — complementary to GRACE-FO’s mass approach.
• SWOT (s11 →): surface water elevation — complementary to GRACE-FO’s total-storage signal.
• SMAP (s13 →): shallow soil moisture — combines with GRACE-FO total to derive deep-storage component.

Related datasets