Sea Surface Height Anomaly Calculator

A radar altimeter measures the range from a satellite to the sea surface, which combined with precise orbit determination gives sea surface height above a reference ellipsoid. That raw height is dominated by the geoid, the equipotential surface of Earth's gravity field, which departs from the ellipsoid by roughly minus 106 meters near southern India to plus 85 meters near New Guinea. Subtracting the geoid N leaves dynamic ocean topography, the ocean-circulation signal, which spans only about 2 meters globally. Because geoid models carry their own error at short wavelengths, operational analysis instead subtracts a mean sea surface computed from many years of altimetry, giving the sea surface height anomaly. SSHA isolates the time-varying part and is typically 5 to 30 cm, reaching 50 cm or more across strong eddies and western boundary currents. Anomalies arise from two distinct causes. Steric change reflects thermal expansion and salinity effects on the water column and dominates seasonal and interannual variability, while mass change reflects water added or removed by ice melt and land storage. The surface geostrophic velocity follows from the anomaly slope as (g / f) times the cross-track gradient of SSHA, so a 10 cm rise over 100 km at mid-latitude implies roughly 0.1 m/s. Corrections are substantial and must be applied before interpretation: ocean and solid earth tides up to several meters, the inverse barometer response of about minus 1 cm per hPa, wet and dry tropospheric delay, and ionospheric delay. Residual accuracy for modern missions is near 3 to 4 cm along-track.

Satellite altimetry began as a proof of concept aboard Skylab in 1973, followed by GEOS-3 in 1975 and then Seasat in 1978, which despite failing after 105 days demonstrated that centimeter-class ocean topography from orbit was achievable. Geosat, launched by the US Navy in 1985, ran a classified geodetic mission before its data were later released and transformed marine gravity and seafloor mapping. The decisive advance was TOPEX/Poseidon, a joint NASA and CNES mission launched in August 1992 into a 66 degree inclination orbit with a 9.9156 day repeat cycle chosen specifically to alias the principal tidal constituents away from important ocean signals. TOPEX/Poseidon reduced orbit error to a few centimeters and established the continuous global mean sea level record that now underpins climate assessment. It was succeeded by Jason-1 in 2001, OSTM/Jason-2 in 2008, Jason-3 in 2016, and Sentinel-6 Michael Freilich in 2020, all flying the same reference ground track to preserve the calibration chain, alongside the European ERS-1, ERS-2, Envisat, and Sentinel-3 series. The geoid problem was greatly reduced by the dedicated gravity missions GRACE, launched in 2002, and ESA's GOCE, which flew from 2009 to 2013. SWOT, launched in December 2022, introduced wide-swath interferometric altimetry, resolving features down to about 15 km rather than the roughly 100 km limit of nadir altimeters.