Satellite Altimetry: Measuring Sea Level to the Centimetre
🌐 Measuring the Earth from space
Question
How do we know the height of the oceans all over the world, to the centimetre?
Radar satellites (TOPEX/Poseidon, Jason, CryoSat, Sentinel-3) fire pulses at the ocean surface and time the return, from a precisely known orbit. Sea level is defined relative to the geoid - the shape the ocean surface would take under gravity and rotation alone, a gravitationally determined surface that is only defined for a sphere. Altimetry requires orbital mechanics, which only work around a sphere.
The math
Sea-surface height is determined to within a few centimetres (about one inch). Jason orbits at 1,336 km with a period of ~112 minutes (~12.5 orbits per day). Global mean sea level has risen ~7.5 cm over the altimetry period (since 1993), at ~3 mm per year and accelerating.
Why it matters
A planet-scale measurement: the entire ocean surface, mapped to the centimetre, matching the predictions of a rotating sphere. The data also caught the sea level rising - which is how we know it is happening, and how fast.