Since the altimeter only measures the range from the satellite to the sea surface, called R, we have to calculate the sea-surface height with respect to a terrestrial reference (see diagram).

To do this, we must first define an arbitrary reference surface. Since the sea depth is not known accurately everywhere, we use a regular, imaginary surface that is a raw approximation of the shape of the Earth, which is a sphere flattened at the poles. This surface is called the reference ellipsoid, which allows us to calibrate data precisely and uniformly.

The satellite’s altitude with respect to the reference ellipsoid, called S, is calculated with an accuracy of 3 cm using the satellite’s orbital parameters and precise positioning instruments.

The sea-surface height is thus the difference between the satellite’s altitude relative to the reference ellipsoid and the altimeter range, in other words S-R.

This height corresponds to the undisturbed sea level, called the geoid, and the ocean circulation or dynamic topography, which is the consequence of the Earth’s rotation, winds and tides.

^{The geoid is a model of the figure of the Earth that mirrors the shape of the sea surface in the absence of the effects of waves, winds, currents and other perturbations. It is a very irregular, undulating surface reflecting global variations in gravity. The dynamic topography (DT), of the order of 1 metre, is the difference between this unperturbed surface and the actual sea surface.
Imagined by Jean-Pierre Penot (CNES) and Bernard Nicolas, illustrated by Bernard Nicolas}