Radio waves transmitted and received by an altimeter do not travel in a vacuum. Signals passing through Earth’s atmosphere may be subjected to path delays, thus introducing errors into measurements. For example, large amounts of electrons at altitudes near 400 km, dry air and water vapour all generate errors ranging from a few centimetres to more than 2 metres.

To achieve the extreme level of accuracy required, we have to identify the source of these perturbations and then calculate the necessary corrections. Special instruments on board the satellite are designed to measure physical parameters for this purpose. After corrections, the satellite-to-ocean range can be estimated with an accuracy of 2 centimetres.

Since the aim is to estimate sea level precisely with respect to a terrestrial reference, measurements must also be independent of the satellite. For this reason, we need to know the satellite’s exact orbital position.

This is the role of the onboard orbit determination systems, supported by a network of ground location beacons and orbital trajectory models. Used in combination with GPS positioning instruments, these systems enable us to calculate the satellite’s position very precisely relative to Earth, in terms of its altitude, longitude, latitude and orientation.