Hydrology and Land

The earliest altimetry missions were dedicated to studying the open ocean and some ice measurements. However, every stretch of water (enclosed seas, lakes, rivers, flooding areas...) or even flat surfaces over lands can give valid data - as long as the satellite fly over them.

Altimetry has the advantage of taking being able to take global, homogeneous, repeated measurements (thus enabling systematic monitoring to be carried out over several years), unhindered by clouds, night or even vegetation. The measured surface heights are referenced to the same frame. However, it this technique is mainly optimized for the ocean (but although specific land re-tracking can be applied) and takes measurements only at the nadir (i.e. just under below the satellite), with a rather narrow footprint -- and averaging everything in that footprint. Over non-ocean surfaces (wet or dry), the accuracy of the altimetry measurements can be degraded to by several centimetres or tens of centimetres, mainly because of the heterogeneity of the reflecting surface (a mix of water and emerged land surfaces). Another important source of error lies in the signal's propagation of the signal through the atmosphere. The satellites repeat-orbits are rather long (10 to 35 days), which do not fit with suit real-time monitoring of river or lake level variations (e.g. flood alerts), but agree do work well with seasonal or interannual monitoring.