The earliest altimetry missions were dedicated to studying the open ocean and some ice measurements. However, as scientists always like to see ‘what happens if…’, they began looking at the levels of lakes, then rivers as measured by altimeters. Some experiments have also been conducted over solid land, to observe and analyse the signal sent back to the altimeter.
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, this technique is mainly optimised 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 by several centimetres or tens of centimetres, mainly because of the heterogeneity of the reflecting surface (a mix of water and emerged landsland surfaces). Another important source of error lies in the signal’s propagation through the atmosphere. The satellites’ repeat-orbits are rather long (10 to 35 days), which do not suit real-time monitoring of river or lake level variations (e.g. flood alerts), but agree do work well with seasonal or interannual monitoring.
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