Poseidon-2 (Jason-1 altimeter) first waveforms. (Credits CNES)
A radar altimeter measures surface characteristics with a high degree of accuracy. This implies high precision in the radar measurements and therefore requires performance far greater than a conventional radar, in particular for classic radar range measurements. The radar altimeter emits a pulse towards the Earth’s surface. The time which elapses from the transmission of a pulse to the reception of its echo reflected off the Earth’s surface is proportional to the satellite’s altitude. Some theoretical details of the principle of radar applied to altimetry will help better understand the different behaviours and characteristics of the pulse in function of irregularities on the surface encountered. The magnitude and shape of the echoes (or waveforms) also contain information about the characteristics of the surface which caused the reflection. The best results are obtained over the ocean, which is spatially homogeneous and has a surface which conforms with known statistics. Surfaces which are not homogeneous, which contain discontinuities or significant slopes, such as some land surfaces, make accurate interpretation more difficult. Even in the best case (the ocean), the pulse should last no longer than 70 picoseconds to achieve an accuracy of a few centimetres. Technically, this means that the emission power should be greater than 200 kW and that the radar will have to switch every few nanoseconds. These problems are solved by the full deramp technique, making it possible to use only 5 W for emission. The range resolution of the altimeter is about half a metre (3.125 ns) but the range measurement performance over the ocean is about one order of magnitude greater than this. This is achieved by fitting the shape of the sampled echo waveform to a model function which represents the form of the echo.