A radiometer is a passive receiver that collects emitted radiation (generally in the microwave, infrared and near infrared regions).
Radiometers on board altimetric satellites detect the level of water vapour and liquid in the atmosphere, allowing appropriate corrections to be applied to the altimetric measurement. Their antenna collects the radiation reflected by the ocean.
The radiation measured by the radiometer depends on the surface wind, the ocean temperature and salinity, whether there is spray, the absorption by water vapour and clouds in the atmosphere etc. If the water vapour content in the atmosphere is to be determined accurately, the contributions from the surface and the clouds must be eliminated from the signal received by the radiometer. This is why several frequencies are used, each one being more sensitive than the others to one of these contributions. By combining the measurements made at each of these frequencies, the signal due to the water vapour can be extracted.