Passive instrument that detects and measures radiant energy, usually in the microwave, infrared and near-infrared wavebands. Radiometers flown on altimetry satellites help us to measure water vapor and liquid water content in the atmosphere. These measurements are used to apply corrections to altimetry data. The radiometer’s antenna collects radiation reflected by the ocean. The amount measured depends on surface winds, ocean temperature, salinity, foam, absorption by water vapor and clouds, and other factors. To determine atmospheric water vapor content accurately, the surface and cloud contributions must be filtered out from the signal received by the radiometer. For this reason, different signal frequencies are used to increase sensitivity to each of these parameters. By combining measurements acquired at both frequencies, we can extract the water vapor signal.