The main objective of the microwave radiometer (MWR) is to measure the integrated atmospheric water vapour column and cloud liquid water content, which are used as correction terms for the radar altimeter signal. In addition, MWR measurement data are useful for determining surface emissivity and soil moisture over land, for surface energy budget investigations to support atmospheric studies, and for ice characterisation.
The Envisat MWR has evolved from the instruments previously flown on ERS-1 and ERS-2. It is a dual-channel nadir-pointing radiometer, operating at frequencies of 23.8 GHz and 36.5 GHz. For Envisat the design of the MWR had to be modified in some areas compared to its ERS predecessors, in order to comply with the different platform and mission requirements.
The MWR measures water vapour content in the atmosphere so that we can determine how it impacts radar signal propagation. Its measurements also can be used directly for studying precipitable water and cloud liquid content along the satellite track.
The MWR is a passive receiver that collects radiation reflected by the oceans at frequencies of 23.8 GHz and 36.5 GHz.
Radiation measured by the radiometer depends on surface winds, ocean temperature, salinity, foam, absorption by water vapour and clouds, and various other factors. To determine atmospheric water vapour content accurately, we need to eliminate sea surface and cloud contributions from the signal received by the radiometer. This is why the MWR uses different frequencies, each of which is more sensitive than the others to one of these contributions. The frequencies 23.8 GHz and 36.5 GHz are the result of a trade-off between instrument (reflector) size required to cover a horizontal area on the Earth’s surface comparable to the RA-2 beam, and the maximum sensitivity to water vapour change in the atmosphere. These frequencies are used to measure the strength of the weak water-vapour emission-line at 22 GHz. In order to eliminate microwave radiation emitted by the Earth’s surface, differential measurements at two frequencies must be made. The optimal choice is to use one frequency at the peak of the line and one at the lowest point.
With one feed horn for each frequency, the MWR points via an offset reflector at an angle close to nadir. The instrument is configured in such a way that the 23.8 GHz channel is pointing in the forward direction, the 36.5 GHz channel in the backward direction, with a footprint of about 20 km diameter for each beam.