4.8 Microwave Radiometer

4.8 Microwave Radiometer

As indicated in section 3.4, a part of the microwave is also radiated by thermal radiation from the objects on the earth. Microwave radiometers or passive type microwave sensors are used to measure the thermal radiation of the ground surface and/or atmospheric condition.

Brightness temperature measured by a microwave radiometer is expressed by Reyleigh-Jean's law (see 1.7), which is the resultant energy of thermal radiation from the ground surface and the atmospheric media. Multi-channel radiometers with multi- polarization are used to avoid the influences of unnecessary factors to measure the specific physical parameter.

Figure 4.8.1 shows the sensitivity of physical parameters in oceanography with respect to frequency and the optimum channels as arrow symbols.

Figure 4.8.2 shows two typical microwave scanning radiometers; the conical scanning type and the cross track scanning type. The former is used for the microwave channel which is influenced by the ground surface, while the latter is used for the channel which can be neglected by the influence of the ground surface.

The most simple radiometer is the total power radiometer, as shown in Figure 4.8.3. This system has a mixer to enable it to mix high frequency of a local oscillator in order to amplify the high signal after transforming to a low frequency. However the influence of system gain variation cannot be neglected in this system.

The Dicke radiometer can reduce the influence of system gain variation by introducing a switch generator which allows it to receive the antenna signal and noise source of constant temperature, alternatively of which antenna signal can be detected later on, synchronously with the switch generator.

The zero-balance Dicke radiometer can reduce the influence of system gain variation and increase the sensitivity further by adding a noise generator to the Dicke radiometer in order to increase the sensitivity about two times higher than total power radiometer.