| Summary: | The microwave radiometer system measures, within its bandwidth, the naturally emitted radiation – the brightness temperature – of substances within its antenna’s field of view. Thus a radiometer is really a sensitive and calibrated microwave receiver. The radiometer can be a basic total power radiometer or a more stable Dicke radiometer. Also correlation receivers play an important role in modern systems. The radiometer system might be single or dual polarized (horizontal and vertical) – or even be polarimetric, i.e. measure all 4 Stokes parameters, thus providing additional geophysical information at any given frequency. The radiometer system will very often be configured as an imaging system on a spacecraft for example. This normally implies scanning the antenna. Then there are certain relationships (or even conflicts) between achievable radiometric sensitivity/ ground resolution/antenna size, and the problem: scanning antenna/space- craft stability. In many cases good compromises have been reached, as evident recalling the many successful missions throughout the recent 30 years. But in some cases the situation calls for special solutions, like the push-broom system or the synthetic aperture radiometer technique, both yielding imaging capability without scanning. Typical applications of microwave radiometry concerning oceans are: sea salinity, sea surface temperature, wind speed and direction, sea ice detection and classification. However, in an attempt to measure properties of the sea from space, the intervening atmosphere will disturb the process, and corrections might be required. Also, at some frequencies and for some applications, the Faraday rotation in the Ionosphere must be taken into account.
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