The impact of antenna performance on microwave sounding from space

The attraction of passive microwave instruments is that they can provide information day or night and in nearly all weathers. It is a major disadvantage of other types of sensor that they are often unable to provide data during the conditions where information is most required because of the presence of heavy cloud. Several passive microwave instruments have been flown on an experimental basis. These can be divided into two categories depending on whether the instrument observes the ground using window frequencies or sounds the atmosphere using frequencies in the electromagnetic spectrum which are absorbed by particular atmospheric constituents. The latter category of instruments provide valuable information on temperature and humidity profiles through the atmosphere and will provide an operational service in the near future. The author discusses the particular antenna characteristics that are required of an instrument of this type