Considerations in High-Sensitivity Microwave Radiometry

This paper discusses considerations in high-sensitivity microwave radiometry, especially as applied to systems having temperature thresholds significantly less than 1°K. Many considerations that have been ignored in previous analyses are shown to be prominent. The influences of the background radiation from space, atmospheric oxygen, atmospheric water vapor, and earth-bound radiators are shown to set a threshold level. Fluctuations in gain and temperature of the antenna, the waveguide system, the comparison source, the noise balancer, the receiver, and other amplifying components are shown to set another threshold. Impedance-modulation effects set still another threshold. The intrinsic internal receiver noise establishes an irreducible threshold. Continuous nonswitched types of radiometers usually are not suited for high-sensitivity applications because the present state of the art in gain stability is not adequate. The square-wave switched system is most likely to yield satisfactory results. In these systems the optimum performance is obtained when the magnitude of the signal within the system is minimized at approximately the level at which measurements will be made. This result establishes the requirement for noise balancing which may be either continuously adjusted or adjusted once for each measurement.