ITU-R RS.515-4 PDF

Introduction

Energy at microwave frequencies is emitted and absorbed by the surface of the Earth and by the atmosphere above the surface. The transmission properties of the absorbing atmosphere vary as a function of frequency, as shown in Figs. 1a and 1b. These Figures depict calculated one-way zenith (90° elevation angle) attenuation values for oxygen, water vapour and minor constituents. The calculations are for a path between the surface and a satellite. These calculations reveal frequency bands for which the atmosphere is effectively opaque and others for which the atmosphere is nearly transparent. For example, for nadir sounding, the regions or windows that are nearly transparent may be used to sense surface phenomena; the regions that are opaque are used to sense the atmosphere.

The surface brightness temperature, the atmospheric temperature at points along the path, and the absorption coefficients are unknown and to be determined from measurements of the antenna temperature, T_A. The surface brightness temperature and the absorption coefficients in turn, depend upon the physical properties of the surface or atmosphere that are to be sensed. A single observation at a single frequency cannot be used to estimate a single physical parameter. Observations must be made simultaneously at a number of frequencies and combined with models for the frequency dependence and physical parameter dependence of the surface brightness temperature and of the absorption coefficient, before solutions can be obtained.

Operating frequencies for passive microwave sensors are primarily determined by the phenomena to be measured. For certain applications, such as those requiring measurements of microwave emissions from atmospheric gases, the choice of frequencies is quite restricted and is determined by the spectral line frequencies of the gases. Other applications have broad frequency regions where the phenomena can be sensed.