resolution with a smaller physical antenna (Fitch, 1988; Atlas and Moore, 1987). However, there are several special demands of weather radars that make SAR applications challenging. For example, most of the SAR operation is done at large incidence angles away from nadir, and ground clutter contamination is a serious problem under these conditions; motion of hydrometeors also forces a reduction in performance. Major innovation is needed prior to utilizing SAR technology extensively for space-borne observation of precipitation.

Refractivity Measurements

Conventional radar data processing is tacitly directed at precipitation, and the other echoes are “clutter” that must be suppressed. Yet, radar information is considerably richer. For example, anomalous propagation ground echoes reveal changes in the vertical profiles of temperature and moisture in the lower troposphere. Specialized processing is required in order to obtain and use these ancillary sources for meteorological information.

An example of such specialized processing is the measurement of the near-surface refractive index of air using fixed ground targets (Fabry et al., 1997). As the refractive index of air in the propagation path changes, the time of travel of radar waves between the antenna and the fixed ground target varies slightly and causes a change in the measured ground target echo’s absolute phase. That variation is large enough to be accurately measured by carefully selecting and processing a large number of specific targets. If an independent set of temperature and pressure measurements are available, the refractivity can then be converted to spatial and temporal variations of moisture, or water vapor fields, in the surface boundary layer.

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