DIGITAL CORRELATORS FOR SYNTHETIC-APERTURE INTERFEROMETRIC RADIOMETRY

A numerical simulator is developed to assess various design implicatio ns of a digital correlator used by a synthetic aperture interferometri c radiometer (SAIR). The simulator permits control of the type of digi tization, the digitization thresholds with respect to noise power, and the degree of correlation between the two antenna signals which are b eing cross correlated. Digitization schemes are considered which use 2 , 3, 4, and 8 levels. Estimates are made of the increase in inherent r adiometer noise (Delta T) due to the digitization. The increase in Del ta T is found to depend strongly on the degree of correlation, with hi gher correlations suffering less increase. In most cases, 3 level digi tization is recommended based on this sensitivity consideration. Two l evels perform significantly noisier and four levels only slightly clea ner. Several case studies are also considered regarding the need to co ntrol the signal level relative to the digitizer thresholds. Automatic gain control circuitry prior to digitization is found not to be neces sary, provided the thresholds are preset within a fairly broad region of minimum sensitivity to variations in signal power, and provided the system noise temperature of the radiometer is monitored with reasonab le accuracy. It is also found that, under conditions of very high corr elation between the two signals, digital correlators hare better SNR p erformance than analog. Reasons for this behavior are discussed.