The source coding theory of information theory is applied to the digital processing of synthetic aperture radar signals. A lower bound is established on the data rate that must exist at any point in the processing flow, in particular at the conversion from analog-to-digital (A/D) signals, to achieve a given output map signal-to-noise ratio. The usual linear uniform-step A/D conversion of in-phase and quadrature coherent video signals is found to provide a given output map signal-to-noise ratio with a data rate within about 20 percent of the derived lower bound.

Straightforward implementation of a maximum likelihood decoder implies a complexity that grows algebraically with the inverse of error probability. Forney has suggested an approach, concatenation, for which error probability decreases exponentially with increasing complexity. This paper presents the results of an evaluation of a particular concatenation system, structurally similar to the hybrid system of Falconer, employing a Reed-Solomon outer code and an inner convolutional code. The inner decoder is a Viterbi decoder of constraint length less than the corresponding encoding constraint length (nonmaximum likelihood). The outer decoder assumes one of three possible forms, all employing the likelihood information developed by the inner decoder to assist in outer decoding. Error corrections and erasure fill-ins achieved by the outer decoder are fed back to the inner decoder. Performance is evaluated through computer simulation. The three outer decoders are found to provide approximately the same performance, all yielding low error probabilities at rates somewhat above Rcompof sequential decoding and at signal energy to noise density ratios per information bit around 1.7 dB.

Intermediate frequency (IF) and video limiting are evaluated and compared for a two-channel coherent digital signal processor, where the video limiting is that of the analog-to-digital (A/D) converters, occurring separately in each channel. The input is a narrow-band random signal. Video limiting is shown to produce considerably higher distortion sidebands outside the input signal band. Within the input signal band, video-limiting distortion is about 1 dB worse. The results are useful for both the radar moving target indicator (MTI) and mapping applications.