VLSI System for SAR Processing

Abstract

Synthetic Aperture Radar (SAR) is a radar system that processes the return signal to achieve the effect of having a larger aperture than the one provided by the physical dimensions of its antenna. The processing consists of a weighted summation of regularly spaced samples from the signal history, hence of logic for the arithmetic and storage for the signal history. LSI and VLSI technology offer some beautiful ways to implement this computation in chips in which the storage and logic functions are commingled. The SAR problem discussed in this paper is based on actual requirements set forth by NASA for a spaceborne application. The requirements for high resolution and high quality necessitate a data sampling rate of 7.5 MHz. For each data value 1,025 4-bit complex multiply+add operations are needed, which is equivalent to 7.7 GHz complex multiply+add operation rate. Since this rate is much too high for general purpose systems, a special-purpose device was sought. This paper discusses two architectures based on parallel operation of 1,025 identical cells, each of which is capable of performing arithmetic, storage, and several control operations. The operation rate in each device is only 7.5 MHz, which is quite manageable, especially with the help of a substantial degree of pipelining. A computational-mathematical analysis is used as a primary tool for evaluating the design and some of its tradeoffs. Two different approaches are discussed and compared; both are based on having 1,025 identical cells working in parallel, but differ in their dual approaches to the flow of data. The mathematics require a relative motion of the data with respect to some (relatively) constant sets of coefficients. In one approach the coefficients are held stationary in space, and the data flows past them; in the other, the data is held and the coefficients flow past. The paper discusses the architecture, both approaches, some of the control issues, and most important, some aspects of the methodology of the design.