A new technique for inversed synthetic aperture radar (ISAR) ranging, which resembles the principle of the Vernier measuring system, is presented. In this technique, the transmitted ISAR pulse comprises a train of chirp subpulses with uniformly stepped up center frequencies. The return ISAR echo is first processed, using hardware, to determine a coarse estimate of the target range. Further refinements of the range estimate are achieved through software processing, consisting of two stages of discrete Fourier transform operation. The ranging accuracy can be increased without the need for increased bandwidth, but at the expense of a slight increase in computational complexity. Numerical evaluation shows that a noiseless system is capable of achieving high-ranging accuracy, of the order of millimeters, even in the presence of dispersion and target motion. From computer simulations, the proposed system is also found to be robust against additive system noise and frequency jitter under practical conditions.
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