Target shifts due to modeling assumptions in inverse synthetic aperture radar

Approaching targets can be imaged, with high resolution, using inverse synthetic aperture radar (ISAR). Image formation via the FFT, however, produces an image where the targets are shifted from their true locations. We begin by reviewing the Fourier model of ISAR image formation, which utilizes a Taylor series approximation to the range function. We then show that this Taylor series approximation is equivalent to the straight-line projection assumption in the tomographic model for ISAR. This approximation, plus the presumption that the acquired Fourier data lie on a Cartesian rather than polar grid, are shown to cause the target shifts. We derive formulas for accurately estimating the target shifts. These formulas can provide the basis for effectively compensating the ISAR image for scenarios with small, widely-spaced targets