Data Collection Eclipsing Effects in the Inverse Polar Format Algorithm (IPFA) for Turntable Spotlight Inverse Synthetic Aperture Radar (ISAR) Imaging Systems

This paper will present the findings associated with data eclipsing effects in the inverse polar format algorithm (IPFA) for turntable spotlight inverse synthetic aperture radar (ISAR) imaging systems. Typical turntable ISAR systems illuminate a scene of interest with an electromagnetic source in order to obtain samples of the reflectivity function over a given set of aspect angles. These samples form a bandlimited portion of the spatial frequency (k_x,k_y) response of the scene, from which a focused image may be generated by using a simple 2D inverse fast Fourier transform (IFFT). However, the collected samples lie on a polar raster in (k_x,k_y) space, which cause defocusing effects in the final image unless interpolation to a rectangular grid of points is first accomplished. The IPFA implements a frequency selective waveform in order to sample the reflectivity function of the scene of interest so that the (k_x,k_y) samples lie on a rectangular grid of points. This is accomplished by "designing" a rectangular grid of (k_x,k_y) sample points based upon the desired image resolution, pixel spacing, and scene size. Once this is accomplished, a corresponding set of frequency/aspect angle (f,thetas) pairs can be generated for data collection. However, the (f,thetas) pairs must first be sorted in order of increasing aspect angle in order to get the correct frequency transmission schedule. This sorting results in a staggered transmission schedule that can result in pulse overlapping (eclipsing) if certain conditions are not met regarding the pulse length and turntable rotation rate. This paper will detail the requirements under which these conditions must be met and will provide simulation results showing the effect of pulse overlap in image formation.