Inverse synthetic-aperture radar
Inverse synthetic aperture radar (ISAR) is a radar technique using Radar imaging to generate a two-dimensional high resolution image of a target. It is analogous to conventional SAR, except that ISAR technology utilizes the movement of the target rather than the emitter to create the synthetic aperture. ISAR radars have a significant role aboard maritime patrol aircraft to provide them with radar image of sufficient quality to allow it to be used for target recognition purposes. In situations where other radars display only a single unidentifiable bright moving pixel, the ISAR image is often adequate to discriminate between various missiles, military aircraft, and civilian aircraft.
Images of the target region produced by ISAR can be a useful tool in locating scattering regions on the target. ISAR images are often produced by rotating the target and processing the resultant Doppler histories of the scattering centers. If the target rotates in azimuth at a constant rate through a 'small' angle, scatterers will approach or recede from the radar at a rate depending only on the cross range position- the distance normal to the radar line of sight with the origin at the target axis of rotation. The rotation will result in the generation of cross range dependent Doppler frequencies which can be sorted spatially by a Fourier transform. This operation is equivalent to (but the inverse of) the generation of a large synthetic aperture phased array antenna formed by the coherent summation of the receiver outputs for varying target / antenna geometries. For small angles, an ISAR image is the 2-dimensional Fourier transform of the received signal as a function of frequency and target aspect angle.
If the target is rotated through 'large' angles, the Doppler frequency history of a scatterer becomes non linear, following a sine-wave trajectory. This Doppler history can not be processed directly by a Fourier transform because of the smeared Doppler frequency history resulting in the loss of cross range resolution. The maximum rotation angle which can be processed by an unmodified Fourier transform is determined by the constraint that the aperture phase error across the synthesized aperture should vary by less than a specified (arbitrary) amount, for example 45 degrees. This occurs when the synthetic aperture to the target range is less than required by the limit where is the required lateral extent of the target. At this point the synthetic aperture is within the target nearfield region and requires focusing. The focusing is accomplished by applying a phase correction to the synthetic aperture.
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