Inverse Synthetic Aperture Radar - Errors in ISAR

Errors in ISAR

Errors in the ISAR imaging process generally result in defocusing and geometry errors in the image. ISAR transform errors include:

• Unknown target or antenna motion: Unmodeled motion will cause the target image to defocus and be at an incorrect location. This error is controlled by suitable mechanical design or by the use of auto-focus techniques. This error can be measured by the analytic signal phase measurement method described earlier.
• Vertical nearfield errors: Unless 3D ISAR is performed, the vertical target extent at right angles to the horizontal synthetic aperture must fit within the vertical far field limit. Tall targets will defocus and move to incorrect positions. The 2D ISAR representation of a target region is a planar surface.
• Integrated sidelobe return: ISAR image quality is degraded by range and azimuth compression side lobes. The sidelobes are due to data truncation and can be reduced by the application of appropriate window functions. The sidelobes can cause significant image degradation. First, the peaks of the stronger sidelobes may cause a string of progressively weaker targets to appear on either side of a strong target. Second, the combined power of all sidelobes tends to fog or washout detail in low RCS areas. The integrated sidelobe level can under poor conditions reach a level 10 dB below the peak target return.
• Frequency and azimuth sampling errors: Incorrectly selected frequency or aspect deltas will result in aliased images, creating spurious targets. The SIM program described earlier specifically monitors for aliening errors effectively eliminating this error source.
• Antenna aberrations: Aberrations in the geometry result when the antenna phase center position is dependent upon the antenna aspect or RF frequency. This error source is normally controlled by using small, simple antennas over narrow frequency bands at long ranges. First order corrections to frequency dispersive antennas such as log periodic can be handled by phase correcting the received signal. Full correction of the aberrations can be accomplished by a direct integration of the ISAR transform using the aberrated geometry.
• Target dispersion: Dispersive targets have a non-minimum phase response, appearing to shift in position with RF frequency. Examples of dispersive targets include RF absorbers in which the absorption depth is a function of frequency and various antenna in which the phase center position is frequency dependent. CW ISAR imaging or in some cases preprocessing prior to a FMCW ISAR transform an eliminate dispersive defocusing of the target image.
• Multipath: Multiple reflections can result in ISAR imaging distortions such as the classic ghost image trails from jet aircraft tail pipes.

Errors in the 2D planar Inverse ISAR transform include:

• Image blocking modeling errors: The Inverse ISAR transform currently assumes that scatters are on a planar surface and cannot block other scatters.
• Image multipath modeling errors: The Inverse ISAR transform currently does not model the multipath environment. Note the current ISAR transforms also do not correctly process multipath.