Inverse Synthetic aperture radar
Inverse Synthetic Aperture Radar (ISAR) is a technique to identify the reflectivity centers of a target with high spatial resolution.
It is possible to achieve the same large synthetic aperture without moving the transmitter/receiver. If the target rotates by a small amount, it has the same effect as if the transmitter/receiver were to travel a distance equal to the arc length at the range R.
Introduction
Inverse Synthetic Aperture Radar (ISAR) is a well-established technique to identify the reflectivity centers of the target with high spatial resolution. A fine two-dimensional reflectivity map of the target is generated by using a large bandwidth transmitted signal in order to achieve high range resolution; and by coherently processing the echoes received from different aspect angles of the target, to achieve fine cross-range resolution. The availability of a two-dimensional high resolution image permits the radar operator/researcher to better identify of the target and it can also be useful for the purpose of target classification.
RCS Imaging
Images produced by ISAR of the target region can be a useful tool in locating scattering regions on the target. ISAR images are 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, scatters 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 by a Fourier transform. This operation is equivalent to 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 scatter 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, usually 45 degrees. This occurs when the synthetic aperture to the target range is less than required by the 2D2/lambda limit where D 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.
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 sidelobes. 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 AZIMUITH 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 an 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.