ISARLAB - Inverse Synthetic Aperture Radar Simulation - Defence ...

DSTO-GD-02102. Inverse Synthetic Aperture Radar (ISAR)2.1 Basic ISAR TheorySince the prospective users of this software will have had some exposure to ISARimaging we shall not embark on a detailed description of the theory. Instead we willgive a brief summary then concentrate on the characteristics particular to our radarsystems and therefore the data we are able to process. In any case, there are manyexcellent references to this form of radar imaging, most of which can be found in or viathe texts by Wehner [6] and Mensa [7].The basic principle of ISAR imaging is to coherently collect the wideband echo signalsthat are produced as the object of interest rotates to present a change in viewing angleto the radar. Processing the individual wideband echo signals provides information asto the relative range of individual scatterers on the target. These range profiles 1 can bethought of as projections of the target’s scattering centres onto the line-of-sight rangevector from the radar and if the aspect angle of the target is known, some dimensionscan be extracted. If the target is assumed to be rotating at a constant rate relative to theradar then Doppler spectral analysis of the time-history of range profiles can provideadditional information about the target’s scattering centres. This cross-rangeresolution is achieved because there is a velocity differential across the target causedby its rotation that manifests itself in the received signal as a Doppler frequencydifferential. By coherently measuring these signals over time, then performing aspectral analysis, the Doppler frequency content and therefore the relative position ofscatterers can be determined. Since the location of target scatterers to within one rangeresolution cell has already been obtained in generating range profiles, this Doppler orcross-range processing makes it possible to separate scatterers that reside in the samerange cell. The eventual result of these two distinct processing steps is a range versuscross-range map of the target’s scattering centres, otherwise known as an ISAR image.To assist in visualising the form of the ISAR image generated in a given scenario, it isuseful to note that the projection plane is simply defined as being parallel to the radarline-of-sight and perpendicular to the effective rotation axis of the target. The termeffective is used here because the target does not actually have to rotate but onlyappear to do so from the radar’s point of view. An example of this is the situation ofimaging an aircraft in level flight as it traverses the radar viewing area while beingtracked by the radar antenna(s). Unfortunately, imaging in this scenario has othercomplications that will be addressed in the section on radial motion compensation(Section 2.2).The resolution achievable in range, also referred to as slant-range resolution, is directlyrelated to the unambiguously sampled bandwidth of the transmitted radar signal, inthe case of coded waveforms (stepped-frequency, chirp, etc.), or the time delay1Range profiles in this context are also often referred to as high range resolution (HRR) profiles.2