Ground mapping by synthetic-aperture radar (SAR) is sensitive to the motion of the airborne radar platform. Current methods for compensation use motion compensation (i.e. correction of the phases of all radar return signals by the phase of the map reference point (MRP) echoes) estimated from inertial sensor outputs, and a variable pulse repetition frequency (PRF). These common approaches are shown to be equivalent to a special case of bifocal mapping: a mapping in which at least two points in space are in sharp focus. A method is presented which generalizes this process to a trifocal process where the use of an additional degree of freedom in the signal design permits specifying three independent focal points in the region to be mapped. The plane defined by these three foci, which need not be a horizontal plane, contains a region of improved map sharpness. The trifocal method is implemented by using inertial sensor outputs, in real time, to modify parameters of the radar signals generated by the radar exciter receiver. The subsequent processing is conventional. Results are presented and compared with performance predictions based on existing analyses.
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