In this paper, these complications are avoided by discussing the SAR fundamentals in terms of the end-to-end convergence angle of the radiation transmitted to and received from the target, since this term determines the SAR focusing action, while rotational ISAR is discussed in terms of its phase excursion. This approach appreciates the similarity in objective and the almost total dissimilarity in implementation of antenna synthesis in radio astronomy and in radar.
Subject to an observation time of at least four complete revolutions, any filter matched to the resulting periodic RF phase excursion of [+ or -]4[Pi][r.sub.i]/[Lambda] would be orthogonal to all similar filters, matched to the centers of other such annuli (whose phase excursions differ by integral multiples of 2[Pi]).
When observing a limited angular increment of a continuous rotation, the same analysis applies, but processing to match to the given pattern of phase excursion will then take a different form.
The location-dependent phase excursions, which control the resolution of rotational ISAR, are determined by each target-point's movement about the center of rotation, and are virtually independent of any change of viewing angle from the sensor.
The angular position [Psi.sub.i] is then determined from the relation of the along-range to the across-range phase excursions, unfortunately with the accuracy of the across-range.