AZIMUTHAL AMBIGUITIES IN SYNTHETIC APERTURE SONAR AND SYNTHETIC APERTURE RADAR IMAGERY

Azimuthal image ambiguities, with few exceptions, are rarely studied at length in the synthetic aperture radar (SAR) literature, and entirely overlooked in the synthetic aperture sonar (SAS) literature. The sonar literature claims that the image ambiguities, or image aliases, are removed by nulls in the beam pattern of the sonar transceiver. This paper shows that azimuthal ambiguities cannot be dismissed out-of-hand, that they are not eliminated by the nulls of a sonar (or radar) beam pattern, and that they have a definite influence on image quality. In synthetic aperture systems that are strongly limited in spatial sampling, particularly in oceanborne SAS and spaceborne SAR, azimuthal ambiguities will corrupt the images unless special measures are taken. These azimuthal aliases may be reduced by emphasizing the centermost portion of the available synthetic aperture length, and de-emphasizing the endmost portions. This minimizes the effects from synthetic array elements that most strongly contribute to aliases.