Aperture Synthesis by Object Rotation in Coherent Imaging

Doppler processing is an established method for obtaining spatial resolution of objects which exhibit radial motion relative to an observing sensor. When applied to rotating objects, Doppler processing yields resolution in cross-range along an axis normal to both object rotation and line-of-sight axes. In this case, the cross-range resolution performance is dictated by the angular interval over which the data are observed and processed. This paper treats a tomographic extension of Doppler processing in the case of a rotating object (or a rotating planar array of point scatterers). It uses a stationary, continuous-wave (CW) irradiating source and sensor, co-located in the plane being imaged. The processing of data from a rotating object is shown to be equivalent to synthesizing an aperture which is a segment of a circle, however, processing over large angular rotations requires a focusing correction. Angular rotations up to and including 2π radians are considered. For this latter condition the system simulates an annular aperture which encloses the object. With this technique, high resolution may be obtained in all directions in the object plane without the use of a wideband signal, and equal point-objects separated by a quarter wavelength may be resolved. The imaging process provides high resolution for sparse arrays of objects of similar strength which are small in comparison to a wavelength. The imaging capability in the case of dense object arrays or large objects is limited and has a restricted dynamic range.