Principles of radar imaging

Synthetic aperture radar (SAR) and inverse synthetic aperture radar (ISAR) are referred to in literature as imaging radars because of their ability to reconstruct electro-magnetic (e.m.) images of natural and man-made objects by coherently processing the echoes coming from the targets at different aspect angles. The comparison between radar and photographic images is quite common in literature, since both systems perform a transformation that maps a 3D object to a 2D space. However, differences exist which concern the mapping transformation and the image feature. While the latter is quite obvious since different imaging systems use different illuminators thus producing images representing different characteristics of the target, the former is of more difficult interpretation. Differently from electro-optic systems, where the image projection plane (IPP), which is the 2D plane which a 3D target is projected onto, coincides with the focal plane of the sensor, for an imaging radar, the IPP depends on the relative motion between the radar and the target. Therefore, the imaging radar IPP can be a 2D plane arbitrarily oriented in the 3D space, which depends on the complexity of the target motions with respect to the radar.