Generalized Polar Format Algorithm for ISAR imaging with complex target motion

Inverse synthetic aperture RADAR (ISAR) imaging produces range-Doppler images by exploiting the motion of the target related to the RADAR. Two cases arise in ISAR imaging: the target motion is known (cooperative target) or not (non-cooperative target); subsequently, the range-Doppler imaging algorithm(s) may not be the same depending on whether we are in one case or the other. In introduction, a brief state-of-the-art presents some algorithms allowing imaging in the general case, i.e. when the target motion is unknown. The two stages involved in ISAR focusing process on this situation are then introduced: range migration compensation and Doppler adjustment. Secondly, synthesis algorithms which exploit target trajectory, are introduced. These algorithms can perform the two steps mentioned above. This is especially the case of polar format algorithm (PFA) which is particularly well suited to ISAR imaging. After a brief overview of PFA along with an example of application, we then highlight a noticeable loss of quality degradation as soon as the target is affected by an oscillatory motion. Indeed, raw data spectrum is naturally expressed in the slant plane which doesn’t contain information concerning target oscillations. The Generalized PFA presented afterwards consists of calculating an optimal focusing plane expressed in a local coordinate system linked to the target. Then, the raw data spectrum is projected on this plane before focusing with the same process as PFA.