Novel side-view imaging of ships at sea for airborne ISAR

For airborne ISAR imaging of ship targets at sea, both ship's three-dimensional rotation driven by waves in the sea surface and the relative tangential translational motion between the platform and the ship contribute to the imaging. At high sea-state and in long-range and low-altitude observation, the rotation component produced by the relative tangential motion between the radar and the ship is negligible. A well-focused side-view image of a ship formed by utilizing the ship's dominant roll and pitch is always available via a suitable imaging time selection scheme. However, the effect of the rotation due to the platform tangential motion with respect to the ship, which provides much top-view information of the ship, has to be taken into account in some cases, such as when the ship is observed in a relatively close range. In these cases, the resulting image presents a mixture of the ship side-view and top-view that is less valuable than a pure side-view image for further classification. We present a ship side-view imaging method in this paper to solve this issue. The Doppler frequency derivative of each scatterer is shown to approximately only contain the height information and is estimated and utilized for the side-view image formation, with the use of 'CLEAN' technique and a concentration measure that is introduced to increase the estimation accuracy of the Doppler frequency derivative. For a short imaging interval, the Doppler frequency derivative is approximate to a constant chirp rate. The corresponding estimation reduces to a chirp rate estimation. The proposed method is verified by experiments on simulated data.