Inverse Synthetic ISAR (ISAR) Imaging

ISAR Imaging

The ISAR Section of the Radar Analysis Branch works on developing advanced algorithms and processing systems for the radar imaging technique known as Inverse Synthetic Aperture Radar (ISAR) which is related to the better known SAR technique. The picture shown above-left is a radar image generated using ISAR imaging. It was taken by the P-3 aircraft (shown below) and is the image of the ship USS Crockett, which is pictured to the right. The ISAR image is a two dimensional representation of the target, with the resolution in the horizontal dimension determined by the short pulse characteristic of the radar and the vertical dimension by the Doppler of the radar returns. The result is a recognizable image at long range under all weather conditions.

Polar Reformatting 

Polar Reformatting is a method of tomographic image reconstruction originally developed for SAR image processing. This technique relies on the projection slice theorem which relates the Fourier transform of a measured response along the radar-line-of-sight of the target to the Fourier transform of the entire target response. When this measurement is performed at a series of aspect angles, the Fourier transform of the target response is characterized over a region. An Inverse Fourier transform provides a spatial reconstruction of the target response. The figure below shows the application of the projection slice theorem for a two-dimensional case.

We can apply this technique to the problem of using a radar to image a moving target. The received radar pulse is a projection of all scatterers perpendicular to the line-of-sight of the radar and the Fourier transform of the pulse represents a line segment in 3-D Fourier space, which sweeps out a data surface as the aspect of the target changes (see below). The offset of the line segment, from the Fourier origin, is caused by the fact that the radar signal is modulated by an RF carrier. This data surface is projected onto a flat surface and processed into an image by performing an Inverse Fourier Transform on the data. The key to using this technique for ISAR is to develop a model for the motion of the target and then to measure the model parameters from the data. The technique developed here at NRL measures and compensates for target rotational acceleration in two dimensions and requires only three measurement points on the target.

This processing is necessary to compensate for the non-linear rotational motion, the scatterer range walk (range cell migration from pulse-to-pulse) and changing Doppler over the image formation period that occurs when imaging small targets or targets with extreme motion. A sample image processed using both traditional and Polar Reformatted ISAR techniques is shown below. In these images, the outline and some of the small features of the ship are much sharper in the Polar Reformatted image (on right) providing finer ship structure detail to the target identification system (Human or Machine).

For further information, email:contact@radar.nrl.navy.mil