Synthetic Aperture Radar and its Specialized Applications

Summary of Hans Hellsten's article in RSAWSPJ no 3 2001.

This article is an attempt to provide a modern overview of radar imaging technologies derived from Synthetic Aperture Radar (SAR). In trying to offer an analysis as technically advanced as possible of SAR, the present writer has nevertheless made a concerted effort to present the subject in a popularized and easy-to-grasp form.

The article starts off with the SAR technique itself, which is discussed from the viewpoint of the modern SAR-processing method of back projection. The difference between stripmap and spotlight SAR as regards the possibility of obtaining very high resolution is discussed. So is the influence on the image by ground topography and motion errors.

The article then discusses GMTI, i.e. the technique for detecting and positioning objects in a state of linear motion by SAR-similar techniques. It is explained how SAR actually focuses not only stationary objects but also certain subclasses of objects in linear motion and how SAR can thus be used to focus on an object in an arbitrary and unknown state of linear motion. GMTI adopts further processes of clutter cancellation and bearing estimation, which are also explained.

The importance of wavelength is touched upon. Attention is drawn to the fact that a SAR can be made to operate at wavelengths of many meters with the advantage of increased stability and interpretability of the SAR-image.

Finally, the article presents a technique for using SAR-similar methods for air surveillance. The advantage of using these methods is that target positioning is obtained by range measurements rather than angular measurements. A large antenna aperture, ordinarily required to obtain an exact bearing estimation, is thus not called for. Consequently, the system can use low radar frequencies without any degradation of measurement accuracy and thus provide for accurate radar surveillance of stealth aircraft.

In the air surveillance system the synthetic radar image is obtained by means of a network of ground-based radar stations. Simultaneous capacity with respect to multiple air targets is obtained applying an association technique, by which detections from different stations are uniquely related to each other by matching combinations of radial and transversal Doppler shifts.

This article is Hans Hellsten’s inaugural lecture, presented to the Military Technical Section of the Royal Swedish Academy of War Sciences on 2nd May, 2000.