Principles of the Synthetic Aperture Radar (SAR)

Synthetic Aperture Radar is an electronic device for imaging the electromagnetic reflectivity of objects or environments with high spatial accuracy.

Conventional radar transmits waves from a fixed position and acquires the response of a scene to the transmitted signals. Measuring the time taken for an object's response to propagate to the radar enables precise measurement of the radar-object distance, while the amplitude of the received signal enables obstacles of interest to be detected and their reflectivity assessed. This type of radar, known as true-aperture radar, cannot discriminate between objects located at identical radial distances, even if their positions are very far apart (by several kilometers in the case of satellite measurements) in the transverse direction.

To overcome this limitation, the principle of aperture synthesis is used, based on analysis of the phase variation of the signal reflected by an object when the measurement position is changed. Signal processing techniques make it possible to focus the response of an object in a two-dimensional plane, with an increased resolution whose order of magnitude was first decametric, then metric and now decimetric. Synthetic Aperture Radar (SAR) is usually mounted on airborne or satellite platforms, and is widely used in remote sensing of natural environments, for monitoring and mapping territories and activities. Because of the carrier frequency range of the signals used, Synthetic Aperture Radar measurements are complementary to optical measurements, which are generally dependent on light and cloud conditions in the broadest sense.

The first part is an introduction to Synthetic Aperture Radar imaging, describing the principles of radar measurement and proposing a simple model of the response of objects and environments to a radar signal. The objectives of Synthetic Aperture Radar Imaging are presented, along with the notions of resolution, ambiguity and signal-to-noise ratio, which are widely used in imaging.

The second part, dedicated to range imaging, introduces the concept of signal focusing by adaptive filtering and describes the associated performances. Radar architecture and range-focused signal characteristics are proposed for various pulse and continuous waveforms.

The final section deals with two-dimensional imaging using aperture synthesis. We first highlight the spectral diversity in azimuth associated with coherent measurement of radar echoes along an aperture, and show how this feature can be exploited to improve resolution by adaptive filtering. The most commonly used techniques for focusing radar signals are then deduced using the azimuth-matched filtering principle. We propose :

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