Introduction to Synthetic Aperture Radar (SAR)

The synthetic aperture radar principle has been discovered in the early 50. Since then, a rapid development took place all over the world and a couple of airand space-borne systems are operational today. Progress made in technology and digital signal processing lead to very flexible systems useful for military and civilian applications. Radar has proved to be valuable before, because of its day-and-night capability and the possibility to penetrate clouds and rain. Optical instruments however had great advantages in the interpretation of depicted objects. The great wavelength of radar signals limits the achievable resolution in cross range direction of real aperture radar systems. Thus, imaging cannot be realized using static radar systems. The idea of SAR was to transmit pulses and store the scene echoes along a synthetic aperture (i.e. the path of the SAR sensor) and to combine the echoes afterwards by the application of an appropriate focussing algorithm. The combination is carried out coherently. As we will see, it is quit easy to understand the basic idea of SAR. We will show also the hardware concept or a SAR system and give an idea for a processing algorithm. 1.0 GENERAL FACTS ABOUT SAR Today, synthetic aperture radar (SAR) plays an important role in military ground surveillance and earth observation. Since the late eighties a couple of SAR-systems have been developed for both space and airborne operation. The underlying radar principle offers advantages compared to competing sensors in infrared or visible spectral area. This chapter describes the field of applications where SAR can be used to gain valuable information. Physical conditions like propagation of electromagnetic waves and scene reflectivity affect the choice of several radar design parameters. 1.1 Applications SAR has got a broad range of applications. For remote sensing a couple of earth observing satellites are currently in operation, having imaging sensors working in different spectral areas. The utilisability of optical sensors depends not only on daylight but also on the actual weather conditions. Clouds and strong 1 Later on, we will see that high resolution radar imaging requires a relative motion between the sensor and the object/scene to be imaged. It is not relevant, whether this relative motion results from scene or object motion. R Berens, P. (2006) Introduction to Synthetic Aperture Radar (SAR). In Advanced Radar Signal and Data Processing (pp. 3-1 – 3-14). Educational Notes RTO-EN-SET-086, Paper 3. Neuilly-sur-Seine, France: RTO. Available from: http://www.rto.nato.int/abstracts.asp.