TABLE OF CONTENTS This Chapter describes some of the work that has been carried out on radar systems for remote sensing below the surface of the earth and the planets. Radar systems can be mounted on aircraft or on satellites. The types of radar used are radically different from typical GPR systems, and the aim of this Chapter is to provide an insight into the remarkable results and sophistication of radar systems. Most of the work is based on synthetic aperture radar (SAR) processing and there is a wealth of material available on this topic.
Sub-surface imaging by radar from satellites is possible where the topographic cover is radar smooth and the material penetrated is fine grained, no more than a few metres thick and very dry. Where the thickness of the cover is less than the skin depth, the return signal can be significantly increased because of refraction of the electromagnetic wave, and reduction of backscatter due to oblique incidence. Kadaba [1] has reported data on the penetration of 0.1 1.5 GHz radiation into the earth's surface for remote-sensing applications. Elachi et al. [2] (Jet Propulsion Laboratory, USA) demonstrated that the satellite radar SIR-A (shuttle-imaging radar) could detect dry river beds beneath the Sahara Desert. This ability is due to the hyper-arid conditions existing in desert regions where the skin depth of the sand can be 5 m or more. Blom et al. [3] (USA) describe the detection of igneous dykes beneath the Mojave Desert. The Seasat satellite L-band...
