The scattering behaviour of the earth's surface at radar wavelengths can provide useful information about many natural processes. Since most applications of remote sensing require large areas to be surveyed, HF radars provide the only useful ground-based systems. These are used for sea sensing. Extensive coverage can be provided by air-borne platforms. Scatterometers mounted on helicopters or aircraft, and air-borne side-looking radars and SARs are employed around the world for a wide range of applications. Perhaps the most exciting prospect is the new generation of space-borne radar instruments, which promise to be a major source of information on global-scale processes. Weather conditions have little effect on these instruments at the range of wavelengths employed (though ionospheric effects have to be allowed for in altimetry and SAR operation). This will allow reliable gathering of surface information, which cannot be guaranteed at optical wavelengths. Radar instruments can also provide information, such as the global wind field over the oceans, that is not available by other means. Much of this information, such as the global wind field or mean sea height, can be gathered at comparatively low spatial resolutions by scatterometry or altimetry (though high-resolution altimetry is attracting much current interest, and we can expect significant progress in the range of problems to which altimetry can be applied). For applications requiring high spatial resolution, SAR provides a possible answer, at the expense of increased system complexity (and cost!) and problems of data interpretation in the presence of speckle. With the launch of ERS-1 and other space-based radars throughout the 1990s, this decade should see major advances in understanding these techniques and their application to monitoring the earth's environment.

Chapter Contents:

• 11.1 High-frequency radar scattering from the sea
• 11.2 Measuring ocean currents
• 11.3 Measuring waves
• 11.4 The future of high-frequency remote sensing
• 11.5 Microwave scatterometry
• 11.6 Radar altimetry
• 11.7 Synthetic aperture radar
• 11.8 Summary
• 11.9 References
• 11.10 Further reading
• 11.11 Problems

Inspec keywords: airborne radar; remote sensing by radar; synthetic aperture radar

Other keywords: ground based system; earth environment monitoring; optical wavelength; air borne side looking radar; space borne radar instrument; earth surface scattering behaviour; air borne platform; radar wavelength; scatterometer; mean sea height; scatterometry; spatial resolution; SAR operation; high resolution altimetry; space based radar; radar remote sensing; global wind field; sea sensing

Subjects: Radar equipment, systems and applications; Geophysical techniques and equipment