ES 771 Lecture
James S. Aber

SPACE-BASED IMAGING RADAR

Introduction to radar remote sensing

RAdio Detection And Ranging--or RADAR--was a military technology based on broadcasting and receiving microwave energy. The possibility for radar was first recognized in the 1930s, and its rapid development played a key role for the Allied victory in World War II. Following declassification in the 1960s, radar has been applied to civilian remote sensing of the Earth's surface. Radar "echoes" may be used to build up images that are quite different from images acquired by passive means. Microwaves (1 mm to 1 m wavelength) have several properties of value for remote sensing. Most importantly, they penetrate the atmosphere with no absorption or scattering effects--even through heavy clouds and dust. Microwaves are not a significant component of solar radiation. Radar can, thus, be used under all weather conditions any time of day or night.

Basic priniciples of radar imagery (see also RSE figs. 9-3, 9-8, 9-9, 9-11, 9-18).

Radar has some obvious advantages as a means for remote sensing; however, radar remains less widely used than photography or multispectral scanning. The reasons are cost and complexity of radar systems--both quite high, plus difficulty of interpreting radar imagery compared to more conventional types of images. Microwaves interact with surface features in ways that are not always fully understood. Roughness, orientation, and water content of objects play large roles, as do microwave bands and polarizations (see RSE fig. 9-19). Thus, many features do not have predictable spectral signatures. Very small objects, such as man-made metal structures, may be quite prominent in radar images, whereas larger natural features are difficult to recognize. In general, radar imagery tends to emphasize landscape topographic features--roughness and slopes, which are excellent for geological applications.

Space-based radar

Manned and unmanned platforms have been used for radar remote sensing of the Earth's surface. Two systems are reviewed here: space-shuttle imaging radar and Radarsat.

The shuttle imaging radar (SIR) program began in the early 1980s with missions A and B. The goals of these experiments were to acquire radar images of the Earth's surface and to demonstrate the potential of operating radar from the space shuttle. The L band (23 cm) was utilized with horizontal polarization and variable look angles (B only). The L band proved to have surprising ability to penetrate subsurface soils of extremely arid lands. As a result, some spectacular discoveries were made--buried river systems were identified in the Sahara region and the ancient city of Ubar was located in Oman.

Spaceborne Imaging Radar C (SIR-C) is the current generation NASA instrument. It operates in L band and C band (6 cm) with the ability to transmit and receive either horizontally or vertically polarized waves in both bands. The multiband, multipolarization capability greatly expands the potential for investigations of Earth-surface materials. SIR-C has been coupled with the German/Italian X-band Synthetic Aperture Radar (X-SAR) on several shuttle missions during the 1990s. SIR-C/X-SAR imagery has proven valuable for applications in many subjects, including: agriculture, archeology, geology, hydrology, oceanography, urban geography, and volcanology.

Examples of SIR-C/X-SAR imagery.

Radarsat-1 is an unmanned satellite that was successfully launched in Nov. 1995, and Radarsat-2 is scheduled for launch in March of 2007. The Radarsat series is a cooperative venture led by the Canadian Space Agency in association with the United States, several Canadian provinces, and the private sector. It carries a synthetic aperture radar (SAR) that operates in the C band (5.3 cm) with horizontal polarization. The antenna can be adjusted for different look angles. Spatial resolution and swath width are also variable over large ranges. These capabilities allow for collection of data to meet many different requirements of users. The satellite follows a near-polar, sun-synchronous orbit that provides for whole-earth coverage every 6 days and more frequent high-latitude coverage.

Overview of Radarsat.

The first image from Radarsat was a view of Cape Breton Island, Nova Scotia. This scene demonstrated the ability of Radarsat to acquire image data under conditions of darkness and adverse weather (Nazarenko et al. 1996). Because of its ability to operate under all conditions, Radarsat can provide data rapidly for disaster response, such as flood monitoring or oil spills. Near-real-time data can be delivered within 4-6 hours. Data may be obtained via governmental agencies or RadarSat International, a private Canadian firm--see RSI.

Reference

ES 771 schedule or ES 771 homepage.

© Notice: ES 771 is presented for the use and benefit of students enrolled at Emporia State University. Any other use of text, imagery or curriculum materials is prohibited without permission of the instructor. Last update 2006.