||ES 771 Lecture|
James S. Aber
SPACE-BASED IMAGING RADAR
Introduction to radar remote sensing
RAdio Detection And Rangingóor RADARówas 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 effectsóeven 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.
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 systemsóboth 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. 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 featuresóroughness and slopes, which are excellent for geological applications.
- See radar science and engineering at NASA JPL.
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 madeóburied river systems were identified in the
Sahara region and the ancient city of Ubar was located in Oman.
Spaceborne Imaging Radar C (SIR-C) was the 1990s generation NASA instrument. It operated 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 expanded the potential for investigations of Earth-surface materials. SIR-C was coupled with the German/Italian X-band Synthetic Aperture Radar (X-SAR) on several shuttle missions. SIR-C/X-SAR imagery has proven valuable for applications in many subjects, including: agriculture, archaeology, geology, hydrology, oceanography, urban geography, and volcanology.
Radarsat-1 was an unmanned satellite that was successfully launched in Nov. 1995, and Radarsat-2 was launched in December 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.
- Examples of SIR-C/X-SAR imagery.
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 private firmsósee MDA.
- Overview of Radarsat-2.
Nazarenko, D., Staples, G. and Aspden, C. 1996. RADARSAT: First images. Photogrammetry and Remote Sensing 62,. p. 143-146.
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