A Synthetic Aperture Radar (SAR), or SAR an airborne or spaceborne sidelooking radar system has been flown on both military and civilian spacecraft because of its ability (for certain wavelengths) to penetrate clouds and generates high-resolution remote sensing imagery. Seasat, the SIR series, and Radarsat are among the instruments used so far. ESA’s ERS-1 and ERS-2 are also radar satellites. Thermal remote sensing, operating primarily in the 8-14 µm but also in the 3-5 µm wavelength region of the spectrum produces diagnostic data that can aid in identifying materials by their thermal properties. Some meteorological satellites have thermal sensors, as does the Landsat TM.
As the radar moves, a pulse is transmitted at each position; the return echoes pass through the receiver and are recorded in an ‘echo store.’ Because the radar is moving relative to the ground, the returned echoes are Doppler-shifted (negatively as the radar approaches a target; positively as it moves away). Comparing the Doppler-shifted frequencies to a reference frequency allows many returned signals to be “focused” on a single point, effectively increasing the length of the antenna that is imaging that particular point. This focusing operation, commonly known as SAR processing, is now done digitally on fast computer systems. The trick in SAR processing is to correctly match the variation in Doppler frequency for each point in the image: this requires very precise knowledge of the relative motion between the platform and the imaged objects (which is the cause of the Doppler variation in the first place).
Radar transmits a pulse Measures reflected echo (backscatter)
Synthetic aperture radar is now a mature technique used to generate high resolution radar images in which fine detail can be resolved. SARs provide unique capabilities as an imaging tool. Because they provide their own illumination (the radar pulses), they can image at any time of day or night, regardless of sun illumination. And because the radar wavelengths are much longer than those of visible or infrared light, SARs can also “see” through cloudy and dusty conditions that visible and infrared instruments cannot.
What is a radar image?
Radar images are composed of many dots, or picture elements. Each pixel (picture element) in the radar image represents the radar backscatter for that area on the ground: darker areas in the image represent low backscatter, brighter areas represent high backscatter. Bright features mean that a large fraction of the radar energy was reflected back to the radar, while dark features imply that very little energy was reflected. Backscatter for a target area at a particular wavelength will vary for a variety of conditions: size of the scatters in the target area, moisture content of the target area, polarization of the pulses, and observation angles. Backscatter will also differ when different wavelengths are used.
A useful rule-of-thumb in analyzing radar images is that the higher or brighter the backscatter on the image, the rougher the surface being imaged. Flat surfaces that reflect little or no microwave energy back towards the radar will always appear dark in radar images. Vegetation is usually moderately rough on the scale of most radar wavelengths and appears as grey or light grey in a radar image. Surfaces inclined towards the radar will have a stronger backscatter than surfaces which slope away from the radar and will tend to appear brighter in a radar image. Some areas not illuminated by the radar, like the back slope of mountains, are in shadow, and will appear dark. When city streets or buildings are lined up in such a way that the incoming radar pulses are able to bounce off the streets and then bounce again off the buildings (called a double- bounce) and directly back towards the radar they appear very bright (white) in radar images. Roads and freeways are flat surfaces so appear dark. Buildings which do not line up so that the radar pulses are reflected straight back will appear light grey, like very rough surfaces.
Backscatter is also sensitive to the target’s electrical properties, including water content. Wetter objects will appear bright, and drier targets will appear dark. The exception to this is a smooth body of water, which will act as a flat surface and reflect incoming pulses away from a target; these bodies will appear dark.
Different observation angles also affect backscatter. Track angle will affect backscatter from very linear features: urban areas, fences, rows of crops, ocean waves, and fault lines. The angle of the radar wave at the Earth’s surface (called the incidence angle) will also cause a variation in the backscatter: low incidence angles (perpendicular to the surface) will result in high backscatter; backscatter will decrease with increasing incidence angles.
Death Valley as seen from the Space Shuttle’s synthetic aperture radar instrument. This image is in false color; the surface color and intensity represent the radar reflection properties of the ground cover. The image is arranged like a map; the image pixels represent (approximately) rectangles on the ground. (NASA)
SAR’s ability to pass relatively unaffected through clouds, illuminate the Earth’s surface with its own signals, and precisely measure distances makes it especially useful for the following applications:
- Lake and river ice monitoring
- Sea ice monitoring
- Change Detection
- Environmental Monitoring
- Surface deformation detection
- Glacier monitoring
- Crop production forecasting
- Forest cover mapping
- Ocean wave spectra
- Urban planning
- Coastal surveillance (erosion)
- Monitoring disasters such as forest fires, floods, volcanic eruptions, and oil spills
For more information on Synthetic Aperture Radar, visit here.
Coming up next Part 3 – Spectral Imaging
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