Synthetic aperture radar or commonly abbreviated to as SAR provides high resolution imagery by taking the advantages of the intricate information processing capacity of today's digital electronics and the long-range propagation characteristics of radar signals,
Due to the minimum constraints on the time-of-day and atmospheric conditions, synthetic aperture radar enhances photographic and other optical imaging capabilities.
Figure 1. Synthetic Aperture Radar image of Washington, D.C. (Source: Sandia National Laboratories)
Synthetic Aperture Radar - How does it Work?
Under normal conditions, a two-dimensional image is generated by synthetic aperture radar. The two dimensions in a SAR image are referred to as azimuth and range.
In order to achieve a fine azimuth resolution, a large antenna is required to bring the transmitted and received energy into focus to a sharp beam. The azimuth resolution is defined by the sharpness of the beam. It is the capability of synthetic aperture radar to generate a relatively fine azimuth resolution which separates synthetic aperture radar from other radar systems.
Compared to optical systems, synthetic aperture radars are in much lower frequency. An antenna length of several hundred meters is often needed. This makes it impractical for the antenna to be carried by an airborne platform. Nevertheless, data could be collected and processed by airborne radar as if it came from a long antenna. The distance that the aircraft flies in synthesizing the antenna is called the synthetic aperture. A fine resolution can be achieved through a relatively long synthetic aperture.
Doppler processing can also be used to attain fine azimuth resolution. The Doppler frequency of the target's echo is determined by its position along the flight path. A positive Doppler offset is produced when the target is ahead of the aircraft. On the other hand, a negative offset is produced when the target is behind the aircraft.
The other dimension is referred to as the range and it is perpendicular to azimuth. Range is the line-of-sight distance from the radar to the target. By precisely measuring the time from the transmission of a pulse to receiving the target's echo determines the range. The range resolution id determined by the width of the transmitted pulse.
Figure 2. Synthetic Aperture Radar Imaging Concept (Source: Sandia National Laboratories)
Applications of Synthetic Aperture Radar
Synthetic aperture radar technology has been used to provide geologists terrain structural information for mineral exploration, information to environmentalists regarding oil spill boundaries on water, ice hazard maps and sea states to navigators, and targeting and reconnaissance information for the military.