Imaging radars are known in the art, e.g., for use in environmental monitoring and earth-resource mapping. Typically imaging radars are active systems which send pulses and receive radial reflected radiation after striking a material in its path. The radiation received is comprised of both radiometry information (e.g., the intensity of the reflection energy) and spatial information (e.g., the distance and the azimuth of the material relative to the sensor). An imaging radar works like a flash camera in that it provides radiation to one or more surfaces and generates an image based on radiation reflected from the surface. However, imaging radar uses microwaves to generate the image rather than visible light.
Because imaging radar systems generally do not require illumination from the sun, but from itself, they can generate images at any time of day or night due to its ability to produce the energy and transmit it. Further, because the radar wavelengths are much longer than those of visible or infrared light, imaging radar systems can often capture images through cloudy and dusty conditions while visible and infrared instruments cannot.
The main difference between radar imaging and optical imaging is the behavior of the reflected beam energy. While optical systems generally moves toward a straight line, imaging radar systems generally move radially to determine the distance and the azimuth angle from the material in addition to the intensity radiation value. The spatial resolution depends on sensor geometrical parameters.
One type of such imaging radar systems is Synthetic Aperture Radar (SAR). As one of ordinary skill in the art will understand, the length of the radar antenna generally determines the resolution in the azimuth (along-track) direction of the image: the longer the antenna, the finer the resolution in this dimension. SAR systems are able to synthesize or simulate a very long antenna by combining signals (echoes) received by the radar as the radar moves along a path or flight track. The aperture, or area used to receive signals, is therefore created artificially during the signal processing.
As the radar moves, a pulse is transmitted at each position. The return signals or echoes pass through the receiver and are recorded. 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.
There are some advanced image processing techniques for improving the spatial resolution by acquiring the images at different phase angles relative to the surface. These techniques can improve the resolution beyond the limitation of the sensor. There are also certain techniques to increase the field of view of the images by acquiring them from spatial different positions, e.g. a sensor which is rapidly moving along a circular rail.