5.4 Radar Systems in Remote Sensing

Radar Systems in Remote Sensing

In order to understand how a radar imaging system produces imagery, a comparison with optical imaging systems is useful. Photographs or scanned images are the product of systems which use visible light and near infrared radiation and are the result of near instantaneous exposure. In contrast, radar imagery is produced by recording microwave pulses travelling to and from a target area over a period of time.

The optics-equivalent in a radar imaging system is a long, rectangular antenna which transmits and receives microwave energy. Resolution, which is the ability of a system to differentiate between two closely spaced objects, is dependent on focal length in optical sensors and in the along-track direction antenna length in radar systems. Antennas are analogous to lens systems in that a long antenna can be compared to a telephoto (long focal length) lens, while a shorter antenna is similar to a wide angled (short focal length) lens. To continue the analogy, a long antenna provides a detailed, or high-resolution image of a small area, while a short antenna provides an image of a large area with less detail.

Range Resolution

• P = pulse length
• 1, 2 = two targets that are too close together to be resolved as individual targets
• 3, 4 = two targets with sufficient range separation to be resolved as individual targets

Resolution in a radar system is controlled by the signal pulse length and the antenna beam width. The signal pulse length dictates resolution in the direction of energy propagation. This is referred to as the range direction. Shorter pulses result in a higher range resolution. The width of the antenna beam determines the resolution in the flight or azimuth direction. The beamwidth is directly proportional to radar wavelength and is inversely proportional to the length of the transmitting antenna. This means that resolution deteriorates with distance from the antenna. In order to have a high resolution in the azimuth direction the radar antenna must be very long.

Resolution in the flight or azimuth direction

• A = antenna beam
• 1, 2, = two targets that can be resolved as being separate
• 3, 4, = two targets that cannot be resolved as being separate

Synthetic Aperture Radar

Remote sensing radars can be divided into two categories - real aperture and synthetic aperture radars (SAR). Real aperture radars transmit and receive microwave signals with a fixed length antenna. They are limited in their ability to produce resolutions fine enough for most remote sensing applications, simply because it is difficult to transport a very long antenna. To solve this problem synthetic aperture radars (SAR) were developed. SARs have physically shorter antennas, which simulate or synthesize very long antennas. This is accomplished through modified data recording and signal processing techniques.