Tactical Synthetic Aperture Radar

The Tactical Synthetic Aperture Radar (TSAR) is a new radar system introduced on the MQ-8B Fire Scout unmanned aerial vehicle. It delivers high-resolution, wide-area radar images day or night, and in all weather conditions. TSAR systems take advantage of the long-range characteristics of radar signals and the information processing capability of modern digital electronics to provide high resolution imagery. The TSAR system can see through dense foliage, or penetrate sand and soil to image targets underground.

How Synthetic Aperture Radar works
SAR produces a two-dimensional (2-D) image. One dimension in the image is called range (or cross track) and is a measure of the “line-of-sight” distance from the radar to the target. Range measurements and resolutions are achieved in synthetic aperture radar in the same manner as most other radars, range is determined by precisely measuring the time from transmission of a pulse to receiving the echo from a target. In the simplest SAR, range resolution is determined by the transmitted pulse width; for example, narrow pulses yield fine range resolution. The second dimension is azimuth, or along track, and is perpendicular to range. It is the ability of SAR to produce relatively fine azimuth resolution that differentiates it from other radars. To obtain fine azimuth resolution, a physically large antenna is needed to focus the transmitted and received energy into a sharp beam. The sharpness of the beam defines the azimuth resolution. Even moderate SAR resolutions require an antenna physically larger than can be practically carried by an airborne platform: antenna lengths several hundred meters long are often required. However, airborne radar collects data while flying this distance and then process the data as if it came from a physically long antenna. The distance the aircraft flies in synthesizing the antenna is known as the synthetic aperture. A narrow synthetic beam width results from the relatively long synthetic aperture, which yields finer resolution than is possible from a smaller physical antenna. [1].

SARs are not simple and transmitting short pulses to provide range resolution is generally not practical. Typically, longer pulses with wide-bandwidth modulation are transmitted. This complicates the range processing but decreases the peak power requirements on the transmitter. For even moderate azimuth resolutions, a target’s range to each location on the synthetic aperture changes along the synthetic aperture. The energy reflected from the target must be “mathematically focused” to compensate for the range dependence across the aperture prior to image formation. Additionally, for fine-resolution systems, the range and azimuth processing is coupled (dependent on each other), which greatly increases the computational processing. The images generated by the TSAR are 3-D rather than 2-D as described above. So how does the TSAR system generate a 3-D image? There are actually two ways to create a 3-D image using SAR. The first is to use two antennas. With new mathematical techniques for relating the radar reflection from the terrain surface to the time delay between radar signals received at the two antenna locations, a 3-D image is created. If only one antenna is available, the aircraft has to fly two slightly offset passes to receive the same data.

As mentioned above, Tactical Synthetic Aperture Radars also offer the capability for penetrating materials which are optically opaque, and thus not visible by optical or infrared techniques. Low-frequency SARs may be used under certain conditions to penetrate foliage and even soil. This provides the capability for imaging targets normally hidden by trees, brush, and other ground cover. To obtain adequate foliage and soil penetration, SARs must operate at relatively low frequencies (10's of MHz to 1 GHz). Studies have shown that SAR may provide a limited capability for imaging selected underground targets, such as utility lines, arms caches, bunkers, land mines, etc. Depth of penetration varies with soil conditions (moisture content, conductivity, etc.) and target size, but individual measurements have shown the capability for detecting 55-gallon drums and power lines at depths of several meters. In dry sand, SAR can penetrate up to ten meters.