Synthetic Aperture Radar

A Synthetic Aperture Radar, or SAR, is a coherent mostly airborne or space-borne side-looking radar system which utilizes the flight path of the platform to simulate an extremely large antenna or aperture electronically, and that generates high-resolution remote sensing imagery. Over time, individual transmit/receive cycles (PRT's) are completed with the data from each cycle being stored electronically. The signal processing uses magnitude and phase of the received signals over successive pulses from elements of a synthetic aperture. After a given number of cycles, the stored data is recombined (taking into account the Doppler effects inherent in the different transmitter to target geometry in each succeeding cycle) to create a high resolution image of the terrain being over flown.

How Does Synthetic Aperture Radar (SAR) Work?

Consider an airborne SAR imaging perpendicular to the aircraft velocity as shown in the figure below. Typically, Synthetic Aperture Radar 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 measurement and resolution are achieved in Synthetic Aperture Radar in the same manner as most other radars: range is determined by measuring the time from transmission of a pulse to receiving the echo from a target and, in the simplest SAR, range resolution is determined by the transmitted pulse width, i.e. narrow pulses yield fine range resolution.

The other dimension is called azimuth (or along track) and is perpendicular to range. SAR ability to produce relatively fine azimuth resolution 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. Similarly, optical systems, such as telescopes, require large apertures (mirrors or lenses which are analogous to the radar antenna) to obtain fine imaging resolution. Since SAR is much lower in frequency than optical systems, even moderate SAR resolutions require an antenna too large to be practically carried by an airborne platform: antenna lengths several hundred meters long are often required. However, airborne radar can collect 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.

Achieving fine azimuth resolution may also be described from a Doppler processing viewpoint. A target's position along the flight path determines the Doppler frequency of its echoes: targets ahead of the aircraft produce a positive Doppler offset, while targets behind the aircraft produce a negative offset. As the aircraft flies a distance (the synthetic aperture), echoes are resolved into a number of Doppler frequencies. The target's Doppler frequency determines its azimuth position.

What I have mentioned above is very basic, in reality SARs are not as simple as described above. Transmitting short pulses to provide range resolution is generally not practical. Typically, longer pulses with wide-bandwidth modulation are transmitted, which complicate 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 are coupled (dependent on each other) which also greatly increases the computational processing.

The SAR works similar of a phased array, but contrary of a large number of the parallel antenna elements of a phased array, SAR uses one antenna in time-multiplex. The different geometric positions of the antenna elements are result of the moving platform now. The SAR-processor stores all the radar returned signals, as amplitudes and phases, for the time period T from position A to D. Now it is possible to reconstruct the signal which would have been obtained by an antenna of length v · T, where v is the platform speed. As the line of sight direction changes along the radar platform trajectory, a synthetic aperture is produced by signal processing that has the effect of lengthening the antenna. Making T large makes the „synthetic aperture” large and hence a higher resolution can be achieved.

As a target (like a ship) first enters the radar beam, the back-scattered echoes from each transmitted pulse begin to be recorded. As the platform continues to move forward, all echoes from the target for each pulse are recorded during the entire time that the target is within the beam. The point at which the target leaves the view of the radar beam some time later, determines the length of the simulated or synthesized antenna. The synthesized expanding beam-width, combined with the increased time a target is within the beam as ground range increases, balance each other, such that the resolution remains constant across the entire swath. The achievable azimuth resolution of a SAR is approximately equal to one-half the length of the actual (real) antenna and does not depend on platform altitude (distance).

The requirements are: stable, full-coherent transmitter an efficient and powerful SAR-processor, and exact knowledge of the flight path and the velocity of the platform. Using such a technique, radar designers are able to achieve resolutions which would require real aperture antennas so large as to be impractical with arrays ranging in size up to 10 m.

A Synthetic Aperture Radar was used on board of a Space Shuttle during the Shuttle Radar Topography Mission (SRTM). SAR radar is partnered by what is termed Inverse SAR (abbreviated to ISAR) technology which in the broadest terms, utilizes the movement of the target rather than the emitter to create the synthetic aperture. ISAR radars have a significant role aboard maritime patrol aircraft to provide them with radar image of sufficient quality to allow it to be used for target recognition purposes.

SAR is often preferred over optical imagers for these applications because its performance is independent of available daylight and visibility. From a high-level perspective, SAR systems emit microwaves at a scene and measure the voltage returned

from a scene of targets. From these voltages, the radar cross-section (RCS) is found, which is a measure of how “large” a target appears in a radar image. High values of RCS show up as bright targets, whereas targets with low RCS are dim. Targets with rough surfaces tend to scatter EM radiation back towards the radar, so manmade targets like buildings, vehicles, and roads have a high RCS. Since the ground is relatively flat, it scatters EM radiation away from the radar and its RCS is low.