Created Jul 21, 2003 | Updated Feb 9, 2007

Theoretical Basics: Ambiguous Measurements | Theoretical Basics: Signals and Range Resolution
How a Bat's Sensor Works | Low Probability of Intercept (LPI) Radar | Electronic Combat: Overview | Electronic Combat in Wildlife

As explained in the entry about radar antennas, the beamwidth and angular resolution of a radar's antenna are determined by its dimensions, expressed in relation to the wavelength used. Synthetic Aperture Radars (SAR) are overcoming this rule. They manage to achieve considerably better angular resolution by exploiting the effects that show up when a radar is mounted on a fast moving platform. SAR also has some more astonishing features, as explained below.

### The SAR Principle

The basic idea is this: if an object is being illuminated by a radar signal then the return signal carries information about it (such as reflecting properties, range profile etc) and is scattered into a large angular sector. This scattering can be thought of as a broadcast. A fixed radar installation receives only a minute part of the reflected signal and therefore only has access to a percentage of the information. But if the radar moves, it can collect all the pieces of information, one after the other. Thus, if the antenna moves a distance of, say, 300m during the measurement then it is doing the same thing as an antenna with a length of 300m would. The difference is that a 300m antenna can collect the information pieces simultaneously, whereas the small but moving one needs to store all the contributions and fiddle them together afterwards. In other words, by moving a radar antenna it is possible to synthesise the aperture of a much bigger antenna. It follows from the above that SAR can only work if it the antenna is pointing sideways from the moving platform. Therefore they are also called Side-Looking Airborne Radar (SLAR).

This explanation is somewhat simplified but it does cover the basics. SAR imagery requires tremendous signal processing power, transmitter signals of extreme purity and a platform that moves precisely in a straight line (unless you're willing to spend additional processing power for compensating deviations from a linear path). Any transmitter instabilities and vibrations of the platform make themselves known through the appearance of noise patterns (called 'speckle') in the images.

### SAR Applications and Features

SAR have been used with great success as imaging and ground mapping devices. Depending on the frequency used, a radar can easily look through clouds and precipitation. In contrast to photography, a SAR uses its own transmitter for illuminating the scene and thus does not rely on daylight conditions. However, photography and SAR imagery are quite different things. Every feature of the ground has its own properties, and sophisticated signal processing is able to sort them all out. Thus it is possible to determine what kind of grain is being grown in a field, whether it is ripe or not, and whether it is suffering from a lack of moisture in the soil. Hence, SAR imagery can be used for land cover classification, estimating annual crops and even for detecting forest fires.

Surface water acts much like a mirror. But, as it is being illuminated from an oblique position, such an area doesn't reflect anything back to the radar and thus it appears as a totally black spot. A bridge spanning a river stands out very clearly against this background.

Ocean waves can be seen on SAR images. These waves exhibit characteristic patterns and reflecting properties that change with the presence of icebergs or spilled oil. Therefore, SAR can be used to give iceberg warnings and to collect proof of environmental pollution.

Everybody knows that tall objects, such as the Pyramids of Giza, cast shadows if they are illuminated from the side. This applies to SAR imagery too, and therefore it is possible to measure the height of a structure by examining the shadow's length.

Something surprising happens if an object moves through the scene. A car on a road is not shown as a dot, nor is it shown as being on the road. A moving car appears as a straight line that indicates the direction it is moving. The length of the line is proportional to the illumination time - ie, it tells something about the radar but not about the object. The line is shown at some offset with respect to the road, and this offset is a measure of the car's speed. The same holds true for a ship in an ocean, but how do you know where the real movement path is? Simply enough, this is indicated by the traces of the ship's wake, which also show up on the SAR image.

NASA's SIR-C SL1 mission of April 1994 employed two SARs to produce a digital map of some 80% of the Earth's surface. This map features a height resolution of 30m and can be used in a wide range of scientific fields such as archaeology, ecology, agriculture and geology. By comparing images that were taken several months apart it is possible to measure the movements of glaciers and to see plate tectonics at work. There is also hope that one day SAR imagery may contribute in the prediction of earthquakes. On top of all that, SAR imagery can be of highly aesthetic value.

### Inverted Synthetic Aperture Radar (ISAR)

So, SAR draws advantages from being carried on a reasonably fast moving platform and looking at a stationary scene on the ground. Now if the situation is just the other way round, the working principle remains intact.

If an object moves through a radar beam then the overall return is composed of contributions from all its components. A slight variation of the aspect angle translates into different Doppler shifts that are obtained from features of the object's body. Say, an aircraft passes by and starts a banking manoeuvre. The fuselage yields an echo that exhibits a Doppler shift determined by the true ground speed of the aircraft. Because of the banking manoeuvre, there is some relative motion of the wings with respect to the fuselage - ie, they are slightly faster, respectively slower. Inverted SAR can isolate and measure these small contributions and synthesise an image of the whole arrangement. Without requiring daylight conditions, ISAR can produce images of aircraft, ships and objects as far away as a space shuttle in orbit.

### Summary

To sum it all up, SAR is probably the most complex area within the radar field. But the results it gives are outstanding and can rarely be matched by anything else, including high-resolution photography.

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