Synthetic aperture radar

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The surface of Venus, as imaged by the Magellan probe using SAR

Synthetic aperture radar (SAR) is a form of radar in which sophisticated post-processing of radar data is used to produce a very narrow effective beam. It can only be used by moving instruments over relatively immobile targets, but it has seen wide applications in remote sensing and mapping.

1 Basic operation
2 More complex operation
3 Doppler Beam Sharpening
4 Chirped (Pulse Compressed) Radars
5 Data collection
6 See also
7 External links

[edit] Basic operation

NASA's AirSAR instrument is attached to the side of a DC-8

In a typical SAR application, a single radar antenna will be attached to the side of an aircraft. A single pulse from the antenna will be rather broad (several degrees) because diffraction requires a large antenna to produce a narrow beam. The pulse will also be broad in the vertical direction; often it will illuminate the terrain from directly beneath the aircraft out to the horizon. However, if the terrain is approximately flat, the time at which echoes return allows points at different distances from the flight track to be distinguished. Distinguishing points along the track of the aircraft is difficult with a small antenna. However, if the amplitude and phase of the signal returning from a given piece of ground are recorded, and if the aircraft emits a series of pulses as it travels, then the results from these pulses can be combined. Effectively, the series of observations can be combined just as if they had all been made simultaneously from a very large antenna; this process creates a synthetic aperture much larger than the length of the antenna (and in fact much longer than the aircraft itself).

Combining the series of observations requires significant computational resources. It is normally done at a ground station after the observation is complete, using Fourier transform techniques. The result is a map of radar reflectivity (including both amplitude and phase). The phase information is, in the simplest applications, discarded. The amplitude information, however, contains information about ground cover, in much the same way that a black-and-white picture does. Interpretation is not simple, but a large body of experimental results has been accumulated by flying test flights over known terrain.

Before rapid computers were available, the processing stage was done using holographic techniques. This was one of the first effective analogue optical computer systems. A scale hologram interference pattern was produced directly from the analogue radar data (for example 1:1,000,000 for 0.6 meters radar). Then laser light with the same scale (in the example 0.6 micrometers) passing through the hologram would produce a terrain projection. This works because SAR is fundamentally very similar to holography with microwaves instead of light.

[edit] More complex operation

SAR image of Death Valley colored using polarimetry

The basic design of a synthetic aperture radar system can be enhanced in various ways to collect more information. Most of these methods use the same basic principle of combining many pulses to form a synthetic aperture, but they may involve additional antennas or significant additional processing.

[edit] Polarimetry

Radar waves have a polarization. Different materials reflect radar waves with different intensities, but anisotropic materials such as grass often reflect different polarizations with different intensities. Some materials will also convert one polarization into another. By emitting a mixture of polarizations and using receiving antennae with a specific polarization, several different images can be collected from the same series of pulses. Frequently three such images are used as the three color channels in a synthesized image. This is what has been done in the picture at left. Interpretation of the resulting colors requires significant testing of known materials.

New developments in polarimetry also include utilizing the changes in the random polarization returns of some surfaces (such as grass or sand), between two images of the same location at different points in time to determine where changes not visible to optical systems occurred. Examples include subterranean tunneling, or paths of vehicles driving through the area being imaged.

[edit] Interferometry

Main article: Interferometric synthetic aperture radar

Rather than discarding the phase data, information can be extracted from it. If two observations of the same terrain from very similar positions are available, aperture synthesis can be performed to provide the resolution performance which would be given by a RADAR system with dimensions equal to the separation of the two measurements. This technique is called Interferometric SAR or InSAR.

If the two samples are obtained simultaneously (perhaps by placing two antennas on the same aircraft, some distance apart), then any phase difference will contain information about the angle from which the radar echo returned. Combining this with the distance information, one can determine the position in three dimensions of the image pixel. In other words, one can extract terrain altitude as well as radar reflectivity, producing a digital elevation model (DEM) with a single airplane pass. One aircraft application at the Canada Centre for Remote Sensing produced digital elevation maps with a resolution of 5 m and altitude errors also on the order of 5 m. This method was used in 2000 by the IFSAR team to map many regions of the Earth's surface with unprecedented accuracy from the Space Shuttle.

If the two samples are separated in time, perhaps from two different flights over the same terrain, then there are two possible sources of phase shift. The first is terrain altitude, as discussed above. The second is terrain motion: if the terrain has shifted between observations, it will return a different phase. The amount of shift required to cause a significant phase difference is on the order of the wavelength used. This means that if the terrain shifts by centimeters, it can be seen in the resulting image (A digital elevation map must be available in order to separate the two kinds of phase difference; a third pass may be necessary in order to produce one).

This second method offers a powerful tool in geology and geography. Glacier flow can be mapped with two passes. Maps showing the land deformation after a minor earthquake or after a volcanic eruption (showing the shrinkage of the whole volcano by several centimeters) have been published.

[edit] Differential Interferometry

Differential interferometry (D-InSAR) requires taking at least two images with addition of a DEM. The DEM can be either produced by GPS measurements or could be generated by interferometry as long as the time between acquisition of the image pairs is short, which guarantees minimal distortion of the image of the target surface. In principle, 3 images of the ground area with similar image acquisition geometry is often adequate for D-InSar. The principle for detecting ground movement is quite simple. One interferogram is created from the first two images; this is also called the reference interferogram or topographical interferogram. A second interferogram is created that captures topography + distortion. Subtracting the latter from the reference interferogram can reveal differential fringes, indicating movement. The described 3 image D-InSAR generation technique is called 3-pass or double-difference method.

Differential fringes which remain as fringes in the differential interferogram are a result of SAR range changes of any displaced point on the ground from one interferogram to the next. In the differential interferogram, each fringe is directly proportional to the SAR wavelength, which is about 5.6 cm for ERS and RADARSAT single phase cycle. Surface displacement away from the satellite look direction causes an increase in path (translating to phase) difference. Since the signal travels from the SAR antenna to target and back again, the measured displacement is twice the unit of wavelength. This means in differential interferometry one fringe cycle -pi to +pi or one wavelength corresponds to a displacement relative to SAR antenna of only half wavelength (2.8 cm). There are various publications on measuring subsidence movement, slope stability analysis, land slide, glacier movement, etc tooling D-InSAR. Further advancement to this technique whereby differential interferometry from Satellite SAR ascending pass and descending pass can be used to estimate 3-D ground movement. Research in this area has shown accurate measurements of 3-D ground movement with accuracies comparable to GPS based measurements can be achieved.

[edit] Ultra-wideband SAR

Normal radar emits pulses with a very narrow range of frequencies. This places a lower limit on the pulse length (and therefore the resolution in the distance direction) but greatly simplifies the electronics. Interpretation of the results is also eased by the fact that the material response must be known only in a narrow range of frequencies.

Ultra wideband radar emits very short pulses consisting of a very wide range of frequencies, from zero up to the radar's normal operating frequency. Such pulses allow high distance resolution but much of the information is concentrated in relatively low frequencies (with long wavelengths). Thus such systems require very large receiving apertures to obtain correspondingly high resolution along the track. This can be achieved with synthetic aperture techniques.

The fact that the information is captured in low frequencies means that the most relevant material properties are those at lower frequencies than for most radar systems. In particular, such radar can penetrate some distance into foliage and soil. (See Ground-penetrating_radar).

[edit] Doppler Beam Sharpening

A commonly used technique for SAR systems is called Doppler Beam Sharpening. Because the real aperture of the RADAR antenna is so small (compared to the wavelength in use), the RADAR energy spreads over a wide area (usually many degrees wide in a direction orthogonal (right angle) to the direction of the platform (aircraft). Doppler Beam Sharpening takes advantage of the motion of the platform in that targets ahead of the platform return a Doppler up-shifted signal (slightly higher in frequency) and targets behind the platform return a Doppler down-shifted signal (slightly lower in frequency). The amount of shift varies with the angle forward or backward from the ortho-normal direction. By knowing the speed of the platform, target signal return is placed in a specific angle "bin" that changes over time. Signals are integrated over time and thus the RADAR "beam" is synthetically reduced to a much smaller aperture - or more accurately (and based on the ability to distinguish smaller doppler shifts) the system can have hundreds of very "tight" beams concurrently. This technique dramatically improves angular resolution; however, it is far more difficult to take advantage of this technique for range resolution. (See Pulse-doppler radar).

[edit] Chirped (Pulse Compressed) Radars

A common technique for many RADAR systems (usually also found in SAR systems) is to "chirp" the signal. In a "chirped" radar, the pulse is allowed to be much longer. A longer pulse allows more energy to be emitted, and hence received, but usually hinders range resolution. But in a chirped radar, this longer pulse also has a frequency shift during the pulse (hence the chirp or frequency shift). When the "chirped" signal is returned, it must be correlated with the sent pulse. Classically, in analog systems, it is passed to a dispersive delay line (often a SAW device) that has the property of varying velocity of propagation based on frequency. This technique "compresses" the pulse in time - thus having the effect of a much shorter pulse (improved range resolution) while having the benefit of longer pulse length (much more signal returned). Newer systems use digital pulse correlation to find the pulse return in the signal.

[edit] Data collection

Highly accurate data can be collected by aircraft overflying the terrain in question. In the 1980s, as a prototype for instruments to be flown on the NASA Space shuttles, NASA operated a synthetic aperture radar on a NASA Convair 990. However, in 1986, this plane crashed on takeoff. In 1988, NASA rebuilt a C, L, and P-band SAR to fly on the NASA DC-8 aircraft. Called AIRSAR, it flew missions at sites around the world until 2004. Another such aircraft, the Convair 580, was flown by the Canada Center for Remote Sensing until about 1996 when it was handed over to Environment Canada due to budgetary reasons. Most land-surveying applications are now carried out by satellite observation. Satellites such as ERS-1/2, JERS-1, Envisat ASAR, and RADARSAT-1 were launched explicitly to carry out this sort of observation. Their capabilities differ, particularly in their support for interferometry, but all have collected tremendous amounts of valuable data. The Space Shuttle has also carried synthetic aperture radar equipment during the SIR-A and SIR-B missions during the 1980s, as well as the Shuttle Radar Laboratory (SRL) missions in 1994 and the Shuttle Radar Topography Mission in 2000.

The Venera 15 and Venera 16 followed later by the Magellan space probe mapped the surface of Venus over several years using synthetic aperture radar.

Synthetic aperture radar was first used by NASA on JPL's Seasat oceanographic satellite in 1978 (this mission also carried an altimeter and a scatterometer); it was later developed more extensively on the Spaceborne Imaging Radar (SIR) missions on the space shuttle in 1981, 1984 and 1994. The Cassini mission to Saturn is currently using SAR to map the surface of the planet's major moon Titan, whose surface is partially hidden from direct optical inspection by atmospheric haze.

The Mineseeker Project ([1]) is designing a system for determining whether regions contain landmines based on a blimp carrying ultra-wideband synthetic aperture radar. Initial trials show promise; the radar is able to detect even buried plastic mines.

SAR has been used in radio astronomy for many years to simulate a large radio telescope by combining observations taken from multiple locations using a mobile antenna.