合成孔径雷达
维基百科，自由的百科全书
合成孔径雷达 (Synthetic Aperture Radar, SAR), 又名微波成像雷达, 是一种可以产生高分辨率雷达图像的机载雷达或星载雷达。它使用复杂的雷达数据后处理方法来获得极窄的有效辐射波束（对产生的雷达图像意味着极高的分辨率）。它只能安装在移动的载体上对相对静止的目标成像，或反之；但自合成孔径雷达发明以来，它被广泛的应用于遥感和地图测绘。
目录 |
[编辑] 基本工作模式
对一个典型的合成孔径雷达来说，天线安装在飞机的侧面。所发出的电磁波波束是相当宽的(可能有几度)，如果想获得极窄的波束，从 衍射 的原理来讲需要非常巨大的天线（一般来说是难以实现的）。在垂直的方向波束也相当宽; 经常天线波束照射的区域会从飞机正下方延伸到遥远的天边。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).
数据的处理使用快速傅里叶变换: 成像计算量是相当巨大的，实时数据处理仍然是一个严峻的挑战，因此数据的精处理通常是观测记录数据后由地面站进行。 The result is a map of radar reflectivity (including both amplitude and phase) on the ground. 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 in what was one of the first effective analogue optic computer systems. A scale hologram interference pattern was produced directly from the analogue radar data (for example 1:1000000 for 0.6 meters radar) and a 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.
[编辑] 更多复杂的工作模式
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.
[编辑] 极化
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 antennas 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 above. 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 occured. Examples include subterranean tunneling, or paths of vehicles driving through the area being imaged.
[编辑] 干涉
Rather than discarding the phase information, information can be extracted from it. If two observations of the same terrain from very similar positions are available, a great deal of interesting information can be extracted. 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 with a single airplane pass. One aircraft application at the Canada Center for Remote Sensing produced digital elevation maps with a resolution of 5 m and altitude errors also on the order of 5 m.
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 obervations, 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.
[编辑] 超宽带合成孔径雷达
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 simplifes 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.
[编辑] 多普勒锐化
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 ortho-normal (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.
[编辑] 线性调频 (脉冲压缩) 雷达
A common techniqe for many RADAR systems (sometimes 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 is passed to a dispersive delay line (often a SAW device (Surface Acoustic Wave) that has the property of varying velocity of propogation 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).
[编辑] 数据采集
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 CV-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 was flown by the Canada Center for Remote Sensing until about 1996 when it was decommissioned for cost 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 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.
[编辑] 参看
[编辑] 外部链接
- The Imaging Radar Home Page (NASA SAR missions)
- Airborne Synthetic Aperture Radar (AIRSAR) ) (NASA Airborne SAR)
- The CCRS airborne SAR page (Canadian airborne missions)
- RADARSAT international (Canadian radar satellites)
- The ERS missions (European radar satellites)
- The ENVISAT mission (ESA's most recent SAR satellite)
- The JERS satellites (Japanese radar satellites)
- Images from the Space Shuttle SAR instrument
- The Mineseeker Project has technical information about ultra-wideband SAR
- The Alaska Satellite Facility has numerous technical documents, including an introductory text on SAR theory and scientific applications
