SYNTHETIC APERTURE RADAR
A BRIEF HISTORY:
Radar has been used in a wide variety of military and non-military applications such as remote sensing, imaging, guidance and global positioning .The radar was used as a tool for detecting ships and aircrafts during the 1920s. The first imaging radar was developed during World War II and it produced rectangular image formats using the B-Scan technique. Distortions in the display were produced by the nonlinear relation between angle and distance to the side of aircraft. This distortion was improved by using the Plan Position Indicator (PPI). Its antenna beam was rotated through 360? about the aircraft and a picture of ground was produced. The Side Looking Airborne Radar (SLAR) was developed in the 1950's. The SLAR achieved scanning by fixed beam pointed to the side with aircraft's motion moving the beam across the land. The image formed by SLAR has poor azimuth resolution. In case of the SLAR the smaller the azimuth beam-width, the finer is the azimuth resolution. To obtain high-resolution image one has to employ either a long antenna or to employ wavelengths so short that the radar must contend with severe attenuation in the atmosphere. The antenna size and weight are restricted in airborne application. Signal processing is another way of achieving better resolution from radar.
Get your grade
or your money back
using our Essay Writing Service!
Synthetic Aperture Radar (SAR) is a method which makes use of the signal processing technique to develop the resolution beyond that of the physical antenna aperture. In SAR, forward motion of actual antenna is used to 'synthesize' a very long antenna. Synthetic Aperture Radar makes use of longer wavelengths and is still able to get good resolutions.
Synthetic Aperture Radar makes use of mathematical techniques which makes use of the phase and amplitude information to synthesize a high resolution image from several adjacent-in-time RADAR pulses, which is almost equal to the quality of the image that is achieved from a larger antenna, without any additional mathematical manipulation. In addition to rendering a different view of the target, SAR can also be used in all weathers and day or at night and still be at much lower electromagnetic frequency than the optical sensors.WORKING: How does SAR work?
SAR is a type of radar in which the antenna used by conventional radar which are large and highly-directional are replaced with many low-directivity small stationary antennas scattered near or around the target area. The echo waveforms which are received at the different antenna positions are processed to decide the target. SAR can only be implemented by placing multiple stationary antennas over a relatively large area or by moving one or more antennas over relatively immobile targets, or a combination of both of them. Remote sensing and mapping are some of the fields were SAR has applications. A single radar antenna which is attached to the side of an aircraft ia a typical example of SAR. Due to the diffraction the pulse from the antenna will be broad because the large antenna has to produce a narrow beam. The pulse will be broad in the vertical direction, and often the terrain from just underneath the aircraft out to the horizon will be illuminated. If it is a flat terrain, the time at which echoes return back allows points at different distance to be eminent. It is not easy to distinguish points that come along the track of the aircraft with the help of a small antenna. But, the phase and amplitude of the returning signal can be recorded and if the aircraft emits a series of pulses as it travels, then the results from these pulses can be combined. This means that the series can be assumed as if they had all been made at the same time from a very large antenna; this method makes a synthetic aperture much larger than the length of the antenna (and much longer than the aircraft itself).The synthesized expanding beam width
The Synthetic Aperture Radar is analogous to a phased array, but SAR uses one antenna in time-multiplex where as the phased array makes use of a number of the parallel antenna elements. The moving platform is caused due to the different geometric positions of the antenna elements.
Always on Time
Marked to Standard
All the radar signals are stored by the Synthetic Aperture Radar processor as amplitude and phases for the time period T from position A to D. The reconstruction of the signal which should have been got from the antenna of length v Â· T, where v is the platform speed is now possible. A synthetic aperture is produced by signal processing as the LOS direction is changed alone the radar platform trajectory which has the effect of lengthening the antenna. Higher resolution can be achieved by making "T" large which also makes the synthetic aperture large.
When a target (say ship) enters into the radar beam the echoes that are backscattered from each of the pulse that are transmitted start to get recorded. All the echoes from the target are recorded the entire time the target is present within the beam and also as the platform continues to move forward. The point at which the target leaves the view of the radar determines the length of the simulated or synthesized antenna. The time a target is within the beam along with the expanding beam width balance each other and the resolution remains constant across the entire path.The realizable azimuth resolution of a SAR is just about equal to 0.5 times 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
- Exactly knowledge of the flight path and the velocity of the platform.
The resolutions that are only achievable with array size ranging up to 10 m are now achievable by radar design engineers by making use of such a technique
The shuttle Radar Topography Mission (SRTM) made use of Synthetic Aperture Radar. The term Inverse SAR (ISAR) technology is used jointly with SAR. It makes use of the movement of the target rather than the emitter to create the synthetic aperture. The Maritime patrol aircraft makes use of the ISAR radars to provide a radar image of low quality which can be used for target recognition purposes.Slant-range distortion
If the radar measures the distance to features in slant range than measuring in the true horizontal range down the floor then slant-range distortion occurs. This produces a varying image scale that moves from near to far range.Foreshortening:
If the radar reaches the top before it arrives at the top before it arrives at the base of a feature (say a mountain) the foreshortening occurs. Because the radar has to measure the distance in slant range the point from 'a' to 'b' will appear compressed and the length of the slope from a' to b' will appear wrongly in the image plain.
Layover: If the radar before it reaches the base (a) arrives at the top of the tall feature (b) then it is known as Layover. In this case the signal from the top of the feature will be expected to arrive early than the signal from the bottom. This causes the displacement of the top of the feature towards the radar from its original position in the ground and "Lays over" at the base.
Shadowing: Just as our shadows lengthen the shadowing effect also lengthens with greater incident angle ?.
Significant computation of resources is necessary to combine the above series of observations. This is often achieved by the use of Fourier transform at the ground station. Now-a-days the SAR aircraft itself performs the SAR processing onboard due to the high computing speed that is available now. The result is a map of both the amplitude and phase which is also known as the radar reflectivity. In some simple applications the phase information is discarded. The amplitude contains information about the ground cover similar to that of black-and -white pictures. It is not very easy to interpret the results. But they can be compared with the previous results and we can come to a conclusion.AN INTUITIVE FEEL OF HOW THE SAR WORKS
The theory of operation of SAR is complex. The figure shown below gives the airborne SAR imaging that is perpendicular to the aircraft velocity. Only two-dimensional image are produced by SAR. One of the dimensions is known as the range which measures the "line-of-sight" dist from the radar to that of the target.SAR achieves the range measurement and resolution similar to that of most of the radars. In the simplest SAR the range resolution is found out by using the transmitted pulse width. In others the range is measured by calculating the time from the transmission of the pulse and the time at which the echo is received back. It must be noted that the narrow pulses yield fine range resolution.
This Essay is
a Student's Work
The second one is known as the azimuth and it is found to be perpendicular to the range. The thing that differentiates the SAR from the other radars is its ability to produce fine azimuth resolution. This resolution can be achieved by focusing the transmitted and received energy into a sharp beam by using a physical large antenna. The more sharper the beam is the more finer the azimuth resolution is. Larger apertures are also required in optical systems such as telescopes to get fine image resolution. But it is to be noted that SAR operate at a much lower frequency than optical systems hence even moderate SAR resolutions needs an antenna that is larger than what can be carries by a plane. Usually antennas of several hundred meters in length are required. But a radar that is airborne will be able to collect data when it is flying and also will be able to process the data as if it came from a long physical antenna. Here the synthetic aperture is the distance the aircraft flies in synthesizing the antenna. Even resolution much finer than that of a small physical antenna is possible by a narrow synthetic beam that has a relatively long synthetic aperture.
One can also describe azimuth resolution from a Doppler processing viewpoint. The dopler frequency of the echoes is determined from the tar gets position along the flight path.A positive Doppler offset is produced by a target that is ahead of the aircraft and a negative offset Is produced by the ones behind.
Once the aircraft goes to a distance the echoes are resolved into a number of Doppler frequencies. These frequencies help us to determine the azimuth position of the target.
The above provides some information about SAR .But it should be noted that SAR are not as easy as they are explained above. It is generally not possible to transmit short pulses to give us range resolution. Usually we transmit longer pulses that create a complication in the range processing but it is to be noted that this also decreases the peak power requirement. The synthetic aperture is changed even for moderate azimuth resolutions due to the targets range. The computational processing for finer resolution systems are increased because the range and the azimuth processing are coupled there.SPOTLIGHT SAR:
One of the modes of operation of the SAR is the spotlight SAR. It is used for obtaining a high resolution. The higher resolution is obtained by the steering the radar beam at the target for a longer duration thus forming a long synthetic aperture.
When comparing to the conventional Synthetic Aperture Radar that operates in strip mapping mode in which there is a fixed pointing direction, the spotlight SAR gives higher resolution. We can achieve this by making the radar beam to fall on the target for a longer duration of time. This results in a larger synthetic aperture which causes a increase in azimuth resolution. The disadvantage of the spotlight SAR is that it cannot illuminate other areas when it is illuminating a particular area, but this was not in the case of strip map SAR. Usually military aircrafts and satellite makes uses of these spotlight SAR system.
By using the Spotlight SAR we are able to achieve a detailed observation of the target area with thw help of beam steering and also increased length of the aperture.A TYPICAL SAR IMAGE:
Interpreting SAR Images
It is not an easy task to interpret the radar images. We have to know the ground conditions to read the image properly. Generally we can say that the higher the backscattered intensity the rougher is the surface being imaged.
It can be seen that flat surfaces like paved roads, runways or calm water usually appear as dark areas in a radar image. This is because most of the incident radar pulses are specularly reflected away.
Specular Reflection: A smooth surface acts like a mirror for the incident radar pulse. Most of the incident radar energy is reflected away according to the law of specular reflection, i.e. the angle of reflection is equal to the angle of incidence. Very little energy is scattered back to the radar sensor.
Diffused Reflection: A rough surface reflects the incident radar pulse in all directions. Part of the radar energy is scattered back to the radar sensor. The amount of energy backscattered depends on the properties of the target on the ground.
When the sea is calm it appears as a dark image in SAR. But it is not so n the case of rough sea surfaces. Due to the small incidence angle the rough sea surfaces usually appear bright. It is to be noted that oil appears as dark images in SAR. Thus if there is oil on the surface of the rough sea it can be seen as dark patches.
Some images appear moderately bright on the image. These include trees and other vegetation. This is because they are moderately rough in the wavelength scale. The tropical rain forests have a characteristic backscatter coefficient of between -6 and -7 dB and this remains stable in time.
Corner-reflector or double-bounce produces some very bright targets in the image. This is because the radar bounces off the sea towards the target and then it gets reflected from the vertical surface of the target toward the sensor .High-rise buildings, ships at sea are some of the examples of corner - reflection. Some man-made buildings also appear bright due to the same corner reflector effect.
Corner Reflection: When two smooth surfaces form a right angle facing the radar beam, the beam bounces twice off the surfaces and most of the radar energy is reflected back to the radar sensor.
The areas that are filled with soil only can be seen as very dark or very bright depending on the soil roughness and the amount of moisture that is present in the soil. Usually we can say that the soils that are rough and the soils that have more moisture in them usually appear bright.
Wet Soil: The large difference in electrical properties between water and air results in higher backscattered radar intensity.
Flooded Soil: Radar is specularly reflected off the water surface, resulting in low backscattered intensity. The flooded area appears dark in the SAR image.SAR SYSTEMS IN THE CURRENT SCENARIO: SAR Benchmark
A sensor processing chain was simulated by the HPCS Scalable Synthetic Compact Application #3 (SSCA #3). This was proposed in the MIT Lincoln Laboratory. It consist of two parts one of them a front-end sensor processing stage which is responsible for the formation of the Synthetic Aperture Radar (SAR) images and the other is the a back-end knowledge formation stage. This is responsible for the detection on the difference that is seen on the SAR images. This stage generates its own scalable raw data. The main goal of the system is to imitate the most difficult systems that are the computation and the I/O requirements that are found in medical/space imaging, or reconnaissance monitoring which are also embedded systems. Its main aim is to maximize the throughput or in other words to increase the speed in which the answers are generated. The computational kernels and the I/O kernels must perform the functions of keeping up with the copious quantities of the data that are given by the sensors and also streaming the data storage.Block diagram of SAR system benchmark.
Templates of rotated and pixilated letters are generated by the Scalable Data Generator (SDG) and it also performs the function of storing the simulated "RAW" Synthetic Aperture Radar complex returns.
The Sensor Processing Stage loops until the specified number of desired images has been reached. Using matched filtering and interpolation method and also by using the 'raw' SAR data the kernel 1 forms a SAR image. The interpolation and matched filtering involves the match filtering of the Fourier transform against the transmitted SAR waveform. The results are then shifted from polar coordinate to rectangular coordinated. Finally an inverse Fourier transform is taken which makes the SAR image to become visible distinctly. After this the pixilated templates are places at SAR locations that are selected randomly. The work of Kernel 2 is to store each populated image in a grid of random image locations.
Until the desired number of images has been got the knowledge formation stage keeps on looping. Then there is random picking of image by Kernel 3 and the image is read through the entire grid depth. The differences between the pair are computed by kernel 4 and the difference is used to identify the location to produce the next set of changed pixels.