A Synthetic Aperture Radar (SAR) achieves high resolution s cross-range dimension by taking advantage of the motion of the vehicle carrying the radar synthesize the effect of the large antenna aperture.
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 instrumented over relatively immobile targets, but it has seen wide applications in remote sensing and mapping. The imaging of the earth’s surface by SAR to provide a map-like display can be applied to military reconnaissance, measurement of sea state and ocean wave conditions, geological and mineral explorations.
History of Synthetic Aperture Radar
Synthetic Aperture Radar (SAR) was first used by NASA on JPL’s Seasat oceanographic satellite in 1978; it was later developed more extensively on the Spaceborne Imaging Radar (SIR) missions on the space shuttle in 1981, 1984, and 1994. Imaging radar is also planned as part of the NASA/JPL Cassini mission to Saturn in 1997 to map the surface of the ringed planet’s major moon Titan.
Basic Operation of Synthetic Aperture Radar
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 because diffraction requires a large antenna to produce a narrow beam.
The pulse will also be broad in the verticle direction; often it will illuminate the terrain from directly beneath the aircraft out to the horizon. However, f 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 signal returning from a given piece of ground are recorded, and if the aircraft emits a series of pulses as it travels, then the result from these pulses can be combined.
Effectively, the series of observations can be combined just as if they have all been made simultaneously from a very large antenna; this process creates a synthetic aperture much larger than the length of the antenna.
Combining the series of observations is done using Fast Fourier Transform techniques; it requires significant computational resources and is normally done at a ground station after observation is complete. The result is a map of radar reflectivity 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 larger body of experimental results has been accumulated y flying test flights over a known terrain.
Working Principle of Synthetic Aperture Radar
The angular resolution is determined by the beamwidth of the antenna. At a given range, R, the ability to resolve object in the cross-range direction, known as the cross-range resolution, it s calculated by,
ΔR_{cross} = Rθ
Where θ is the beamwidth expressed in radians. This is merely the arc length swept out by the angle θ at radius R. It is also the width of the radar beam at the range R.For example, at 6o beamwidth (0.1 radians) will be 10 m wide at a range of 100m.
For most radar antennas the beamwidth is sufficiently large so that the cross-range resolution is fairly large at normal detention ranges. As such, these systems cannot resolve the detail of the object they detect.
The narrower the beamwidth, the better the resolution. If antenna beamwidth were small as 0.2 degrees, the resolution ΔR_{cross} at a range of 10Km would be about 350 m, which is far greater than the part of a meter resolution possible with the use of pulse compression radar.
Synthetic Aperture Radar (SAR) uses the motion of the transmitter/receiver to generate a large effective aperture as shown in the figure below. In order to accomplish this, the system must store several returns taken while the antenna is moving and then reconstruct them as if they came simultaneously. If the transmitter/receiver moves a total distance ‘S’ during the period of data collection, during which several returns pulses are store, then the effective aperture upon reconstruction is also ‘S’.
The large synthetic aperture creates a very narrow beamwidth which can be calculated by the usual beamwidth formula, substituting the synthetic aperture for the physical antenna aperture.
An aircraft moving with a constant velocity V along a straight path as shown in the figure below.
The radar antenna is mounted in such a manner si that it can radiate in the direction, which is perpendicular to the direction of motion. This type of radar is known as Side Looking Radar (SLR). The position of the radar antenna is represented by Xs, each time a pulse is transmitted. All the received echoes are stored and added up to the last n pulse. The spacing of the element of the synthesized antenna is equal to the distance traveled by aircraft between pulse transmission and dc.
dc = V \times PRT
dc = \frac{V}{PRF}
where,
- PRT = Pulse repetition time
- PRF = Pulse repetition Frequency
- V = Velocity of the aircraft
The beamwidth of a conventional antenna of width (D) at a wavelength is given by,
θ = \frac{k\lambda}{D}
Where k is the constant and depends upon the shape of the current distribution across the aperture. The value of k might vary from 0.9 to 1.3 or even greater. For the easy analysis, the k = 1. By replacing the value we get,
ΔR_{cross} = R . (\frac{k\lambda}{D})
ΔR_{cross} = R . (\frac{\lambda}{D})
If the length of the SAR antenna is L then beamwidth will be,
θ = \frac{k.\lambda}{2L}
Application of Synthetic Aperture Radar
It has seen wide application in remote sensing and mapping. The imaging of the earth’s surface by SAR to provide a map-like display can be applied to military reconnaissance, measurement of sea state and ocean wave condition, geological and mineral explorations.
The space shuttle has also carried synthetic aperture radar equipment, and the Magellan space probe mapped the surface of Venus over several years. The land deformation after a minor earthquake can also be mapped by SAR.
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