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9. Real Aperture Radar (RAR)
A narrow beam of energy is directed perpendicularly to the flight path of the carrier platform (aircraft or spacecraft). A pulse of energy is transmitted from the radar antenna, and the relative intensity of the reflections is used to produce an image of a narrow strip of terrain.
Reflections from larger ranges arrive back at the radar after proportionately larger time, which becomes the range direction in the image. When the next pulse is transmitted, the radar will have moved forward a small distance and a slightly different strip of terrain will be imaged.
These sequential strips of terrain will then be recorded side by side to build up the azimuth direction. The image consists of the two dimensional data array.
In this figure, the strip of terrain to be imaged is from point A to point B. Point A being nearest to the nadir point is said to lie at near range and point B, being furthest, is said to lie at far range.
The distance between A and B defines the swath width. The distance between any point within the swath and the radar is called its slant range.
Ground range for any point within the swath is its distance from the nadir point (point on the ground directly underneath the radar).
10. Real Aperture Radar: Range resolution
For the radar to be able to distinguish two closely spaced elements, their echoes must necessarily be received at different times.
In the upper part of the figure, the pulse of length L is approaching buildings A and B. The slant range distance between the two buildings is d.
Since the radar pulse must travel two ways, the two buildings lead to two distinguished echoes if:
d > L/2
The part of the pulse backscattered by building A is PA, and the part of the pulse backscattered by building B is PB.
It appears in the lower part of the figure that to reach the target and come back, PB has covered an extra distance 2d, and thus is at a slightly shorter distance than L behind PA..
Because of this, the end of PA and the beginning of PB overlap when they reach the antenna. As a consequence, they are imaged as one single large target which extends from A to B.
If the slant range distance between A and B were slightly higher than L/2, the two pulses would not overlap and the two signals would be recorded separately.
Range resolution (across track resolution) is approximately equal to L/2, i.e. half the pulse length.
Ground range resolution is:
c speed of light
t pulse duration
q incidence angle
Incidence angle is the angle between the vertical to the terrain and the line going from the antenna to the object.
To improve range resolution, radar pulses should be as short as possible. However, it is also necessary for the pulses to transmit enough energy to enable the detection of the reflected signals.
If the pulse is shortened, its amplitude must be increased to keep the same total energy in the pulse.
One limitation is the fact that the equipment required to transmit a very short, high-energy pulse is difficult to build.
For this reason, most long range radar systems use the "chirp" approach which is an alternative method of pulse compression by frequency modulation.
In the case of the chirp technique, instead of a short pulse with a constant frequency, a long pulse is emitted with a modulated frequency.
The frequency modulation must be processed after reception to focus the pulse to a much shorter value. For the user, the result is the same as if a very short pulse had been used throughout the system.
11. Real Aperture Radar: Azimuth resolution
Azimuth resolution describes the ability of an imaging radar to separate two closely spaced scatterers in the direction parallel to the motion vector of the sensor.
In the animation when two objects are in the radar beam simultaneously, for almost all pulses, they both cause reflections, and their echoes will be received at the same time.
However, the reflected echo from the third object will not be received until the radar moves forward. When the third object is illuminated, the first two objects are no longer illuminated, thus the echo from this object will be recorded separately.
For a real aperture radar, two targets in the azimuth or along-track resolution can be separated only if the distance between them is larger than the radar beamwidth. Hence the beamwidth is taken as the azimuth resolution depending also slant-range distance to the target for these systems.
For all types of radars, the beamwidth is a constant angular value with range. For a diffraction limited system, for a given wavelength l, the azimuth beamwidth b depends on the physical length dH of the antenna in the horizontal direction according to:
b = l/dH
For example, to obtain a beamwidth of 10 milliradians using 50 millimetres wavelength, it would be necessary to use an antenna 5 metres long. The real aperture azimuth resolution is given by:
raz = R* b
raz azimuth resolution
R slant range
For example for a Real Aperture Radar of beamwidth 10 milliradians, at a slant range R equal to 700 kilometres, the azimuth resolution raz will be:
raz = 700 x 0.01
raz = 7 km
Real Aperture Radars do not provide fine resolution from orbital altitudes, although they have been built and operated successfully (for example COSMOS 1500, a spacecraft built by the former Soviet Union).
For such radars, azimuth resolution can be improved only by longer antenna or shorter wavelength. The use of shorter wavelength generally leads to a higher cloud and atmospheric attenuation, reducing the all-weather capability of imaging radars.