Side Looking Airborne Radar (SLAR)
Figure 1: Side-looking viewing geometry of imaging radar system.
Side Looking Airborne Radar (SLAR)
Figure 1: Side-looking viewing geometry of imaging radar system.Swath width refers to the strip of the Earth’s surface from which data are collected by a side-looking airborne radar. It is the width of the imaged scene in the range dimension. The longitudinal extent of the swath is defined by the motion of the aircraft with respect to the surface, whereas the swath width is measured perpendicularly to the longitudinal extent of the swath.
The SLAR is a real aperture radar primarily. This requires a reasonable large antenna for adequately angular resolution. The azimuth resolution, Ra, is defined as
| Ra = | H · λ | H is the height of the antenna, (height of the airplane) L is the geometric length of the antenna, λ is the wavelength of the transmitted pulses, and θ is the incidence angle | (1) |
| L · cos θ |
Figure 2: Resolution cell variation.
Figure 2: Resolution cell variation.The equation shows, that with increasing altitude decreases the azimuthal resolution of SLAR. A very long antenna (i.e., large L) would be required to achieve a good resolution from a satellite. Synthetic Aperture Radar (SAR) is used to acquire higher resolution.
The size of the ground resolution cell increases on the side of the nadir as the distance between radar platform and the ground resolution cell increases. This means that the ground resolution cells are larger towards the edge of the image than near the middle. This causes a scale distortion, which must be accounted for.
At all ranges the radar antenna measures the radial line of sight distance between the radar and each target on the surface. This is the slant range distance. The ground range distance is the true horizontal distance along the ground corresponding to each point measured in slant range. The cross-track resolution, Rr, is defined as:
| Rr = | c0 · tp | c0 is the speed of light tp is the pulse duration of the transmitter, and θ = incidence angle | (2) |
| 2 sin θ |
For an SLAR with the following characteristics:
λ = 1 cm,
L = 3 m,
H = 6000 m,
θ = 60°, and
tp = 100 ns,
has got a resolution of
Ra = 40 m and
Rr = 17.3 m
λ = 1 cm,
L = 3 m,
H = 6000 m,
θ = 60°, and
tp = 100 ns,
has got a resolution of
Ra = 40 m and
Rr = 17.3 m
The platform (aircraft or satellite) of an side-looking airborne radar (SLAR) travels forward in the flight direction with the nadir directly beneath the platform. The microwave beam is transmitted obliquely at right angles to the direction of flight illuminating a swath. Range refers to the across-track dimension perpendicular to the flight direction, while azimuth refers to the along-track dimension parallel to the flight direction.
Side-Looking Airborne Radar (SLAR) is an aircraft- or satellite-mounted imaging radar pointing perpendicular to the direction of flight (hence “side-looking”). A squinted mode is possible also. SLAR can be fitted with a real aperture antenna (Real Aperture Radar, RAR) or an antenna using synthetic aperture (SAR).
The platform of the radar moves in direction of the x-axis. The radar “looks” with the looking angle θ (or so called off-nadir angle). The angle α between x-axis and the line of sight (LOS) is called cone angle, the angle φ between the x-axis and the projection of the line of sight to the (x; y)-plane is called azimuth angle. Cone- and azimuth angle are related by cosα = cosφ ∙ cosε. On the earth surface the wave comes in at the (nominal ellipsoidal) incident angle β with respect to the vertical axis at this point. (In some publications the incident angle is denominated to as θi.) The antenna illuminates an area, the so called footprint. The direction of the incoming wave relative to the horizontal plane may be measured also. This angle γ = 90° − β is called grazing angle. The angle ϑ = ε + 90° is used for a mathematical description in a spherical coordinate system.
For the approximation of a flat earth – which is usual for airborne radar with short to medium range – the grazing angle and the depression angle can be assumed to be equal γ = ε and the incident angle is β = 180° – ϑ. The so called LOS-vector is a unit vector
(in the figures shown as a red arrow) pointing from the antenna to a ground scatterer. The variables u, v, w are directional cosines with respect to the x; y; z axes. The variable u is u = cosα with α as the azimuth angle between the line of sight and the x-axis (direction of flight)..png)
Range resolution (across track)The range resolution (the ability to separate the pixels of the image perpendicular to the direction of flight) of an SLAR depends on the length of the transmitted pulse. At the ground of Earth the range resolution has got an inverse relationship with the depression angle:
is typically 0.4 … 1 µs, i.e. 
= 8 … 200 m. The shorter the pulse width the lower and the higher the range resolution, but the lower the echo signal. This limitation can be overcome using intra-pulse modulation. Using a step-frequency waveform of bandwidth B the range resolution is 
.
-
= speed of light
= depression angle
is typically 0.4 … 1 µs, i.e. = 8 … 200 m. The shorter the pulse width the lower and the higher the range resolution, but the lower the echo signal. This limitation can be overcome using intra-pulse modulation. Using a step-frequency waveform of bandwidth B the range resolution is .Azimuthal resolution (along track)
The azimuthal resolution (better known to as crossrange resolution) depends on the beamwidth of the radar antenna. It is derived from the ratio of the physical size of the antenna (the real aperture) to the wavelength used. By the spreading of the beam it is also dependent on the slant range.
-
= wavelength
= antenna length (in direction of flight)
= slant range
= height of the platform
http://www.radartutorial.eu/20.airborne/ab06.en.html
http://www.geo.hunter.cuny.edu/terrain/radarii.html
http://www.geo.hunter.cuny.edu/terrain/radarii.html
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