AirborneWeather Radar 13
Principle of Operation
Primary Radar
AWR is a primary radar and both of its functions, weather detection and ground mapping, use the echo principle to depict range and the searchlight principle to depict relative bearing of the targets. For this purpose range lines and azimuth marker lines are available (see Figure 13.2). It should be noted that the range of ground targets obtained from the display will be the slant range and the Pythagoras formula should be used to calculate the ground range.
Antenna
The radar beam is produced by a suitable antenna in the nose of the aircraft. The antenna shape can be parabolic or a flat plate which produce both a conical or pencil-shaped beam as well as a fan-shaped or cosecant squared beam. The type of radiation pattern will depend upon the use; the pencil beam is used for weather and longer range (> 60 NM) mapping while the fan-shaped beam is used for short range mapping. It is usually necessary to tilt the antenna down when using the radar in the mapping mode. The radar antenna is attitude-stabilized in relation to the horizontal plane using the aircraft’s attitude reference system otherwise the presentation would become lopsided during manoeuvres.
Radar Beam
The pencil beam used for weather depiction has a width of between 3° and 5°.
The beamwidth must be as narrow as possible for efficient target resolution. For example, two clouds at say 100 NM might appear as one large return until, at a closer range, they are shown correctly in Figure 13.5, as separate entities.
A narrower beam would give better definition but would require a larger antenna which becomes impractical in an aircraft. Therefore, in order to produce the narrower beams it is essential to use shorter wavelengths.
Figure 13.5 Effect of Beamwidth
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Radar Frequency
The optimum radar frequency is one that has a wavelength comparable to the size of the objects which we wish to detect, namely the large water droplets and wet hail which in turn are associated with severe turbulence; these droplets are about 3 cm across.
The typical frequency adopted by most commercial systems is 9375 MHz, +/- 30 MHz as it produces the best returns from the large water droplets and wet hail found in convective clouds. With this frequency it is also possible to produce narrow efficient beams. The wavelength, λ is:
λ = |
300 |
m |
= 3.2 cm |
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9375 |
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A frequency higher than 9375 MHz would produce returns from smaller droplets and cause unnecessary clutter whereas a lower frequency would fail to produce sufficient returns to highlight the area of turbulence.
Radar Weather Airborne 13
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54 000 |
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24 000 |
9000 |
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3° |
0 NM |
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30 NM |
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80 NM |
180 NM |
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Figure 13.6 Radar beam coverage at varying ranges
Water and Ice in the Radome
Some of the energy of the radar waves is absorbed by water and ice as happens in a microwave oven. If there is water in the antenna radome or ice on the outside of it, the energy absorbed will cause the water to evaporate and the ice to melt. This means that less energy is transmitted in the forward direction resulting in weaker returns and a degradation of performance.
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AirborneWeather Radar 13
Weather Depiction
The equipment is designed to detect those clouds which are likely to produce turbulence, to highlight the areas where the turbulence is most severe and to indicate safe routes to avoid them, where possible.
The size and concentration of water droplets in clouds is an indication of the presence of turbulence (but not of clear air turbulence - CAT). The shorter the distance, in continuous rainfall, between light and strong returns, the steeper the rainfall gradient and the greater likelihood of turbulence. Figure 13.7 depicts the reflective levels of different precipitation types. For a given transmission power a 3 cm wavelength will give the best returns from large water droplets. Wavelengths of 10 cm and above produce few weather returns.
Airborne Weather Radar 13
Figure 13.7 Reflective levels
In colour weather radar systems the weather targets are colour-coded according to the intensity of the rainfall as follows:
BLACK |
Very light or no returns |
Less than 0.7 mm/h. |
GREEN |
Light returns |
0.7 - 4 mm/h. |
YELLOW |
Medium returns |
4 - 12 mm/h. |
RED |
Strong returns |
Greater than 12 mm/h. |
MAGENTA |
Turbulence |
Due to rainfall intensity. |
On colour systems without magenta the RED areas may have a CYCLIC function, which causes them to alternate RED/BLACK in order to draw the pilot’s attention.
The areas of greatest potential turbulence occur where the colour zones are closest together i.e. the steepest rainfall gradient. Also turbulence is associated with the following shapes on the display as shown in Figures 13.8 - 13.11: U-shapes, Fingers, Scalloped edges and Hooks. These are areas to avoid.
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Figure 13.8 U-shape indicating hail activity
Figure 13.9 Finger indicating hail activity
Figure 13.10 Scalloped edge indicating hail activity
Figure 13.11 Hook indicating hail activity
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