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Radar energy disturbed
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As the beam continuously travels from right to left and back we get a complete image
of water reflections in front of us.
4.2 What produces reflections?
In an aviation based weather radar, which is much less sensitive than a
meteorological weather radar, only water and wet hail produces reflections.
I found this image in the Collins WXR2100 manual which says it best:
As you will see later, ground obstacles also will produce reflections, if the beam is
focused on them.
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4.3 The beam
Beam Rotation Angle = 160°
Beam Tilt angle from –15° up to 15° in 0.25° steps
Beam width is 3.5° (+/- 1.75°)
The beam is rotated by 160° from left to right and back, and while rotated it
sends continuously pulses out and receive them. So the beam covers a large
lateral area in front of the aircraft. This movement can not be controlled by the
pilot. The speed of the movement depends on the range and sensitivity
positive tilt angle
Zero tilt angle
negative tilt
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settings. (In the FSWXR it needs 160 frames for one rotation, which needs
about 8 seconds@ 18fps as in reality)
The beams tilt angle need to be set by the pilot. Only in OFP Mode the tilt is
swept automatically (see later). The tilt is independent of the pitch angle that is
automatically compensated.
With the tilt you can vary the sensing altitude of at a certain point of interest.
Since tilting is a little complex, it is described in the “usage” section.
The beam is not ideally focused, for Collins WXR2100 the beam width is 3.5°.
You will see all reflections within this beam triangle. (A triangle is only a
simplified 3db slope).
4.4 Attenuation effects
Attenuation is one of the major effects that have to be considered by the pilot.
Well understanding of the following section is mandatory!
Radar shadowing:
If the beam hits dense water bodies (almost thunderstorms) a great amount of the
beam energy is disturbed. As you see, the green arrow is the remaining beam
energy after the first particle. It is the fully disturbed at the next particle, leaving no
energy for the last particle.
So the first two particles are drawn on the screen but the last one is not, because
it is not sensed.
That is important to know! Imagine there is a heavy thunder cloud followed by an
other. The second one is not displayed, since the first one disturbs all the energy.
Examples are shown later below.
Path attenuation:
Since the radar beam gets weaker by distance, the reflected energy of far
particles is lower than that of near particles. That means that the same particle
seems to be less dense at high distances. This could cause that a thunderstorm
be displayed in green, becoming red as it gets closer.
The WXR-2100 radar has a path attenuation compensation within 80NM, that
means within 80NM it all particles are displayed in their real colour.
A warning is displayed when operating at distances greater than 80NM.
Radar beam
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4.5 Ground clutter reflections
As the beam is hits the ground, it is very well reflected from water, metal
concrete and so on. So ground reflections are mostly shown as yellow and red
reflections.
In the example above the Thunderstorm is sensed by the radar, but it is also
surrounded by red and yellow reflections from the mountain. So increasing the
tilt would solve the problem. But as you see later, thunderstorms are only
visible at the lower 2/3 of the cloud, the top 1/3 is invisible for radar. So a tilt
increase would maybe also eliminate the storm on the display. Collins found
out an algorithm to compensate the ground reflection and so make only
weather data visible. This mode is called the Ground Clutter Suppression
(GCS)
Without GCS and finally with GCS
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4.6 Meteorological Basics
What can be displayed on the radar? As said above only wet particles
(raindrops, wet hail) produce good reflections. Now it is time to interpret
different precipitation densities and reflections.
Aviation based weather radars are as sensible to show only reflections that
are relevant for flight operations. Meteorological weather radars are much
more sensitive which may display nearly everything that’s in the sky.
But aviation related weather phenomena are of course thunderstorms due to
　
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