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AOG AOG SA_WXR Users Manual Revision 1.6
http://members.chello.at/addongauges Page - 8 -
4.2 The Beam
Beam Rotation Angle = 160°
Beam Tilt angle from –15° up to 15° in 0.25° steps
Beamwidth is 3.5° (+/- 1.75°)
positive tilt angle
zero tilt angle
negative tilt
AOG AOG SA_WXR Users Manual Revision 1.6
http://members.chello.at/addongauges Page - 9 -
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
settings.
The beam 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.
The beam is not ideally focused, for Collins WXR2100 the beamwidth is 3.5°.
You will see all reflections within this beam triangle. (a triangle is only a
simplified 3db slope)
4.3 Weather attenuation and PAC
It is very important to know that the radar signal gets weaker with increasing
distance from the aircraft. Additional the signal is attenuated by the reflection
and temperature and so on. That means that at high distance a thunderstorm
seems to produce less reflection as in near distance. You can see that
reflections getting more intense as coming closer to the object of interest.
The radar can compensate this effect within 80NM of range, that means within
this range reflections are true an will not change any more due to the
attenuation effects.
This range is called the Path Attenuation Compensation (PAC) range.
A warning is displayed when operating in ranges higher than 80NM to be aware
of this effect.
Radar setup 320 NM with a single thunderstorm coming toward for attenuation
demonstration. Tilt is adjusted to sense the lower third of the thunder cloud.
weak reflections 320NM away reflections get more intense as getting
closer
AOG AOG SA_WXR Users Manual Revision 1.6
http://members.chello.at/addongauges Page - 10 -
In 120-140NM distance first red re- And true image is drawn within the
flections 80NM PAC range.
4.4 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 a algorithm to compensate the ground reflection and so make only weather
data visible. This mode is called the Ground Clutter Suppression (GCS)
AOG AOG SA_WXR Users Manual Revision 1.6
http://members.chello.at/addongauges Page - 11 -
hmm, find the storm in the ground Here it is, switch on GCS
clutter
4.5 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
their high turbulence inside and the relative heavy ice particles (hail). Also
clouds that may develop to a storm.
Other clouds are not really a hazard for aircrafts, and most of them do not
generate any reflections.
To find out thunderstorms on the radar it is important to know that such a
cloud has three precipitation stages.
The lowest third of the cloud consist of turbulent and fast moving heavy
raindrops. They are of course producing very good reflections. So you see a
red area when your beam crosses the bottom third of a cloud.
altitude
1/3
2/3
3/3
AOG AOG SA_WXR Users Manual Revision 1.6
http://members.chello.at/addongauges Page - 12 -
As you increase the tilt and your beam cross the mid third of the cloud, where
the up moving raindrops freeze, you have also good reflections, but the area is
much smaller than in the bottom third. This because of the shape of a thunder
cloud.
　
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