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The aircraft is approaching a line of thunderstorms. Although it is not
readily apparent, attenuation is masking weather in the top left hand
area of the display (top left image). As the aircraft penetrates the storm
front, attenuation increases. Storms to the left disappear and storms
in the upper right hand area of the display are diminished (top right
image). Mid way through penetration of the initial storm cell a small
weather return becomes visible in the center of the display (middle left
image). As the aircraft nears the trailing edge of the initial storm cell,
several additional returns become visible (middle right image). At the
trailing edge of the initial cell, the new returns form a new thunderstorm
line (bottom left image). After exiting the first line of thunderstorms, the
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COLLINS HOW RADAR WORKS
MultiScan™ Radar Radar Beam Characteristics
true extent of the previously attenuated weather is apparent (bottom
right image).
Figure 6-30 Radar Displays of Penetration of Thunderstorm
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HOW RADAR WORKS COLLINS
Radome Characteristics MultiScan™ Radar
RADOME CHARACTERISTICS
The radome is an integral part of the radar system and serves as the
radar’s “window” to the outside world. Repeated painting, repairs or
cracks that allow water into the radome honeycomb structure can
significantly lessen the radome’s transmitivity and create extensive
side lobes.
CAT’S EYES/GHOST TARGETS
“Cats’ Eyes” or “Ghost Targets” may appear in a narrow green or green
yellow arc or as two small targets at +/- 45°. They normally occur
between the 4 and 8 mile ranges and when the aircraft altitude is above
approximately 3,000 feet. Cats’ Eyes are ground returns that result from
side lobe or main lobe reflections. The distance to the return will be the
same as the aircraft altitude. It is not uncommon to see Cat’s Eyes
when MAX gain is selected, even with new radomes. However, they
should not be visible in the CAL (calibrated) gain position. If Cat’s Eyes
are visible during CAL gain operation, it is an indication that repeated
painting and/or patching has significantly reduced radome transmitivity.
N NOTE
Because Cat’s Eyes are a result of ground returns, they are not
visible when the aircraft is on the ground.
1st Edition, 1st Revision
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COLLINS HOW RADAR WORKS
MultiScan™ Radar Radome Characteristics
Figure 6-31 Cat’s Eyes Returns on Radar
RADOME INSPECTION
Water can get trapped in the radome honeycomb structure due to small
cracks in the outer skin. Water that accumulates in various parts of the
honeycomb structure can greatly decrease transmitivity. Two methods
can be used to check for the existence of water in the radome:
1. Shine a flashlight through the front of the radome. A second
person behind the radome can identify water trapped in the radome
honeycombs.
2. A Q-meter can be used to check for moisture in the radome.
A general knowledge of the radome’s transmitivity can be ascertained
by inspecting maintenance records to:
1. Ensure excessive patching or repairs have not occurred.
2. Ensure radome has not been subject to repeated painting and has
been painted with non-lead based paint.
1st Edition, 1st Revision
18 Sep 03 6-31
HOW RADAR WORKS COLLINS
Doppler Turbulence MultiScan™ Radar
N NOTE
A windshear radome that has had numerous repairs or had several
coats of paint should be tested at a qualified radome repair facility
in order to ensure the radome meets windshear Class C radome
requirements.
DOPPLER TURBULENCE
Doppler turbulence detection is designed to detect water droplet
horizontal velocities of 5 meters/second (m/s) or greater. However,
water droplet velocity alone does not necessarily indicate an area of
turbulence. Of more importance is the “spectrum” of velocities. In other
words, turbulence is indicated not only by the velocity of the water
droplets but also by how much the water droplets vary in velocity.
In figure 6-32, target A contains several droplets with velocities of 5 m/s
or greater. However, the variance in the velocities is not pronounced
and the radar does not consider this target as turbulent. Target B, on the
other hand, contains droplets with velocities equal to or greater to 5 m/s
plus a wide spectrum of velocities and would be considered turbulent.
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COLLINS HOW RADAR WORKS
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Collins Weather Radar operator’s guide(40)
