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clutter, the radar beam will scan over the top of long range weather and
distant thunderstorms will remain undetected (see figure 4-75).
The radar horizon is the point where earth’s surface has dropped below
the radar beam and ground return is no longer displayed.
Figure 4-75 TILT Set To Scan Radar Horizon
If common practice is followed and the tilt is adjusted to eliminate the
majority of ground clutter, the radar beam will scan over the top of long
range weather (see figure 4-76). In most cases, eliminating ground
clutter from the radar display limits weather detection to between 120
and 140 NM.
1st Edition, 1st Revision
18 Sep 03 4-79
MULTISCAN OPERATION COLLINS
MultiScan Manual Operation MultiScan™ Radar
Figure 4-76 Long Range Scan With Minimal Down Tilt
To detect long range weather, the radar beam should be adjusted so
that it “peeks” over the radar horizon. Adjusting the tilt so that the radar
beam is centered on the horizon directs the center of the beam towards
the threat weather and allows long range weather to be displayed (see
figure 4-77).
Figure 4-77 Long Range Scan With Increased Down Tilt
One technique that can be used for long range weather detection
is to adjust the tilt downwards until ground clutter first begins to
appear at the radar horizon. Then adjust the beam an additional
2° down. This will position the center of the beam near the
radar horizon. If you are over water and ground clutter is not
present, the approximate tilt angle to center the beam on the
radar horizon can be calculated using the following formula:
Angle to the horizon = – 0.0167 times the square root of the altitude.
1st Edition, 1st Revision
4-80 18 Sep 03
COLLINS MULTISCAN OPERATION
MultiScan™ Radar MultiScan Manual Operation
Tilt Angle to Horizon Settings
Altitude (feet) Angle to Horizon
44,000 – 3.50°
38,000 – 3.25°
32,000 – 3.00°
27,000 – 2.75°
22,000 – 2.50°
18,000 – 2.25°
Figure 4-78 shows the end result. The aircraft is at 23,000 feet. A
down tilt of -2° has been selected by the pilot. Intermediate weather is
masked by the ground, but long range strategic weather is now clearly
visible at 300 NM. The radar horizon is at 186 mile (see formula above
for calculations). Ground clutter is not displayed beyond this point.
Figure 4-78 Weather Return Visible At Edge of Radar Horizon
1st Edition, 1st Revision
18 Sep 03 4-81
MULTISCAN OPERATION COLLINS
MultiScan Manual Operation MultiScan™ Radar
N NOTE
Automatic MultiScan operation uses a low beam that skims the
radar horizon to detect long range weather. Ground returns are
eliminated using Collins’ patented ground clutter suppression
algorithms. The result is optimized long range weather detection
from mid and high altitudes without compromising short and
intermediate range weather detection (♦page 4-22).
GAIN
The WXR-2100 MultiScan radar allows the flight crew to select full
above and below CAL gain control in all modes (automatic and manual).
The CAL gain position sets the radar sensitivity to the standard
calibrated reflectivity levels (♦page 6-4) and is the recommended
position for normal operations.
During low altitude operations where thunderstorm energy is easily
reflected, gain can be momentarily decreased to determine areas of
greatest thunderstorm intensity. During high altitude operations where
thunderstorms exhibit minimal reflectivity, gain can be increased to
better reflect thunderstorm tops.
N NOTE
For information on gain controls and additional operating techniques
see the sections Gain PLUS™ (♦page 4-35) and Gain (♦page 6-4).
THE TOTAL WEATHER PICTURE
In general, an experienced pilot mentally assembles the total weather
picture by combining weather pictures taken at various tilt angles and
with different gain settings. Scanning with various tilt angles allows the
pilot to see weather at different ranges and varying the gain increases
or decreases receiver sensitivity to best respond to the reflective nature
of thunderstorms at the aircraft’s altitude.
1st Edition, 1st Revision
4-82 18 Sep 03
COLLINS MULTISCAN OPERATION
MultiScan™ Radar MultiScan Manual Operation
N NOTE
MultiScan works exactly like an experienced pilot operates the
radar. Multiple scans are taken at different tilt angles in order to
view weather at different ranges. Gain is varied automatically to
compensate for areas of lesser and higher reflectivity. The total
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Collins Weather Radar operator’s guide(26)
