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1st Edition, 1st Revision
6-12 18 Sep 03
COLLINS HOW RADAR WORKS
MultiScan™ Radar Antenna Characteristics
some “look down” capability below the aircraft and can help prevent
inadvertent thunderstorm top penetration at high altitudes (figure 6-13).
Figure 6-13 Parabolic Radar Antenna Beam Patterns
Flat-plate antennas produce a much more focused beam of energy with
small side lobes. Since nearly all of the energy is focused in the main
beam, this kind of radar produces better target definition (i.e., a more
accurate display of precipitation). Ground clutter is significantly reduced
and forward-looking windshear applications are possible. However, due
to the fact that the side lobes do not provide any “look down”
capability, tilt control becomes much more important in preventing
inadvertent thunderstorm top penetrations (figure 6-14).
Figure 6-14 Flat-Plate Radar Antenna Beam Patterns
1st Edition, 1st Revision
18 Sep 03 6-13
HOW RADAR WORKS COLLINS
Antenna Characteristics MultiScan™ Radar
ANTENNA SIDE LOBES AND FALSE WINDSHEAR
WARNINGS
Echoes from strong targets detected by antenna side lobes (figure 6-15)
can mimic a microburst signature and contribute to false alerts. Side
lobes occur with all radar antennas but may be more pronounced if the
antenna is damaged or the radome is degraded (♦page 6-31). Figure
6-16 shows a main radar beam with minimal side lobes. Figure 6-17
shows an antenna that has been modified to simulate severe damage.
The result is extensive side lobes. The effects of side lobes can be
reduced with precision antenna manufacturing. For instance, Collins’
windshear antenna reduces side lobes 10-15% below those of previous
generation non-windshear antennas.
Figure 6-15 Weather Radar Antenna Side Lobes
1st Edition, 1st Revision
6-14 18 Sep 03
COLLINS HOW RADAR WORKS
MultiScan™ Radar Antenna Characteristics
Figure 6-16 New Radar Antenna Showing Minimal Side Lobes
Figure 6-17 Degraded Radar Antenna Showing Extensive Side Lobes
1st Edition, 1st Revision
18 Sep 03 6-15
HOW RADAR WORKS COLLINS
Antenna Characteristics MultiScan™ Radar
ANTENNA CALIBRATION
Air transport ARINC 708A radars are bore sighted using laser calibration
at the aircraft OEM’s facility during installation. The radar contains bore
sight and incremental sensors that detect variation from the original
factory setting. If the sensors detect a variation in the tilt or horizontal
angle, an antenna fail message is sent to the display. In other words, it
is not necessary to attempt a manual calibration with ARINC 708A
radars. The tilt that is displayed on the display will be accurate.
N NOTE
If the tilt displayed on the EFIS and the control knob tilt disagree,
the displayed EFIS tilt is the correct indication.
1st Edition, 1st Revision
6-16 18 Sep 03
COLLINS HOW RADAR WORKS
MultiScan™ Radar Antenna Characteristics
ANTENNA STABILIZATION
For the WXR-2100, antenna stabilization (roll and pitch axis) is always
active. Stabilization enables the antenna to maintain a sweep parallel
to the horizon regardless of the aircraft’s attitude (aircraft image, figure
6-18). If ground clutter is set in the outermost range scale (radar image,
figure 6-18), stabilization will keep the clutter ring equal across the
display even when the aircraft is maneuvering.
Figure 6-18 Stabilized Radar Antenna Orientation versus Aircraft
Attitude
1st Edition, 1st Revision
18 Sep 03 6-17
HOW RADAR WORKS COLLINS
Antenna Characteristics MultiScan™ Radar
If for any reason stabilization should fail, the radar antenna defaults to
a sweep that is parallel to the wings. If an aircraft with an unstabilized
antenna makes a turn, the antenna will scan towards the sky during
half of the sweep and towards the earth for the other half of the sweep
(aircraft image, figure 6-19). When the antenna is pointed towards the
earth it will “dig” into the ground and produce a red “cone” of ground
clutter (radar image, figure 6-19).
Figure 6-19 Unstabilized Radar Orientation versus Aircraft Attitude
If a combination of antenna tilt, aircraft roll, and aircraft pitch exceeds
45°, then the stabilization stays at the 45° limit. As the total decreases
below the 45° limit, normal stabilization operation resumes.
1st Edition, 1st Revision
6-18 18 Sep 03
COLLINS HOW RADAR WORKS
MultiScan™ Radar Radar Beam Characteristics
RADAR BEAM CHARACTERISTICS
The MultiScan radar uses a 28-inch antenna that produces a 3.5°-wide
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Collins Weather Radar operator’s guide(37)
