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beam. As the beam sweeps through a thunderstorm at close range, it
takes a “slice” out of the target and then displays that slice on the radar
display (figure 6-20). The displayed weather presentation can change
significantly based on the selected radar tilt and from where in the storm
the slice is taken from (♦page 5-7).
Figure 6-20 Weather Radar Beam “Slice” and Resulting Display
1st Edition, 1st Revision
18 Sep 03 6-19
HOW RADAR WORKS COLLINS
Radar Beam Characteristics MultiScan™ Radar
BEAM DIAMETER
A 28-inch flat-plate antenna produces a 3.5°-wide beam. At ranges
less than 80 NM, this produces a fairly narrow and well-focused beam.
Beyond 80 NM, the beam diameter increases until at 300 NM it is
equal to 105,000 feet (figure 6-21). To put this into perspective, at this
distance, it would take a storm cell over 22 NM tall and wide to fill the
beam.
Because the beam remains fairly focused within 80 NM of the aircraft
(and for reasons that will be mentioned in the next section), it is
recommended that weather evaluation be done only when the weather
is within 80 NM of the aircraft. Beyond 80 NM, the radar should be used
primarily for strategic planning and weather avoidance.
The following formula can be used to calculate the approximate beam
width at any range:
Beam width (in feet) = (Distance in NM + “00”) x 3.5
For example, to determine the width of the radar beam at 50 NM out
from the aircraft, take the 50 NM distance and then add “00” to it for a
result of 5,000. Multiply this figure by 3.5 to yield an approximate beam
width of 17,500 feet at 50 NM.
Figure 6-21 Weather Radar Beam Width Increases Over Distance
1st Edition, 1st Revision
6-20 18 Sep 03
COLLINS HOW RADAR WORKS
MultiScan™ Radar Radar Beam Characteristics
RANGE AND AZIMUTH RESOLUTION
Range and azimuth resolution are affected by the length of the pulse
width and the width of the radar beam, respectively. For long-range
weather detection, the radar uses a longer pulse width to put more
energy on the target. The longer pulse can cause targets to merge into
a single target due to the fact that the front of the pulse may already
be in contact with the next target before the trailing edge of the pulse
leaves the previous target (see figure 6-22). Thus, the pulse appears to
be painting one continuous target. Shorter pulse widths are used for
close range targets and are thus able to distinguish more precisely
between the different weather targets. Often on the display, this is
manifested by more “blocky”-looking weather at extended ranges and a
more refined weather picture at shorter ranges.
1st Edition, 1st Revision
18 Sep 03 6-21
HOW RADAR WORKS COLLINS
Radar Beam Characteristics MultiScan™ Radar
Figure 6-22 Range Resolution versus Azimuth Resolution
In a similar manner, long-range weather targets can be merged into a
single target due to the large beam width diameter at extended ranges.
In this case, the leading edge of the beam comes in contact with a new
target before the trailing edge of the beam leaves the previous target
(see figure 6-29). As the aircraft nears the weather targets, the beam
narrows and the leading edge of the beam will not contact the next
target until the trailing edge has left the previous target. Thus it is not
unusual to see a storm cell separate into two cells as it nears the aircraft
and the beam becomes narrow enough to distinguish between them.
1st Edition, 1st Revision
6-22 18 Sep 03
COLLINS HOW RADAR WORKS
MultiScan™ Radar Radar Beam Characteristics
BEAM ATTENUATION
Significant attenuation of the radar signal due to absorption and
scattering occurs as the transmitted pulse moves to its furthest range
and again during transit back to the receiver from a radar target. In
addition, beyond 80 NM a normal thunderstorm (defined as a 3-NM
sphere of water) no longer fills the radar beam. As a consequence,
significant amounts of radar energy bypass the target entirely (figure
6-23). The end result is that, for weather targets detected at extended
ranges, the signal received back at the aircraft is significantly weaker
than the original radar pulse. In the case of previous generation
radars, the effects of attenuation were visible as a distant thunderstorm
approached the aircraft. The storm would tend to be green at longer
ranges but steadily grow in intensity until it turned red close into the
aircraft (see figure 6-24).
Figure 6-23 Radar Beam Attenuation
1st Edition, 1st Revision
18 Sep 03 6-23
　
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本文鏈接地址：Collins Weather Radar operator’s guide(38)