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HOW RADAR WORKS COLLINS
Frequency Comparison MultiScan™ Radar
The block diagram shown in figure 6-2 identifies the major components
of the WXR-2100 weather radar system.
Figure 6-2 WXR-2100 Weather Radar System Block Diagram
FREQUENCY COMPARISON
A comparison of several different frequencies provides a better
understanding of both the capabilities and limitations of radar. Consider,
for example, the warning buoy pictured in figure 6-3. The bell or
horn in the buoy emits a low-frequency audio signal that penetrates
all forms of weather, which is just what you would expect since its
primary job is to be heard and warn of impending danger. Medium
and high frequencies such as those emitted by airborne HF radios
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COLLINS HOW RADAR WORKS
MultiScan™ Radar Frequency Comparison
also experience minimal interference from precipitation (figure 6-4).
However, very high frequencies (VHF) begin to be scattered or reflected
by very heavy precipitation (figure 6-5). This is the reason that pilots
sometimes experience static on their VHF radios during very heavy
rain. When the frequency is high enough that it begins to be affected
by weather, it then becomes possible to start considering weather
detection and interpretation.
Figure 6-3 Low Audio Frequency Propagation Through Weather
Figure 6-4 Medium and High Frequency (MF/HF) Propagation
Through Weather
Figure 6-5 Very High Frequency (VHF) Propagation Through
Weather
Radar, which is an acronym for Ra(dio) D(etecting) A(nd) R(anging),
utilizes high frequency transmissions that are optimized to reflect a
particular type of target. For instance, air controllers use primarily L
- Band radars that do a good job of penetrating most weather and
detecting aircraft (figure 6-6). This makes sense since the controller’s
primary job is aircraft separation, not weather detection and avoidance.
Since an air controller’s radar detects only the heaviest precipitation, it
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HOW RADAR WORKS COLLINS
Gain MultiScan™ Radar
is also the reason pilots should take seriously a controller’s warning of
weather in the aircraft flight path.
Most airborne weather radars operate in the X-Band and are specifically
designed to both penetrate and reflect weather. In figure 6-6, the
reflection of light rain can be equated with green on the radar display.
Moderate rain can be equated with yellow and heavy rain can be
equated with red. Note, however, the compromise that is involved.
In order to be able to penetrate heavy rain, radar will not be able to
detect fog, very light rain or relatively dry clouds. On the other end of
the spectrum, radar’s ability to reflect light rain may prevent the radar
beam from penetrating heavy rain and thus may mask weather behind
a storm cell (♦page 6-27).
Figure 6-6 Radar Operating Bands
N NOTE
The L, S, C, X and K Band designations for radar frequencies have
no intrinsic meaning. These letters were chosen at random during
Word War II to represent the different frequency ranges.
GAIN
CALIBRATED GAIN COLOR SCHEME
At a basic level, weather radar measures the amount of moisture
present in the atmosphere. Calibrated gain within the weather radar’s
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COLLINS HOW RADAR WORKS
MultiScan™ Radar Gain
circuitry associates these different amounts of moisture (or rainfall
rates) with a particular color level on a weather radar display (see
figure 6-7). For instance, green represents a weak rainfall rate of 0.03
to 0.15 inches/hour (in/hr), while red represents a rainfall rate that is
greater than 0.5 in/hr. Note that black is also a color level. Black on
a weather radar display does not mean that weather is not present
(although this may be the case), it simply means that the rainfall rate is
less than 0.03 in/hr.
Also note that each color level represents a change of 10 dBz (green
is 20 dBz, yellow is 30 dBz, and red is 40 dBz or greater). Therefore,
changing the gain by 10 dBz above or below the CAL setting will
change the display by one color level.
Magenta represents turbulent airflow that, in essence, represents
variations in raindrop movement of greater than 5 meters/second.
Doppler turbulence detection is described in detail later in this section
(♦page 6-32).
Figure 6-7 Calibrated Gain Color Scheme
GAIN CONTROL SETTINGS
The GAIN control allows manual adjustment of the radar sensitivity for a
more detailed assessment of weather conditions. The Calibrated (CAL)
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Collins Weather Radar operator’s guide(35)
