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PRIMUS
r
880
Digital
W
eather
Radar
System
A28-1146-102-00
Radar Facts
5-4
When the antenna is tilted downward for ground mapping, two
phenomena mayoccur that can confusethe pilot. Thefirst iscalled ”The
Great Plains Quadrant Effect” that is seen most often when flying over
the great plains of central United States. In this region, property lines
(fences), roads, houses, barns, and power lines tend to be laid out in
a stringent north-south/east-west orientation. As a result, radar
returns from these cardinal points of the compass tend to be more
intense than returns from other directions and the display shows these
returns as bright north/south/east/west spokes overlaying the ground
map.
The second phenomenon is associated with radar returns from water
surfaces (generally called sea clutter), as shown in figure 5-3. Calm
water reflects very low radar returns since it directs the radar pulses
onward instead of backward (i.e. the angle of incidence from mirrored
light shone on it at an angle). The same is true when viewing choppy
water fromthe upwind side. The downwind side ofwaves, however, can
reflect a strong signal because of the steeper wave slope. A relatively
bright patch of sea return, therefore, indicates the direction of surface
winds.
REFLECTION
CALM WATER OR WATER WITH
SWELLS DOES NOT PROVIDE
GOOD RETURN.
CHOPPY WATER PROVIDES
GOOD RETURN FROM
DOWNWIND SIDE OF WAVES
WIND DIRECTION AT
SURFACE OF WATER
PATCH
OF SEA
RETURNS
AD-12056-R2@
Sea Returns
Figure 5-3