8.2 Interference Sources
When properly installed and operated, Doppler radar technology is
extremely accurate and reliable. However, variations in the environment
can cause situations and circumstances which can cause spurious (erratic
and unusually low or high) speeds to display. Signs that a speed is spurious
can include the following characteristics:
• A reading appears when no target is in the operational range of the
antenna.
• A target entering the operational range overrides the interference
signal, causing the displayed speed to change suddenly.
• Speeds are irregular and do not provide a valid reading.
8.2.1 Angular Interference (Cosine Error Effect)
The cosine error effect causes the radar device to display a speed, which is
lower than the actual pitch-release speed. This condition exists whenever
the target’s path is not parallel to the gun’s antenna. As the angle between
the beam of the antenna and the target increases, the displayed speed
decreases. An angle of zero (0) is best.
The following table shows the effect that an increasing angle has on a
displayed speed.
Horizontal Angle Degrees
0˚ 1˚ 3˚ 5˚ 10˚ 15˚ 20˚ 30˚ 45˚ 60˚ 90˚
Actual Speed
Displayed Speed
31 mph
31 30 30 30 30 30 29 27 22 16 0
41 mph
41 40 40 40 40 39 38 35 28 20 0
50 mph
50 49 49 49 49 48 46 43 35 25 0
56 mph
56 55 55 55 55 54 52 48 40 28 0
63 mph
63 61 61 61 61 60 58 54 44 31 0
69 mph
69 68 68 68 67 66 64 59 48 34 0
Table 8.2.1b: Actual and displayed speeds at different antenna-to-target angles.
Figure 8.2.1a
An angular error occurs when the target’s path is not parallel to the radar antenna
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