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Contents -- Alignment Speed Errors | Speed Measurement Uncertainty

Operation

The antenna must be located such that traffic is within the minimum and maximum design limits. Too close and traffic passes through the beam too fast, too far and reflected signals may be too weak or processing algorithm errors occur. The camera must be aligned for approaching or receding traffic.

Setup Requirements
Antenna Distance From Traffic Lanes: Minimum distance 18 feet
Maximum distance 50 feet
Antenna Height above Ground: 3 - 6 ft relative to traffic lanes
Camera Alignment: Set to photograph approaching or receding traffic

 Alignment Speed Errors

Antenna alignment effects accuracy, pointing into traffic, greater align angle, causes low speed readings and pointing away from traffic, smaller angle, causes high speed readings.

Alignment Error

Radar Calculated Speed: vc = vm / cos ß
Radar Measured Speed: vm = vo cos (ß ± ßerr)

 ß = alignment angle ± ßerr = alignment angle error vc = radar calculated speed v = traffic speed

Alignment and Speed

 Speed Measurement Uncertainty

Measured speed is a function of vehicle speed, alignment angle and beamwidth. Speed measurements are spread, the cosine effect, as an object passes through the angled beam.

vm = v cos ( ß ± ßw / 2 )
 ß = alignment angle ± ßerr = alignment angle error vm = radar calculated speed v = traffic speed

Table 4.1-1 -- Speed Error Spread for a 5° Beamwidth
Design
Alignment
Close
Beam Edge
Beam
Center
Distant
Beam Edge
Speed
19.5° -7.3 % -5.7 % -4.4 % 2.91 %
20.0° -7.6 % -6.0 % -4.6 % 2.98 %
20.5° -7.9 % -6.3 % -4.9 % 3.06 %
21.0° -8.3 % -6.6 % -5.2 % 3.13 %
21.5° -8.6 % -7.0 % -5.4 % 3.20 %
22.0° -9.0 % -7.3 % -5.7 % 3.27 %
22.5° -9.4 % -7.6 % -6.0 % 3.34 %
23.0° -9.7 % -7.9 % -6.3 % 3.41 %
23.5° -10.1 % -8.3 % -6.6 % 3.48 %

In reality the reflected spectrum has a greater spread than can be accounted for by the cosine effect and vehicle speed. Empirical data was taken using a radar with a 5° beamwidth angled 20° across the road against a 20 mph vehicle. The cosine effect predicts a spread of 18 - 19 mph, however the data has a spread of 13 - 21 mph. The difference is due to target vehicle wheel rotation adding Doppler reflections.