Sunday, May 5, 2013

Photo Radar NOT Ready for Prime Time

Photo radar angles the beam across-the-road unlike standard down the road police radar.  Standard down-the-road radar receives a nice clean reflection.  A vehicle in an angled beam reflects a sloppy spread out reflection due to the cosine effect.   Additionally the rotating wheels add to an already excessive unsymmetrical reflection.

Across the road data from the National Institute of Standards and Technology, NIST Technical Note 1398, May 1998. K band radar, Vehicle traveling at 20 mph.

Larger vehicles have a greater spread than smaller vehicles, The faster a vehicle is traveling the greater the reflection spread. The larger the wheels the greater the spread.  Hub cap shape is another factor that contributes to the spread. Vehicle size, shape, speed, wheel diameter and hub cap shape are all factors that determine reflection magnitude and spread.

Photo radar measures relatively close targets, vehicle reflections are relatively strong.  If the receiver sensitivity, range control, is set too sensitive it will be operating in the nonlinear receiver region. The spread out spectrum will generate multiple false signals in the nonlinear region and produce high false speed readings.  

The harmonic problem is common to all receivers and well understood, but difficult to predict.  There is very little empirical data on vehicle reflections from angles photo radar operates. This lack of data makes photo radar unreliable for speed measurements.

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