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7.2 Measurement angle
The angle between detector beam axis and the movement axis of the detected vehicles has de-
cisive influence on the measured speed. Microwave detectors and all on the physical „Doppler“
principle based measurement systems (some laser, most infrared and ultrasonic) do not meas-
ure the absolute speed of a vehicle. They measure the relative speed or exactly the angular
speed vector component of a vehicle moving towards the radar beam axis.
This is expressed by following formula:
v
measure
= v
absolute
x cos
with : measuring angle
The ideal measurement angle is 0°, this means
v
measure
= v
absolute
and the movement axis and the
radar beam axis are identical in this case.
In practice the system board can only be mounted laterally, besides the driveway. But you
don’t get a measurement error outside the specified accuracy, if you are aware the limits of the
measurement angle (see chapter 5.2).
Please notice the following:
A too large measurement angle can only have a too small speed value as result!
Up to a measurement angle of 4° (equal to which direction) the relative speed meas-
ured by the detector and the absolute speed are the same through the rounding
of
the not displayed digits.
7.3 Accuracy
The accuracy of the speed value is:
2% and 1 digit
, for the whole measurement range
Herewith the accuracy is better than specified for repressive velocity measurement systems in
the German „Eichordnung“ of the „Physikalisch-Technische Bundesanstalt“ ( 3% for v > 100
km/h and 3 km/h for v < 100 km/h).
Despite this fact, this system is not used in Germany for repressive measurements, because it is
not equipped with a electronic calibration circuitry, what is claimed by the „Eichordnung“.
Also in future we won’t design the system in this way, because the pedagogical intention of
this system for visual speed indication would be mislead.
7.4 Display field and visibility
The display field consists of the 300 mm high LED digits and the 190 mm high digits of the
integrated warning symbols. The chosen letter sizes guarantee readability above 100 Meter.
The angle of readability of the LED digits is with 15° sufficiently high to be seen by the vehicle
drivers with full brightness also in bright sunlight. However for spectators looking from aside
the LED display just seem to glow depending on the viewing angle and may even disappear in
bright sunlight (similar to the fiber optic signs on highways). In common the visibility decreases
for LED with rising brightness of the environment a little bit.
A „phantom light mask“ is placed in front of the LED, which suppress a seemingly glowing of
the LED to a large extent when they reflect the sun light.
