You beat me to it..... I was the International Technical Support Manager for the European Provida range of products and installed both that system with Stalker Radar and trained the Police in Sofia, Bulgaria on the use of them.... did the Provida system all over Europe as well as other places.... my ex-boss is now the sales manager for Stalker in Europe....
The Stalker could be used when mobile to doubly verify a target speed which was being calculated by the T&D system on the Provida...... you didn't need road markings, the target always set the distance between two bridges or light poles or marks on the road, the host vehicle operator set the distance over the same distance as the target.... and measured the time separately for the target, a sensor on either a shaft or off the pulses from the speed detection for the speedometer were used for speed measurement... which also gives distance by calculating pulses against time...
System calibrated over an accurately pre-measured distance for pulses per kilometre or mile....
All accompanied by video showing either Provida speed check or Stalker speed check....
You could also park up and measure speed over a distance between marks on the road, the distance between which was known and entered into the system.... no active radar, lidar or anything else required.....
Like shelling peas...... drivers would often plead innocence right up to being shown their performance on review...... and only one review was permitted before the tape was removed, marked and bagged for evidence in case it went to court.....
If there was a dispute over accuracy the PLL (phase locked loop) for the video frame rate could be used as a stopwatch for intervals down to 1/25 of a second. A frame counter was included on screen for this purpose, all in screen graphics were white on black semi-transparent backgound, you could see the picture through this, it prevented tampering.....
Far more acceptable in court, if it got that far for normal speeding....
It was also good for driving offences, as well as turning the camera into the car when the offender was in the back to record the subsequent conversation...
Bazinga!