While they can be affected by poor reception or noise in the radio signal, GPS-based speedometers are generally considered the benchmark for measuring velocity. On the other hand, radar measurements are also considered very accurate, as long as the human at the other end of the device is correctly trained and is tracking the proper vehicle. At stake is the credibility of radar. As Doppler-based speed enforcement has become a primary source of revenue for law enforcement agencies nationwide, police don't want their technology challenged. With that in mind, the local Petaluma police have spent a reported $15,000 on the trial. The case is to be decided in the coming weeks.
[Source: Christian Science Monitor]