Calibration of Speed Enforcement Down-The-Road Radars

IntroductionDown-the-road (DTR) radar (radio detection and ranging) is the most common type of speed measurement device used in the United States and in the rest of the world for traffic speed enforcement.There are approximately 150 000 DTR radar units in use in the United States, and almost every legal jurisdiction in the United States accepts DTR radar for evidentiary purposes in speed enforcement cases in traffic court.Consequently, the performance of these devices must be sufficient to assure the courts' confidence in using these devices.This assurance comes from 1) compliance to established minimum performance requirements, 2) scheduled calibration by testing labs to these requirements, and 3) routine calibration in the field by the radar operator.DTR radars are provided as either hand-held radar "guns," dashboard-mounted units, or similar vehiclemounted units.The DTR radar has three basic components: the transceiver/mixer head, the processing component, and the display/human interface component.DTR radars typically are found in one of two forms, one in which the display and transceiver head are combined into a common unit, and another form in which the two heads are separate but connected by an appropriate electrical interconnect.DTR radars operate within narrowly allocated (by the Federal Communications Commission) spectral bands in one of the following three broad frequency bands: X (nominally 8 GHz to 12 GHz in the United States), K (nominally 18 GHz to 26 GHz in the United States), and Ka (nominally 26 GHz to 40 GHz in the United States).The transceiver head contains the microwave source that illuminates the target vehicle and the mixer that is used to detect the Doppler-shifted microwave signal reflected from the target vehicle.As will be discussed in the next section, the signal output from the mixer circuit (which contains a diode) is at a frequency that is the difference between the Doppler-shifted signal and the incident (illuminating) signal.This difference frequency is much lower than the frequency of the Volume 114, Number 3,