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One of the technologies that law enforcement agencies can use to measure the speed of a moving vehicle uses doppler radar to beam a radio wave at the vehicle, and then infer the vehicles speed by measuring the Doppler effect moderated change in the reflected waves frequency. Radar guns can be hand held, vehicle mounted or strategically mounted.

Radar detectors use a superheterodyne receiver to detect these electromagnetic emissions from the radar gun, and raise an alarm to notify the motorist when a transmission is detected. False alarms can occur however due to the large number of devices, such as automatic door openers, that operate in the same part of the electromagnetic spectrum as radar guns.

In recent years some radar detectors have added GPS technology. This allows users to manually store the locations where police frequently monitor traffic, with the detector sounding an alarm when approaching that location in the future this is accomplished by pushing a button and does not require coordinates to be entered. These detectors also allow users to manually store the coordinates of sites of frequent false alarms, which the GPS enabled detector will then ignore. Some GPS enabled detectors can download the GPS coordinates of speed monitoring cameras and redlight cameras from the internet, which are contained in the Trinity database. A traveler from out of state, passing through Arizona for example, would receive an alarm when approaching the location of a speed monitoring camera.



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