The Doppler Principle and Radar
To understand how the radar gun determines the speed of a moving vehicle one must first understand relative motion.
Patrol Reflected Frequency
- Relative motion is the
changing distance between two objects. In other words, it is the relative
speed between these objects.
- Stationary Targets - A radar signal reflected off a stationary object (no relative motion) will return at the same frequency as it was transmitted.
Some Police Radars, such as Decatur GVP-D Genesis VersaPak Directional Police Radar Gun, feature Direction Selectivity (Isolates Traffic in One Direction)
and can display Approaching or Receding Targets, or Both!
- Moving (Approaching) Target - A radar signal reflected off an object which is moving towards the transmitter/ receiver will be compressed. The returned frequency (received by the radar antenna) will be higher than the frequency which was transmitted by the same antenna. The amount of change is electronic component, located in the radar antenna, called the mixer diode.
- Moving (Receding) Target - Conversely, a radar signal reflected off an object moving away from the transmitter/receiver will be stretched. The reflected (returned) signal will be of a lower frequency than the transmitted signal.
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