Doppler Effect
Definition
The change in observed frequency when the source and the observer move relative to each other. A radar gun emits a known microwave frequency and measures the frequency of the reflected wave; the shift converts directly to vehicle speed.
- Field
- Physics of wave frequency shift
- Forensic use
- Speed detection radar guns
- Mechanism
- Emits known microwave frequency, measures reflected shift
- Output
- Frequency shift converts to vehicle speed
Common questions
Why can a radar gun give a false reading in traffic?+
Radar beams spread out and can reflect off multiple vehicles at once, so the gun may lock onto the largest or closest reflecting surface rather than the intended target. This is the basis of common radar defence challenges, and why operator training on beam angle and target selection matters for admissibility.
How does LIDAR speed measurement differ from Doppler radar?+
LIDAR does not rely on the Doppler shift at all. It times pulses of laser light to and from the vehicle and calculates speed from the change in distance over successive readings, giving a narrower beam and more precise single-vehicle targeting than radar.
Related terms
- ANPR
- Automatic Number Plate Recognition. A camera plus OCR pipeline that reads vehicle number plates at speed and logs the plate, timestamp, and...
- Calibration Cycle
- The periodic re-certification of a speed-measurement device against a traceable reference, usually annually for radar guns and every six months for LIDAR....
- Cosine Error
- The under-reading of true speed that occurs when the radar or LIDAR beam is not perfectly aligned with the vehicle's direction of...
- FASTag-ANPR Fusion
- Indian highway tolling pairs RFID FASTag reads with ANPR cameras at every electronic toll gantry, creating a national vehicle-position log that investigators...
- Time-of-Flight (LIDAR)
- A measurement principle where the device emits short laser pulses and times how long each pulse takes to return. Multiple pulses over...