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Radar speed guns use Doppler radar to perform speed measurements.. Radar speed guns, like other types of radar, consist of a radio transmitter and receiver.They send out a radio signal in a narrow beam, then receive the same signal back after it bounces off the target object.
The most common wheel speed sensor system consists of a ferromagnetic toothed reluctor ring and a sensor (which can be passive or active).. The tone wheel is typically made of steel and may be an open-air design, or sealed (as in the case of unitized bearing assemblies).
A small permanent magnet affixed to the speedometer cable interacts with a small aluminium cup (called a speedcup) attached to the shaft of the pointer on the analogue speedometer instrument. As the magnet rotates near the cup, the changing magnetic field produces eddy current in the cup, which itself produces another magnetic field.