Wheel speed sensors measure the rpm of the wheels, or a distance travelled per unit of time, or an angle. The various systems operate on the basis of the signals from the wheel speed sensors. Thus the functioning of both the anti-lock brake system (ABS) and traction control system (TCR) and the electronic stabilization program (ESP) are dependent on the information about the wheel speed. Navigation systems too, benefit from the signals from these sensors. From them they calculate the distance travelled.
Wheel speed sensors are divided into so-called passive and active sensors, although today active sensors outweigh in use due to their technical properties such as accuracy and small size. Active sensors need an additional power source to operate; passive sensors manage without an external power source.