Radar
The radar transmitter sends radio pulses. By controlling how often the rapid pulses of radar energy are sent out by a radar transmitter (called the transmitter's "pulse repetition rate"), and how long it takes for the reflected pulse energy to come back to the radar receiver, one can tell where objects are, and how far away.  Digital circuits in a radar receiver calculate the distance to an object by knowing the time interval between energy pulses. The radar receiver's digital circuits count how long it takes between pulses for an object's reflected energy to be detected by the radar receiver. Since radar pulses are sent and received at approximately the speed of light, the distance to an object can easily be calculated. This is done in digital circuits by multiplying the speed of light by the time it takes to receive the radar energy reflected back from an object.