Time Domain Reflectometry (TDR) techniques are used in various applications to calculate the position of damage within a cable. A cable is a linear transmission system and the information from the events within a cable travels along the transmission line to both ends. Any discontinuities in the cable (joints, terminations) will modify the properties of the travelling wave and this can be identified and measured by equipment installed at either end of the cable. The most simple relationship v = d/t is then used to calculate distances.

The wave travels within the cable at a fraction of the speed of light calculated from the relative permittivity of the material. The speed of light in free space is approximated at 300m/us and in cables this may be reduced to anywhere from 150-250 m/us. Often this speed is referred to as v/2 and half the actual speed is used. This is because the TDR system is located at only one end and the reflected wave has travelled twice the distance to reach the TDR equipment. Using v/2 in the equation mathematically compensates for the fact that the distance required is half the distance calculated.