The Interrogation signal travels from the Airborne Interrogator to the Ground Transponder where the frequency is adjusted by +/- 63 MHz, a process which takes 50 microseconds; the Reply signal then returns to the Interrogator.

Using the process of Propagation Delay (Distance = Speed * Time) the Interrogator then calculates the distance to the Transponder, i.e. the direct line-of-sight DME Slant Range to the DME Ground Station.

The simplest way of calculating this is to remember that 1 Radar Mile = 12.36 microseconds, which is the time taken for a signal to travel from a Radar to a Target and back to the Radar.

For DME distance, simply take the time delay, subtract 50 microseconds and divide by 12.36 microseconds and you have your answer.

2000 - 50 = 1950

1950 / 12.36 = 158 NM