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