Distance Calculation
A radio pulse takes around 12.36 microseconds to travel 1 nautical mile (1,852 m) to and from; this is also referred to as a radar-mile. The time difference between interrogation and reply 1 nautical mile (1,852 m) minus the 50 microsecond ground transponder delay is measured by the interrogator's timing circuitry and translated into a distance measurement (slant range), stated in nautical miles, and then displayed on the cockpit DME display.
The distance formula, distance = rate * time, is used by the DME receiver to calculate its distance from the DME ground station. The rate in the calculation is the velocity of the radio pulse, which is the speed of light (roughly 300,000,000 m/s or 186,000 mi/s). The time in the calculation is (total time – 50µs)/2.
Read more about this topic: Distance Measuring Equipment
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