a radio signal takes approximately 12.36 microseconds to travel to the target and back. the time difference between interrogation and reply, minus the 50 microsecond ground transponder delay and the pulse width of the reply pulses (12 microseconds in x mode and 30 microseconds in y mode), is measured by the interrogator's timing circuitry and converted to a distance measurement (slant range), in nautical miles, then displayed on the cockpit dme display.