Subelement K: K – Aircraft— Topic 68: Distance Measuring Equipment
Question 3-68K3
Element 3 (GROL)The Distance Measuring Equipment (DME) ground station has a built-in delay between reception of an interrogation and transmission of the reply to allow:
Explanation
The Distance Measuring Equipment (DME) system calculates distance by measuring the time difference between an aircraft's interrogation pulse and the ground station's reply. At very close ranges, the actual propagation time (the time for the radio signal to travel to and from the aircraft) would be extremely short, potentially near zero.
A built-in, fixed delay (typically around 50 microseconds) in the ground station's reply ensures that there is always a sufficiently long time interval for the aircraft's DME receiver to accurately measure. Without this delay, at close ranges, the measured time might be too short to resolve accurately, or it could be confused with the aircraft's own transmitted pulse. The aircraft's DME unit is designed to subtract this known, constant delay from the total measured time to determine the true round-trip propagation time, allowing for accurate distance calculation even when operating very close to the ground station.
Options A, B, and D describe irrelevant or incorrect functions regarding DME system timing and operation.
Related Questions
3-68K1 What is the frequency range of the Distance Measuring Equipment (DME) used to indicate an aircraft’s slant range distance to a selected ground-based navigation station?3-68K2 The Distance Measuring Equipment (DME) measures the distance from the aircraft to the DME ground station. This is referred to as:3-68K4 What is the main underlying operating principle of an aircraft’s Distance Measuring Equipment (DME)?3-68K5 What radio navigation aid determines the distance from an aircraft to a selected VORTAC station by measuring the length of time the radio signal takes to travel to and from the station?3-68K6 The majority of airborne Distance Measuring Equipment systems automatically tune their transmitter and receiver frequencies to the paired __ / __ channel.