Definition
A coded radio pulse pair transmitted by an aircraft's DME (Distance Measuring Equipment) on a UHF frequency to a ground-based DME station, requesting a reply that the aircraft can use to calculate slant-range distance to the station.
Plain English
A signal the aircraft sends out to ask the ground station 'how far am I?' The ground station hears the signal and sends one back, and the aircraft uses the round-trip time to work out the distance.
Context Anchor
Seen in DME system descriptions, especially when explaining how the aircraft unit and the ground station work together to show distance.
Derivation
From the Latin 'interrogare,' meaning 'to question.' The aircraft is literally questioning the ground station -- 'how far am I from you?' -- and waiting for an answer.
Why Pilots Care
Without a properly transmitted interrogating signal, the DME cannot receive the return pulse needed to calculate and display distance to the station.
Analogy
It is like pressing a call button and waiting for an answer. The DME does not care what the answer says; it cares how long the answer takes to come back.
Intuition Check
Do not read “interrogating” as a person asking a spoken question. Here it means an aircraft radio signal that triggers an automatic reply from a DME ground station.
Example Sentence 1
The aircraft's DME sends out an interrogating signal, and the ground station replies on a different frequency.
Example Sentence 2
If the interrogating signal is blocked by terrain, the cockpit DME display will show dashes instead of a distance.