Definition
Paired radio frequency pulses transmitted by an aircraft's DME (Distance Measuring Equipment) unit to a ground station, asking the station to respond. The ground station receives these pulses, waits a fixed delay, then transmits reply pulses on a different frequency. The aircraft's DME measures the round-trip time between sending the interrogating pulses and receiving the replies, then converts that time into a slant-range distance to the station.
Plain English
Short radio signals the aircraft sends out to a ground station to ask, in effect, 'how far away are you?' The station answers, and the equipment uses the time it took to work out the distance.
Context Anchor
Seen in DME system descriptions, especially when explaining how airborne DME equipment communicates with a ground station to produce a distance readout.
Derivation
Interrogate comes from the Latin interrogare, meaning 'to question.' The aircraft is literally questioning the ground station, and the station answers back. That's why the signals are called interrogating pulses rather than just transmitted pulses.
Why Pilots Care
These pulses are required for DME to calculate slant-range distance, enabling accurate position awareness during instrument flight.
Analogy
Like sonar or an echo: you send a sound out, wait for it to bounce back, and the time it takes tells you how far away the object is. DME does the same thing with radio pulses, except the ground station deliberately replies rather than passively reflecting.
Grounding Statement
Picture the DME unit sending quick radio blips to the station and timing how long it takes for the replies to come back.
Intuition Check
“Interrogating” does not mean a person is questioning someone; here it means the aircraft equipment is sending a radio request that expects an automatic reply. “Pulses” are not a continuous transmission; they are brief bursts.
Example Sentence 1
When the pilot tuned the DME to the VORTAC, the unit began sending interrogating RF pulses and displayed the distance a few seconds later once replies were received.
Example Sentence 2
Each set of interrogating radio frequency pulses triggers a reply that allows the aircraft to compute distance.