🔍 Key Concepts

• Distance Measuring Equipment (DME) basic function in radionavigation • Relationship between radio signal travel time and distance • Why DME needs both interrogation (from aircraft) and reply (from ground station) signals

💭 Think About

• Think about how fast radio waves travel and what measurable quantity could relate directly to distance. • Ask yourself which option actually describes a realistic way radio navigation equipment could work in three‑dimensional airspace. • Consider what kind of signal (DC vs AC, timing vs voltage difference) can be reliably measured over long distances between an aircraft and a ground station.

✅ Before You Answer

• Which option mentions something that can be precisely measured over the air between aircraft and a ground station (time of travel, frequency, phase, or DC voltage levels)? • Which choices clearly do not describe standard radio‑navigation techniques used in aviation? Eliminate those first. • Verify that the principle you pick would work over tens of nautical miles and does not depend on physical wiring or direct electrical connection.