Bats, submarines, and self-driving cars all measure distance the same way: send a pulse, time how long until the echo returns, and halve it — the round trip divided by two, times the wave speed, is the range. The clock becomes a tape measure. Slide the target and watch the echo come back later.
Time-of-flight (echo) ranging: emit a pulse, measure the round-trip time t for its reflection to return, and the one-way distance is d = c·t/2 (c the wave speed). Sonar uses sound (~343 m/s), radar and lidar use light (3×10⁸ m/s), so a light echo from 150 m returns in a microsecond — the whole difficulty is timing that precisely. A fail-loud self-check throws unless a 0.01 s sound echo gives 1.72 m. Combine three such ranges and you have trilateration; this is the ruler each of those distances is measured with. ◆ real ranging physics, node-verified.
Constant wave speed and a sharp echo assumed (the exact d=ct/2); real ToF fights multipath, attenuation and timing jitter — the halve-the-round-trip rule is exact.