VOR, DME and TACAN
Core concepts
VOR gives magnetic bearing from the station (the radial), measured as the phase difference between a rotating or electronically variable signal and a reference signal. The aircraft receiver derives the radial, and the CDI centres when the selected OBS matches the radial. Important quantities are the magnetic variation at the station (radials are magnetic), the cone of silence overhead, and the line-of-sight range limitation. DME measures slant range by timing the round trip of pulse pairs; the displayed distance is always greater than or equal to the horizontal distance, and the error grows with height above the station. TACAN provides bearing and distance for military users and supplies the DME element of a VORTAC.
Traps in questions
Classic traps include: a radial is FROM the station, so being on the 090 radial means the aircraft is east of the station on a heading that is not necessarily 090; reverse sensing when tracking outbound with the wrong OBS; the cone of silence producing a momentary TO/FROM ambiguity overhead; and DME showing ground speed and time-to-station only when tracking directly to or from the station. Also remember that DME distance at high altitude is slant range, so station passage shows a small residual distance rather than zero.
Worked example
An aircraft at 30 000 ft is directly over a DME station. Slant range equals height, about 5 NM. If the aircraft is 20 NM ground distance from the station at that height, slant range = √(20² + 5²) ≈ 20.6 NM, an error of about 0.6 NM. The error is negligible at long range but significant close in.
Rule of thumb
Slant range error is important only within about 10 NM of the station when high; treat DME as ground distance at long range.