DME
DME operates in UHF (962-1213 MHz) and measures slant range from the aircraft to the ground transponder by timing the round trip of pulse pairs. Know that the displayed range is slant range, which is greater than ground range at high altitude or close to the station. The relationship is ground range = sqrt(slant^2 - height^2).
Candidates must understand the DME frequencies paired with VOR/ILS (frequency pairing), the use of DME for position fixing (rho-rho or rho-theta), and the accuracy and update rate. Know that DME has a limited number of aircraft it can serve (about 100-200) and that range is line-of-sight. Understand that DME is often colocated with VOR (VOR/DME) or ILS (DME for glide path fix).
Traps: assuming DME reads ground range; forgetting slant range error is greatest close in and at high altitude; confusing DME with a bearing aid. Another trap is the assumption that DME works beyond line of sight.
Worked example: at 30,000 ft (about 5 nm) and slant range 10 nm, ground range = sqrt(100 - 25) = sqrt75 = about 8.7 nm.