AIM ¶ 1-1-7 — Distance Measuring Equipment
AIM 1-1-7 explained for pilot students: how DME works, slant range, accuracy, identification tones, and frequency pairing with VOR/VORTAC/ILS.
In Plain English
Distance Measuring Equipment (DME) gives you a continuous readout of your distance from a ground station in nautical miles. Your airborne unit sends paired interrogation pulses to the ground transponder, which replies on a different frequency. The DME measures the round-trip time and converts it to distance.
Key points to remember:
- DME is line-of-sight and operates between 960–1215 MHz.
- Reliable signals out to 199 NM with accuracy better than ½ mile or 3% of distance, whichever is greater.
- DME shows slant range, not horizontal distance — directly overhead a station, the readout equals your altitude in NM (most error close-in and at high altitude).
- VOR/DME, VORTAC, ILS/DME, and LOC/DME are paired facilities — selecting the VOR/LOC frequency auto-tunes the DME.
- Identification: VOR/LOC ident is a 1020 Hz tone; DME/TACAN ident is a 1350 Hz tone, transmitted once for every 3–4 VOR idents. A single DME ident roughly every 30 seconds means the VOR is out but the DME is still working.
- TACAN-equipped aircraft get distance from VORTAC automatically; VOR-only aircraft need a separate DME unit.
Disregard DME displays when tuned to a VOR/ILS that has no DME installed.
AIM Source Text
FAA AIM ¶ 1-1-71-1-7. 1-1-7. Distance Measuring Equipment (DME)
In the operation of DME, paired pulses at a specific spacing are sent out from the aircraft (this is the interrogation) and are received at the ground station. The ground station (transponder) then transmits paired pulses back to the aircraft at the same pulse spacing but on a different frequency. The time required for the round trip of this signal exchange is measured in the airborne DME unit and is translated into distance (nautical miles) from the aircraft to the ground station. Operating on the line-of-sight principle, DME furnishes distance information with a very high degree of accuracy. Reliable signals may be received at distances up to 199 NM at line-of-sight altitude with an accuracy of better than / 2 mile or 3 percent of the distance, whichever is greater. Distance information received from DME equipment is SLANT RANGE distance and not actual horizontal distance. Operating frequency range of a DME according to ICAO Annex 10 is from 960 MHz to 1215 MHz. Aircraft equipped with TACAN equipment will receive distance information from a VORTAC automatically, while aircraft equipped with VOR must have a separate DME airborne unit. VOR/DME, VORTAC, Instrument Landing System (ILS)/DME, and localizer (LOC)/DME navigation facilities established by the FAA provide course and distance information from collocated components under a frequency pairing plan. Aircraft receiving equipment which provides for automatic DME selection assures reception of azimuth and distance information from a common source when designated VOR/DME, VORTAC, ILS/DME, and LOC/DME are selected. Due to the limited number of available frequencies, assignment of paired frequencies is required for certain military noncollocated VOR and TACAN facilities which serve the same area but which may be separated by distances up to a few miles. VOR/DME, VORTAC, ILS/DME, and LOC/DME facilities are identified by synchronized identifications which are transmitted on a time share basis. The VOR or localizer portion of the facility is identified by a coded tone modulated at 1020 Hz or a combination of code and voice. The TACAN or DME is identified by a coded tone modulated at 1350 Hz. The DME or TACAN coded identification is transmitted one time for each three or four times that the VOR or localizer coded identification is transmitted. When either the VOR or the DME is inoperative, it is important to recognize which identifier is retained for the operative facility. A single coded identification with a repetition interval of approximately 30 seconds indicates that the DME is operative. Aircraft equipment which provides for automatic DME selection assures reception of azimuth and distance information from a common source when designated VOR/DME, VORTAC and ILS/DME navigation facilities are selected. Pilots are cautioned to disregard any distance displays from automatically selected DME equipment when VOR or ILS facilities, which do not have the DME feature installed, are being used for position determination.
Oral Exam Questions a DPE Might Ask
Q1What is slant range error and when is it greatest?
Per AIM 1-1-7, DME measures SLANT RANGE — the direct line from aircraft to station — not horizontal distance. The error is greatest when the aircraft is close to and high above the station (e.g., directly overhead the readout equals altitude in NM).
Q2How can you tell if the DME is operative when the VOR portion has failed?
Per AIM 1-1-7, VOR/LOC and DME share synchronized identifications. The VOR/LOC ident is a 1020 Hz tone and the DME ident is a 1350 Hz tone sent once for every 3–4 VOR idents. A single coded ident repeating about every 30 seconds indicates the DME is operative while the VOR is not.
Q3What is the maximum reliable range and accuracy of DME?
Per AIM 1-1-7, reliable DME signals may be received at distances up to 199 NM at line-of-sight altitude with accuracy better than ½ mile or 3 percent of the distance, whichever is greater.
Related Paragraphs in AIM Chapter 1