Paired pulses at specific spacing (interrogation) are sent to a ground station from the aircraft via the antenna
The ground station (transponder) sends the same pulses back to the aircraft at a different frequency
Time time it takes is interpreted as the distance, usually in Nautical Miles (NM)
Distance is measured in slant range (not horizontal range!) but some units can correct this
Slant range error minimized at lower altitudes
Operates on the line-of-site principle
Reliable up to 199 NM accuracy of better than 1/2 mile or 3% of the distance, whichever is greater (more accurate)
Due to the limited number of available frequencies, assignment of paired frequencies is required for certain military non-collocated VOR and TACAN facilities which serve the same area but which may be separated by distances up to a few miles
DME is not available on a VOR and would require a separate receiver
DME paired with a VOR constitutes a VORTAC
Can be identified every 30 seconds or about every 3rd to 4th VOR identification
Required above FL 240 when VOR navigation required (under IFR) as per FAR 91.205(e)
If DME fails above FL 240, continue to next airport where repairs or equipment replacement can be done (must still report as per AIM 5-3-3)
GS values, if displayed, are only accurate when flying directly to / from the station
Service volume applies:
Note that Standard Service Volume (SSV) does not apply to airways as they've been certified at their respective distances
Frequency:
960 MHz to 1215 MHz in accordance with ICAO Annex 10
Frequency Pairing:
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 non-collocated VOR and TACAN facilities which serve the same area but which may be separated by distances up to a few miles
Identification of Distance Measuring Equipment Stations:
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
Slaved Compass Systems:
Aircraft equipped with slaved compass systems may be susceptible to heading errors caused by exposure to magnetic field disturbances (flux fields) found in materials that are commonly located on the surface or buried under taxiways and ramps
These materials generate a magnetic flux field that can be sensed by the aircraft's compass system flux detector or "gate," which can cause the aircraft's system to align with the material's magnetic field rather than the earth's natural magnetic field
The system's erroneous heading may not self-correct
Prior to take off pilots should be aware that a heading misalignment may have occurred during taxi
Pilots are encouraged to follow the manufacturer's or other appropriate procedures to correct possible heading misalignment before take off is commenced
Private Pilot (Airplane) Radio Communications, Navigation Systems/Facilities, and Radar Services Airman Certification Standards:
Objective: To determine that the applicant exhibits satisfactory knowledge, risk management, and skills associated with radio communications, navigation systems/facilities, and radar services available for use during flight solely by reference to instruments
Private Pilot (Airplane) Radio Communications, Navigation Systems/Facilities, and Radar Services Knowledge:
The applicant demonstrates understanding of:
PA.VIII.F.K1:
Operating communications equipment to include identifying and selecting radio frequencies, requesting and following air traffic control (ATC) instructions.
PA.VIII.F.K2:
Operating navigation equipment to include functions and displays, and following bearings, radials, or courses.
PA.VIII.F.K3:
Air traffic control facilities and services.
Private Pilot (Airplane) Radio Communications, Navigation Systems/Facilities, and Radar Services Risk Management:
The applicant is able to identify, assess, and mitigate risk associated with:
PA.VIII.F.R1:
When to seek assistance or declare an emergency in a deteriorating situation.
PA.VIII.F.R2:
Using available resources (e.g., automation, ATC, and flight deck planning aids).
Private Pilot (Airplane) Radio Communications, Navigation Systems/Facilities, and Radar Services Skills:
The applicant exhibits the skill to:
PA.VIII.F.S1:
Maintain airplane control while selecting proper communications frequencies, identifying the appropriate facility, and managing navigation equipment.
DME stations are their own set of equipment, and are not necessarily part of the NAVAID, hence association with various other ground-based systems like VORs and localizers
DME is therefore not an indication the NAVAID it is tied to is in service
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
Note, GPS-based guidance is an acceptable substitute for most uses of ground-based navaids (i.e., if DME is out-of-service), except for lateral guidance on the final approach segment, unless the navaid is in service and can be monitored simultaneously
