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Distance Measuring Equipment

Introduction:

  • Distance Measuring Equipment (DME) is a measuring device using ground and air components to determine the slant range of an aircraft from a point

Function:

  • 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 but some units can correct this
  • Slant range error minimized at lower altitudes

  • DME Slant Range Distance
    Figure 1: DME Slant Range Distance
  • 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

Identification:

  • 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 self­correct
  • 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

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

Frequency:

  • 960 MHz to 1215 MHz in accordance with ICAO Annex 10

Conclusion:

  • 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

References: