DME AST 241 DME Theory Most VORs in

DME AST 241

DME Theory • Most VOR’s in the US have DMEdistance measuring equipment or are colocated with a military TACAN facility which provides the same information. • VOR navigation is much improved with DME as it only provides: • Azimuth information

DME Theory • DME is required for aerial navigation above? • FL 240 or 24, 000 ft. • Especially useful in IFR operations • VOR and DME are essentially unified systems which are seamless to the pilot when co-located

DME theory • Each VOR frequency from 108. 00 to 117. 95 MHz (VHF) has a corresponding UHF freq. Ranging from 962 – 1213 MHz • Pilots usually have to tune only one freq. With newer receivers • DME’s can be received independent of the VOR if the tuner is separate but the information is of little use for most pilots

DME Theory • For civilian users VOR/DME and VOR’s co-located with TACAN (VORTAC’s) are essentially the same thing providing pilots with the same information. • TACAN was developed by the military to: – Rectify terrain interference problems – Rectify portability problems – Rectify the need for a level platform- (carriers)

DME Theory • Civilian users can’t take advantage of TACAN’s azimuth information and military pilots can’t use DME information

DME Operation • Similar to the ATC Radio beacon system or Transponder system • VOR’s are a Passive System giving continuous information • DME’s respond only when interrogated making them an Active System

DME Operation • Each DME receiver emits two distinctly (to that receiver) spaced pulses which are replied to in identical fashion by the ground based unit. • The DME receiver ignores all other replies. • This is why the DME can take up to 10 seconds to “lock on” to its unique reply • Once locked it updates every 1/10 th mile

DME Operation • The time difference between interrogation and reply corresponds to the aircraft’s straight line distance from the station. • 12/1, 000 of a second in time corresponds to 1 NM in distance

DME Operation • DME accuracy varies from 3 % of total distance (6 NM at 200 miles out) up to being within. 1 NM at all distances for the higher end receiversas good as GPS • The most common operational mistake is to forget to select the correct VOR if the aircraft is equipped with 2 VOR’s or leaving it in the “hold” mode which holds the last frequency tuned rather than the current one.

DME operation • The most common units today are selectable between the dual VOR receiversif not then the DME signal is usually taken from the #1 VOR but BE SURE! • ALWAYS VERIFY!!

DME Operation • Groundspeed and time to Station- Since DME uses rate of response information to get distance it is a small step for the receiver to calculate groundspeed and time to the station. • This information is usually displayed in a separate window labeled- KTS. MIN. OFF

DME Operation • It is important to remember that this only works if heading directly to or from the station- angular motion w. r. t. the station makes all DME information USELESS!

DME identification • DME stations are identified in the same coded manner using the same identification as the corresponding VOR station. • The DME portion is broadcast at a higher pitch and is less frequent than the VOR identification- every 30 seconds.

DME overload • DME transmitters can become overloaded during heavy usage causing the “late comers” not to lock on the freq. • It is courteous to always turn off the DME receiver when not in use such as when on the ground or when DME info. Is not needed.

DME error • Slant range error occurs because the DME calculates its straight line or “slant range” distance from the station which is not as useful to pilots as ground distance. • This causes all DME indications to be inaccurate when greater than 1, 000 ft. AGL for each NM from the station- otherwise it can be ignored.

DME Error • For example if a pilot is at 3, 000 ft AGL at 2 miles from the station the error is out of tolerance. • Remember for accuracy be at least 1 NM from the station for each 1, 000 ft AGL

DME Arc • Older instrument approach procedure where the DME information is used to fly a constant radius around the transmitter • This becomes easier when VOR “lead radials” are used to prompt the turn

DME Operation • • • DME is Rho-Rho navigation Rho = distance Theta = azimuth VOR/DME together is theta – rho nav. 2 VOR’s are theta - theta nav. Rho – rho nav. Requires 3 DME readings to determine position

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