GONE OF CONFUSION
The line-of-sight transmission pattern from a VOR or TACAN station creates an area
directly overhead the facility where azimuth signal reception is very weak or random. This
area is called the "cone of confusion". Although the azimuth information displayed is
considered to be unreliable, the DME is not affected and will continue to display slant range to
the station. The cone of confusion for a TACAN is much larger than that of the VOR, and at
40,000 feet, the TACAN's cone of confusion encompasses a 15 nm diameter about the TACAN
When you analyze the diagram, it should be obvious that the diameter of the cone is a
function of altitude. Understanding this concept will help eliminate potential navigational
problems in close proximity of the NAVAID.
At 30,000 feet, we could expect less time in the cone; however, at 50,000 ft. we could
expect a long time period of unreliable TACAN or VOR azimuth information. Within the cone
of confusion, the behavior of the TACAN or VOR azimuth will be completely random. It will
oscillate from side to side and will not present accurate information. It is important to
recognize when you have entered the cone, so as not to follow the unreliable indications of the
The DME, however, will continue to behave in a predictable manner and the numbers
displayed will continue to decrease until the aircraft is directly overhead the TACAN station.
At this point you have reached "minimum DME". As noted earlier, this number will be equal
to your AGL altitude in nautical miles. Upon reaching this point your DME will not continue
to decrease. Rather, after minimum DME is observed, it will again increase in the same
predictable manner that it decreased as you were headed inbound to the station.
NAVAIDS POSITIONING 2-9