misinterpretation of the received signal may occur and the displayed bearing will be 40° (or
some multiple of 40°) off the proper bearing.
Although some newer TACAN receivers have built-in self-test equipment which displays a
warning to the aircrew whenever a 40° lockoff occurs, most TACAN receivers in present usage
do not have this capability; therefore, it is imperative that a 40° lockoff be recognized by the
aircrew. A 40° lockoff can be confirmed by comparing the TACAN bearings generated by
ADF or VOR receivers which are not susceptible to the 40° lockoff. The aircrew should
always maintain situational awareness (i.e., a rough DR position) which will enable them to
recognize a 40° lockoff on the TACAN in the event of nonavailability of ADF, VOR, or other
crossreferences to the TACAN bearing. Once a 40° lockoff is recognized, rechannelizing the
airborne equipment will cause unlock and allow the set another chance to lock onto the proper
bearing. A forty-degree lockoff will have no effect upon distance indications.
6. TACAN incorporates Distance Measuring Equipment (DME)
DME is an integral part of a TACAN. When you select a TACAN channel you are
simultaneously tuning your TACAN to the azimuth and DME of that particular TACAN
In order for you to utilize DME, your TACAN must be in the T/R (transmit/receive) mode.
The DME is obtained by transmitting a signal to the ground TACAN station. That signal is
processed by the ground station which then sends your TACAN box a reply signal. Based on
the relationship between your aircraft position, the position of the ground station and the time
it takes for these signals to travel back and forth, your DME indicates the actual "SLANT
RANGE" between your aircraft and your selected TACAN station.
RADIO AIDS TO NAVIGATION 1-9