Figure 8 - P-8010015

INSTRUMENT NAVIGATION

misinterpretation of the received signal may occur and the displayed bearing will be 40 (or

some multiple of 40) off the proper bearing.

Figure 8

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

station.

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