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distance, whichever is greater.
5.13.2.2. Since a large number of aircraft could be interrogating the same station, the aircraft TACAN
must sort out the pulses which are replies to its own signal. Interrogation pulses are transmitted on an
irregular and random basis by the airborne set which then searches for replies synchronized to its own
interrogations. If the signals are interrupted, a memory circuit maintains the last distance indications on
the range indicator for approximately 10 seconds to prevent the search operation from recurring. This
process starts automatically when a new station is tuned or when there is a major interruption of signals.
160 AFPAM11-216 1 MARCH 2001
Depending upon the actual distance from the station, the searching process may require up to 22
seconds. The maximum number of aircraft which can be accommodated by one station at any one time
is 100. With the development of the X and Y bands, this number can be doubled.
5.14. TACAN Characteristics:
5.14.1. Bearing and/or Distance Unlock. Since TACAN bearing and DME are subject to LOS
restrictions, this information could be lost any time signals are blocked. Temporary obstructions can
occur in flight any time any part of the aircraft gets between the ground and aircraft antenna. Other
aircraft, terrain, and buildings are external causes for unlock. Any time the signal is obstructed for more
than 10 seconds for DME and 2 seconds for azimuth, the unlock conditions will be indicated by a
rotating bearing needle and a tumbling DME readout.
5.14.2. Azimuth Cone of Confusion. TACAN antennas transmit radio energy in circular patterns out
from the transmitter. However, waves are not transmitted directly above the station. Therefore, as the
aircraft approaches a TACAN station, signals are lost. This is indicated by a rotating TACAN bearing
needle in the RMI. The azimuth cone can be up to 100o or more in width or approximately 15 NM wide
at 40,000 feet. Thus, one may enter the cone of confusion at approximately 7.5 DME at this altitude.
Approaching the station, usable TACAN information is lost before the cone is reached as aircraft
memory circuits maintain the last information.
5.14.3. Range Indicator Fluctuations. Slight oscillations up to approximately one forth NM are normal
for range indicator operation due to the pulses generated by the transmit or receive function. When a
usable signal is lost, the memory circuit maintains the indicated range for about 10 seconds, after which
unlock will occur unless usable signals are regained.
5.14.4. Forty Degree Azimuth Error Lock-on. The construction of the TACAN ground antenna is
such that it transmits a series of nine signal lobes (eight auxiliary and one main reference pulse) 40o
apart. With the airborne receiver working correctly, these pulses lock on the airborne equipment with the
main reference at 090o. With a weak signal, the main reference pulse may slide over or miss the 090o
slot and lock on at one of the auxiliary positions. When this occurs, azimuth indications will be 40o or a
multiple of 40o in error. Forty degrees azimuth lock-on error will not cause a course warning flag to
appear on the indicator. Rechanneling the airborne receiver may give the set another chance to lock on
properly.
5.14.5. Co-Channel Interference. Co-channel interference occurs when an aircraft is in a position to
receive TACAN signals from more than one ground station on the same frequency. This normally
occurs only at high altitudes when distance separation between like frequencies is inadequate. DME,
azimuth, or identification from either station may be received. This is not a malfunction of either
airborne or ground equipment, but a result of position.
5.14.6. Air-to-Air (A/A) TACAN. This function is provided to give distance information between two
aircraft, working in the same manner as a regular ground-based TACAN station. Some sets provide only
DME information. Newer sets provide both distance and bearing information to other aircraft. In order
to obtain useful information, the A/A function should be selected by both aircraft with a 63-channel
frequency separation.
AFPAM11-216 1 MARCH 2001 161
5.14.6.1. In addition, each aircraft must have the same frequency band (X or Y) selected. Therefore, if
one aircraft sets A/A Channel 4 and the other sets A/A Channel 67 in the X band useful information
should be obtained.
5.14.6.2. A/A TACAN is primarily used during the rendezvous portions of air refueling operations or
formation flights. A prescribed turn range (DME) and offset (bearing, if available) between the two
aircraft are used to effect the rendezvous. Proper A/A channels for each air refueling route are found in
the FLIP Planning Document.
5.14.7. Tuning and Controls. The basic controls of most TACAN systems are shown in Figure 5.11.
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