Figure 2-5.—TACAN radiation pattern: A. cardioid
pattern; B. Ampltitude-modulated pulse pairs.
The aircraft TACAN equipment obtains bearing
information by comparing the 15-Hz modulated signal
with a 15-Hz reference burst signal it receives from the
ground facility. The phase relationship between the
15-Hz modulated signal and the 15-Hz reference burst
signal depends on the location of the aircraft in the
cardioid pattern. The 15-Hz reference burst signals
are transmitted when the maximum signal of the
cardioid pattern aims due East. This group of 12 pulse
pairs is commonly referred to as the North or main
reference burst. You can see the relationship between
the reference pulses and the cardioid pattern by
comparing view A and view B of figure 2-5.
135-HZ BEARING INFORMATION
Errors arising from imperfections in the phase
measuring circuits and radio propagation effects are
known as site error. These errors are significantly
reduced by the addition of 32 outer parasitic elements
added to the electronically scanned antenna. (See the
section on the OE-273(V)/URN antenna group).
Electronically switching these elements modifies the
antenna cardioid pattern. Though the cardioid pattern
is still predominant, it is altered by superimposed
ripples. The aircraft now receives the 15-Hz signal
with a 135-Hz ripple amplitude modulated on the
distance data pulses (figure 2-6).
To furnish a suitable reference for measuring the
phase of the 135-Hz component of the envelope wave,
the transponder is designed to transmit a coded 135-
Hz reference burst similar to that explained for the 15-
Hz reference.
The 135-Hz reference group is
commonly referred to as the auxiliary or aux reference
b u r s t .
The composite TACAN signal is composed of
2700 interrogation replies and noise pulse
pairs-per-second, plus
180 North burst pulse
pairs-per-second, 720 auxiliary burst pulse pairs-per-
second, for a total of 3600 pulse pairs-per-second, or
7200 pulses-per-second.
TACAN SIGNAL PRIORITIES
Priorities have been established for transmission of
the various types of TACAN signals. These priorities
are as
1.
2.
3.
4.
follows:
Reference bursts (North and auxiliary)
Identification group
Replies to interrogations
Squitter
Therefore, the identification group, replies, or
squitter will be momentarily interrupted for the
transmission of either the main or auxiliary reference
group. The transmission of replies or squitter will be
interrupted every 37.5 seconds during the transmission
of an identification code dot or dash.
CHARACTERISTICS OF
RADIO BEACON SIGNALS
Depending on what channel (X or Y) the TACAN
is on, the number of pulses-per-second and the pulse
2-4
