Figure 2-6.—TACAN modulation envelope
spacing are a characteristic of that particular TACAN
signal element. However, it is important to understand
that proper spacing between pulses and pulse pairs is
what actually provides the aircraft with the means to
distinguish between the TACAN pulses and any other
pulses that might be present on the received radio
frequency. Check the reference data in the appropriate
technical manual for specific pulse characteristics and
spacing.
TACAN EQUIPMENT
Many different types of TACAN equipment have
been used for air navigation. Today, the AN/URN-25
is taking over the task of tactical air navigation from
the older AN/URN-20 on new construction ships and
as ships complete overhaul. Two types of antennas
are used with the AN/URN-25. They are the OE-
273(V)/URN, used primarily in shipboard installations,
and the OE-258/URN, which is used primarily ashore.
Because both antenna systems are similar in theory of
operation, we will discuss only the OE-273/URN. In
the following paragraphs, we will discuss the
AN/URN-25 and the antenna group 0E-273(V)/URN,
and then we will briefly discuss the AN/URN-20.
TACAN SET AN/URN-25
The AN/URN-25 TACAN is used as a ground-
based or shipborne beacon transponder to provide
range and bearing information to aircraft equipped
with TACAN equipment. It consists of two major
units: the Transponder Group OX-52/URN-25,
commonly referred to as unit 1, and the Control-
Indicator C-10363/URN-25, commonly referred to as
unit 2. These units are shown in figure 2-7. Each
transponder is housed in a cabinet with two vertical
drawers, one containing a coder keyer and the other
containing a receiver-transmitter.
The control-indicator displays the status of the
transponder(s) and failure alarms, and allows limited
control of the transponder(s) from a remote location.
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