TACAN SET AN/URN-20
Though not modern by any standard, the
AN/URN-20 TACAN set is reliable and operates
similarly to the AN/URN-25. Shown in figure 2-9, it
uses the same electronically-scanned antenna and
control-indicator as the AN/URN-25. The AN/URN-
20 is being replaced by the AN/URN-25.
CAPABILITIES AND LIMITATIONS
In the X mode of operation, the TACAN set
transmits on one of 126 discrete channel frequencies
(which are 1-MHz apart) from 962 to 1024 MHz and
from 1151 to 1213 MHz. In the Y mode of operation,
the set transmits on one of 126 discrete channel
frequencies (which are 1-MHz apart) within the range
of 1025 to 1150 MHz. The navigation set receiver,
operating in the 1025- to 1150-MHz range for both
the X and Y modes, is always displaced 63 MHz from
the transmitter frequency.
The TACAN set can simultaneously provide
individual distance measuring service for up to 100
interrogating aircraft.
Of the 3,600 pulse
pairs-per-second transmitted by the TACAN, 900
pulse pairs (MAIN and AUXILIARY bursts) contain
the bearing information; the remaining 2,700 pulse
pairs are either random noise pulses, identity pulses, or
replies to interrogating aircraft.
Once every 30
seconds, the interrogation replies and random noise
pulses are interrupted for the transmission of identity
pulses.
The navigation set has a receiver sensitivity of -92
dBm or better and a nominal peak power output of 3
kilowatts at the transponder cabinet output. (Power
output may limited to less than peak by directives).
Since the bearing and identification signals are
delivered spontaneously and not in response to
interrogations,
an unlimited number of properly
equipped aircraft can derive this information from the
TACAN set over a line-of-sight (los) range up to 200
nautical miles.
2-9
