is assured. The TACAN set receives coded AM RF
pulse pairs from the ground station. It then detects,
decodes, and demodulates the signal to extract the
bearing and identity signals. The bearing signal is
then sent to the indicator group, and the identity signal
is sent to the ICS group.
TRANSMIT RECEIVE MODE. In the T/R
mode, the system provides range and bearing to the
ground station and station identity information. To
measure the slant-range to the ground station, the
TACAN transmits an interrogation signal to the
station. The station then sends a reply signal, which is
detected and decoded by the TACAN set. The range
is computed by measuring the elapsed time between
transmission of the interrogation pulse and the
reception of the reply.
The range and bearing
information is then sent to the indicator group, and the
station identification signal is sent to the ICS group.
AIR-TO-AIR MODE. The TACAN set is used
to measure range between two or more similarly
equipped aircraft in the air-to-air mode. To
accomplish this, each TACAN set transmits
interrogation pulses, receives interrogation pulses
from other TACAN sets, and transmits a reply pulse
when interrogated. Interrogation signals consist of a
coded pulse pair, and the reply is a single pulse. The
system will measure the elapsed time between the
transmitted interrogation and the reply received to
compute range. When using this mode, one aircraft
must use a channel that is either 63 channels higher or
lower than the other aircraft.
There are two types of self-test used by the
TACAN system. They are the readiness monitoring
and the interruptive self-test.
READINESS MONITORING. Readiness
monitoring is continuously ongoing self-test. The
self-test module is monitoring critical system
performance parameters without interrupting system
operation. If any one of the continuously monitored
parameters degrades beyond limits, a NO-GO
indicator will light on the control panel, as well as on
the RT faceplate.
When the power supply reaches
+ 125°C (±5°C), the self-test module will light the
TEMP indicator on the RT faceplate.
INTERRUPTIVE SELF-TEST. This self-test
is initiated by the operator depressing either of the
BIT switches on the control panel or the RT. The test
can be performed on any of the channels or in any of
the modes of operation. If there are no faults found,
range will display 1.8 nautical miles, bearing will
display 4 degrees, and the GO indication will be
displayed for 9 seconds on the TACAN control panel.
If there is a fault, the NO-GO indicators will light in
the same manner as in readiness monitoring.
LONG RANGE NAVIGATION (LORAN)
The name loran is an appropriate description of
the hyperbolic system of electronic navigation. It
provides lines of position over the surface of the
earth. Over water, usable loran signals can be
received at ranges up to 2,800 miles. This is done
with low-frequency radio waves. At these operating
frequencies, radio waves are capable of following the
curvature of the earth.
Loran lines of position can be crossed with each
other, or with lines of positions determined by other
means, to provide fixes. Loran lines are stationary
with respect to the earths surface. Their determ-
ination is not dependent upon compass or
chronometer, and it is not necessary to break radio
silence to obtain them. Loran signals are available for
reception in all types of weather, except during very
severe electrical storms.
For more information on the theory of operation
of loran, refer to Aviation Electronics Technician 2
(Organizational), NAVEDTRA 12330, chapter 3.
The AN/ARN-81 loran receiving set consists of
three major components. They are the R-1336/
ARN-81 receiver, the IP-796/ARN-81 display
indicator, and the C-6604/ARN-81 control indicator.
The receiver processes the input signals and
routes them to the display. The display provides a
means to align the pulses to determine the aircraft
position. The control indicator provides selection of
power, operating modes and channels, and delay
The loran receives signals through the ADF sense
antenna. The signals are routed from this antenna to
the R-1336/ARN-81 receiver. At the receiver, the
signals are amplified, heterodyned, and detected to
provide video output signals, which are applied to the
This signal is then applied to a
high-gain amplifier that supplies two outputs. One of