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to a TACAN ground station or range to a suitably equipped aircraft. The TACAN system operates in
the L-Band frequency range, limiting the operating range to line of sight, which depends upon aircraft
altitude. The maximum operating range is 390 nm when the selected TACAN station is a surface
beacon and 200 nm when the selected TACAN station is an airborne beacon. The aircraft receives a
three letter Morse code signal to identify the beacon being received. When operating in conjunction
with aircraft having air-to-air capability, the A/A mode provides line of sight distance between two
aircraft operating TACAN sets 63 channels apart. Up to five aircraft can determine line of sight
distance from a sixth, lead aircraft in the A/A mode.
A1-F18EA-NFM-000
VII-24-46 ORIGINAL
To preclude MIDS interference to the IFF system, notch filters centered at 1030 MHz were placed
in the antenna lines. The upper filter is fixed, while the lower filter is switched in for Link-16
transmissions, and out for TACAN transmissions and all reception. Because it's fixed, the upper filter
interferes with A/A TACAN on channels 1 to 36 and 64 to 99. There is also attenuation on A/G TACAN
channels 1 to 36, 40 to 63, and 64 to 99 on the upper antenna, but is less of a problem due to A/G
TACAN using primarily lower antenna. If the upper antenna filter remains installed with AN/ARN-
118 TACAN installed, this would affect the AN/ARN-118 as well.
24.4.1 TACAN BIT. A TACAN BIT check is manually initiated by ensuring the TACAN is turned on
and pressing the TCN/IFF option on the BIT sublevel display. If the TACAN is good, the DDI shows
the BIT status as GO. If the TACAN does not pass the BIT check, BIT status is DEGD. The TACAN
system also has an automatically initiated BIT. If the automatic BIT check detects a wrong signal or
a failure, TACAN DEGD is displayed on the BIT display and the BIT line on the left DDI. If no fault
is detected, nothing is displayed next to TCN.
24.4.2 TACAN Mode Selection. To enable the TACAN system, select the TCN option on the UFCD
top level CNI display. The TACAN channel number is displayed on the TCN option. If the TACAN
is powered the TCN option is corner highlighted. If TCN is selected with no data entered in the
scratchpad the TACAN sublevel is displayed, see figure 24-16. TACAN operation is controlled from
the TACAN sublevel. The TACAN system is turned on by pressing the ON/OFF option on the TACAN
sublevel.
When the TACAN sublevel is selected, the following TACAN mode options appear: T/R (transmit/
receive), RCV (receive), A/A (air-to-air), and X and Y channel. Border highlights indicate which
TACAN mode is operating. In the T/R mode the TACAN computes bearing and measures slant range
from the selected TACAN station. In the RCV mode only bearing from the selected TACAN station
is computed. In the A/A mode, interrogations and replies are single pulse from one aircraft to another.
The TACAN channel mode is indicated in the X/Y option which is always border highlighted in the
TACAN sublevel and toggles between X and Y mode with each selection.
Figure 24-16. TACAN Mode Selection
A1-F18EA-NFM-000
VII-24-47 ORIGINAL
24.4.3 TACAN Programming. TACAN station data is entered by selecting the DATA option on the
MPCD top level display. Selecting the TCN option on the data sublevel display provides the TACAN
data sublevel display. This display shows the TACAN station lat/long position, elevation, and magnetic
variation. TACAN station data is entered by selecting the desired TACAN station number on the
UFCD keyboard. Pressing the X or Y option to select the TACAN channel, the POSN option enters
lat/long data, the ELEV option to enter elevation data, and the MVAR option to enter magnetic
variation. TACAN data is entered on the UFCD keypad for up to 252 TACAN stations (126 on X
channel, 126 on Y channel). See figure 24-17.
24.4.4 TACAN Position Keeping. The TACAN system may be used for position keeping purposes. To
do this the TACAN system must be in the T/R mode with the proper channel (X or Y) and channel
number selected. The TACAN station selected must be one of the prestored stations. Selecting the
POS/XXX option on the MPCD top level display provides the position keeping option display.
Selecting the TCN option as the position keeping source causes the MPCD top level display to be
returned and POS/TCN to be displayed as the position keeping source.
24.4.5 TACAN Position Updating. The TACAN system may also be used for position updating
purposes. To do this the TACAN system must be in the T/R mode with the proper channel (X or Y)
and channel number selected. The TACAN station selected must be one of the prestored stations.
Selecting the UPDT option on the MPCD top level display provides the UPDT option display.
　
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