TM 1-1520-240-10
3-4-1
SECTION IV. TRANSPONDERS
3-4-1. Transponder System (AN/APX-100).
The transponder system provides automatic radar identification
of the helicopter. The system receives, decodes,
and replies to interrogations on modes 1, 2, 3/A, 4, TEST
and C from all suitably equipped challenging helicopter
or ground facilities. The receiver section operates on a
frequency of 1,030 MHz and the transmitter section operates
on a frequency of 1,090 MHz. Because these frequencies
are in the UHF band, the operational range is
limited to line-sight. Power to operate the system is supplied
by the No. 2 DC bus through the COMM IFF circuit
breaker on No. 2 PDP.
The integral receiver-transmitter-control panel is on the
console. It provides the control switches for application
of power, setting of the modes and codes, modes of
operation, identification of position, and emergency functions
of the set. It receives coded interrogating pulses
and tests them for validity. If the signals conform to the
preset mode and code, a codes reply is transmitted.
Additional preset codes for emergency use are available
when selected. These emergency codes are transmitted
on modes 1, 2, 3/A and 4 regardless of codes selected.
Transponder functions are continuously monitored by
built-in-test circuits. Each mode also has a self-test feature
which can be selected by the pilot to verify operation.
The RT-1285/APX-100 is the interim NVG compatible
control. The RT-1558/APX-100 is the NVG Blue Green
version. Both units are form, fit and functionally interchangeable.
The receiver-transmitter must be removed
from the console to replace the fuse.
3-4-2. Controls and Function, Transponder Control
(TR-1285/APX-100) (RT-1558/APX-100). (fig. 3-4-1)
CONTROLS/
INDICATOR
FUNCTION
MASTER Control
Switch:
OFF Removes power from
transponder set and
computer
STBY Power is applied to transponder
receiver and
computer; transmitter inoperative.
Set ready for
operation after 2 minute
warmup in STBY.
NORM Transponder in normal
operation.
CONTROLS/
INDICATOR
FUNCTION
EMER Transmits emergency reply
signals to mode 1, 2,
or 3/A and 4 interrogations,
regardless of mode
control settings.
M-1, M-2, M-3/A
Switches
At ON position, the applicable
mode is selected
for operation. OUT position
turns off applicable
mode. AT TEST position,
that particular mode sis
tested for proper operation.
If test is valid, TEST
lamp will illuminate.
M-C In mode C, altitude information
is provided to
ground controllers by the
AAU-32A encoding altimeter.
Switch position
functions are the same
as for M-1, M-2, and
M-3A switches.
MODE 1 Selector
Switches
Selects and indicates a
two digit 32 code reply
number.
MODE 2 Selector
Switches
Selects and indicates a 4
digit 4096 code reply
number. (Switches are
preset and covered by a
guard. Only the reply
code can be seen.)
MODE 3/A Selector
Switches
Selects and indicates a 4
digit 4096 code reply
number.
MODE 4 CODE Control
Switch
Selects type MODE 4 operation.
HOLD This function is enabled
only when the right aft
landing gear strut is compressed.
It holds the
MODE 4 code which
would otherwise be
cleared when the set is
turned off or electrical
power is removed.
Switch is spring-loaded
to A. HOLD function is
reset when the transponder
is turned on.
TM 1-1520-240-10
3-4-2
CONTROL/
INDICATOR
FUNCTION
A Enables transponder to
reply to code A interrogations.
B Enables transponder to
reply to code B interrogations.
ZERO Clears (zeroizes mode 4
code settings in transponder
computer.
MODE 4 AUDIO LIGHT
OUT Switch
AT AUDIO, the REPLY
indicator light will illuminate
when the transponder
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