TM 1-1520-238-10
3-66
Section IV.
TRANSPONDER AND RADAR
3.17 TRANSPONDER RT-1296/APX-100(V)1(IFF) AND
RT-1557/APX-100(V)(IFF).
The transponder set (fig 3-26) provides automatic radar
identification of the helicopter to all suitably equipped
challenging aircraft as well as surface or ground facilities
within the operating range of the system. The
RT-1296/APX-100(V) and the RT-1557/APX-100(V) re-
ceives, decodes, and responds to the characteristic
interrogation of operational MODE 1, 2, 3/A, C, and 4.
Specially coded identification of position (IP) and emer-
gency signals may be transmitted to interrogation stations
when conditions warrant. The transceiver can be oper-
ated in any one of four master modes, each of which may
be selected by the operator at the control panel. Five cod-
ing modes are available to the operator. The first three
modes may be used independently or in combination.
MODE 1 provides 32 possible code combinations, any
one of which may be selected in flight. MODE 2 provides
4096 possible code combinations, but only one is avail-
able and is normally preset before takeoff. MODE 3/A pro-
vides 4096 possible codes, any one of which maybe se-
lected in flight. MODE 4 is an external computer mode and
can be selected to provide any one of many classified op-
erational codes for security identification. Power to oper-
ate the IFF system is provided from the dc emergency bus
through the IFF circuit breaker on the pilot overhead cir-
cuit breaker panel.
3.17.1 Antenna.
Two blade antennas (fig 3-1) are used
to receive interrogating signals and to transmit reply sig-
nals. The upper IFF antenna is installed on top of the fuse-
lage area aft of the canopy, and the lower IFF antenna is
installed on the bottom of the fuselage as an integral part
of the UHF-AM antenna. Some helicopters have the up-
per IFF antenna installed on the work platform forward of
the main rotor mast and the lower IFF antenna installed on
the bottom of the fuselage aft of the tail boom jack pad.
3.17.2 Controls and Functions.
All operating and
mode code select switches for transceiver operation are
on identical control panels (fig 3-26) for the
RT-1296/APX-100(V) and the RT-1557/APX-100(V). Con-
trol and indicator functions of the RT-1296/APX-100(V)
and the RT-1557/APX-100(V) transponder are described
in table 3-23.
