Section IV TRANSPONDER AND RADAR
3.21 TRANSPONDER AN/APX-100(V)1 (IFF).
The transponder set (Figure 3-33) provides automatic radar identi.cation of the helicopter to all suitably equipped challenging aircraft and surface or ground facilities within the operating range of the system. AN/APX-100(V) receives, decodes, and responds to the characteristic inter-rogations of operational modes 1, 2, 3/A, C, and 4. Spe-cially coded identi.cation of position (IP) and emergency signals may be transmitted to interrogating stations when conditions warrant. The transceiver can be operated in any one of four master modes, each of which may be selected by the operator at the control panel. Five independent cod-ing modes are available to the operator. The .rst three modes may be used independently or in combination. Mode 1 provides 32 possible code combinations, any one of which may be selected in .ight. Mode 2 provides 4096 possible code combinations, but only one is available and is normally preset before takeoff. Mode 3/A provides 4096 possible codes any one of which may be selected in .ight. Mode C will indicate pressure altitude of the helicopter when interrogated. Mode C is only available if both mode 3/A and mode C switches are placed to the ON position. Mode 4 is the secure mode of cooperative combat identi.-cation, IFF operational codes are installed, the current pe-riod’s code and either the previous or the next period’s code. Power to operate the IFF system is provided from the No. 1 dc primary bus through a circuit breaker marked IFF. Refer to TM 11-5895-1199-12 and 11-5895-1037-12.
3.21.1 Antenna.
The transponder will ignore (and not re-spond to) interrogations received from the ground if the ANT switch is in the TOP position and will ignore interrogations re-ceived from above if the ANT switch is in the BOT position.
Antennas are installed on the top fairing between engine exhaust ports, (Figure 3-1) and under the transition section behind the UHF-AM antenna. They receive signals of in-terrogating stations and transmit reply signals. The AN/ APX-100(V) is a diversity transponder, functioning to re-ceive the rf interrogation from two antennas and transmit the reply via the antenna from which the stronger interro-gation signal was received. If the ANT switch is in the TOP position and the stronger signal was received from the bottom antenna, no rf reply will be transmitted. If the ANT switch is in the BOT position and the stronger signal was received from the top antenna, no rf reply will be transmit-ted. Therefore the ANT switch must be in the DIV position to insure the IFF will reply to all valid interrogations.
3.21.2 Controls and Functions. All operating and mode code select switches for transceiver operation are on Control Panel RT-1296/APX-100(V) (Figure 3-33).
CONTROL/ INDICATOR FUNCTION
TEST GO Indicates successful BIT.
TEST/MON NGO O Indicates unit malfunction .
ANT-DIV switch Allows the pilot to select (upper antenna), BOT antenna), or DIV (diversity, both antennas) of the aircraft. the TOP (bottom
NOTE
The ANT-DIV switch placed in the DIV posititimes. so hall n at be all
MODE 3A FUNCTION SWITCH
MODE 1 SWITCH
MODE 4 CODE
SWITCH
MODE 4
AUDIO.LIGHT.OUT
SWITCH
AA0363A
SA
CONTROL/ INDICATOR
MASTER/OFF/ STBY/NORM/ EMER
M-1, M-2, M-3/A, M-C
switches
RAD TEST/ OUT
RAD TEST
OUT FUNCTION
Selects operating condition.
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