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 identi- fication 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 oper- ates on a frequency of 1,090 MHz. Because these fre- quencies are in the UHF band, the operational range is limited to line-sight. Power to operate the system is sup- plied 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 func- tions 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 fea- ture 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  inter- changeable. 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 trans- ponder receiver and computer; transmitter in- operative. Set ready for operation after 2 minute warmup in STBY. NORM Transponder in normal operation. CONTROLS/ INDICATOR FUNCTION EMER Transmits emergency re- ply signals to mode 1, 2, or 3/A and 4 interroga- tions, regardless of mode control settings. M-1, M-2, M-3/A Switches At ON position, the appli- cable mode is selected for operation. OUT posi- tion turns off applicable mode. AT TEST position, that particular mode sis tested for proper opera- tion. If test is valid, TEST lamp will illuminate. M-C In mode C, altitude infor- mation is provided to ground controllers by the AAU-32A encoding alti- meter. 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 op- eration. HOLD This function is enabled only when the right aft landing gear strut is com- pressed. 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 trans- ponder is turned on.