Transponder Set Functional Description

transmit a reply when RF interrogation is received from an IFF interrogator. If the interrogation is valid, a coded reply is transmitted. This reply is received by the interrogator and processed for display for aircraft identification and location. The transponder is capable of operating in five modes and superimposing four special signals on the mode replies. COMPUTER KIT-1A/TSEC.— This computer allows the IFF transponder to respond to mode 4 interrogations. Mode 4 is a secure mode of operation. TRANSPONDER TEST SET TS-1843/ APX.— The test set (fig. 3-19) generates properly coded test signals for the desired mode. These interrogation signals are then applied to the transponder. The test set then checks the replies for frequency, bracket-pulse spacing, power, and antenna standing-wave ratio. The resulting IFF system check will provide a GO/NO-GO indication on the IFF transponder control box. Transponder Set Functional Description The IFF transponder control box allows an automatic IFF capability when the aircraft is interrogated by a valid interrogation. Special modes and codes can be manually set on the IFF control box, receiver-transmitter, and on the computer. The control box also initiates the self-test function through the test set. RECEIVED SIGNALS.— The interrogator- transmitted signals are received by the aircraft through the UHF L-band blade antennas. These signals are on a frequency of 1030 MHz. The receiver-transmitter recognizes the signals through pulsewidth and spacing detection. Modes 1, 2, 3/A, C, and TEST use two interrogation pulses and one side-lobe suppression pulse that are 0.8 (±0.1) microsecond wide. Pulse spacings between the two interrogation pulses are slightly different, depending on the mode. These spacings are as follows: Mode 1: 3.0 (±0.2) microseconds Mode 2: 5.0 (±0.2) microseconds Mode 3/A: 8.0 (±0.2) microseconds Mode C: 21.0 (±0.2) microseconds TEST: 6.5 (±0.2) microseconds Figure 3-19.-TS-1843/APX test set. IFF TRANSPONDER RECOGNITION.— Recognition of the interrogation mode is done by passing the first interrogation pulse through a time-delay circuit and matching this first pulse with the second pulse position. The delays are of 3, 5, 8, or 21 microseconds. Mode 4 capability is provided when the computer is operating in the system. Mode 4 interrogation pulse characteristics consist of four pulses 0.5 (±0.15) microsecond wide, referenced from the leading edge of the first pulse in multiples of 2 microseconds. The 4 pulses maybe followed by as many as 33 additional pulses spaced as close as 2 microseconds. The side lobe suppression pulse is placed 2 (±0.15) microseconds from the leading edge of the fourth pulse. SIDE-LOBE SUPPRESSION.— The side-lobe suppression pulse allows the transponder to accept the main lobe and to reject minor lobe signals from the interrogation stations. This ensures correct operation of the system. 3-17