synchronizer, and a delayed radar trigger is sent back to the radar set after the interrogation trigger is generated. The synchronizer develops three basic signals: . The interrogator trigger which, via the receiver-transmitter modulating circuit, modulates the transmission. The interrogation trigger consists of three pulses. P1 and P3 are interrogation pulses spaced either 3, 5, or 8 microseconds apart. In mode 1, the spacing is 3 microseconds, mode 2 has a 5-microsecond space, and mode 3/A has an 8-microsecond space. The ISLS pulse (P2) is always 2 microseconds after the P1 pulse. . The gain time control (GTC) trigger, which initiates the receiver gate and the GTC circuits in the receiver-transmitter. . The ISLS control gate, which controls switching of the antenna pattern for ISLS purposes. The ISLS control gate, which precedes the P2 ISLS pulse by about 0.6 microsecond, is applied to the switch amplifier. When triggered by the interrogation trigger, the receiver-transmitter generates RF pulses at the transmitting frequency of 1030 MHz at a nominal peak power level of 2 kW. These pulses are applied to the sum channel of the switch-amplifier. The difference channel path between the receiver-transmitter and the switch-amplifier is not used during transmission. When the ISLS control gate is not present at the switch-amplifier, the RF pulses from the RT sum channel are passed straight to the antenna via the sum channel line for radiation out of the sum antenna pattern. This pattern is a main lobe with its axis along the antenna’s boresight, and small side lobes on either side of the boresight. The P1 and P3 interrogation pulses are radiated in the sum pattern. W h e n  t h e  I S L S  g a t e  a r r i v e s  a t  t he switch-amplifier, shortly before the P2 pulse, the sum channel input at the switch-amplifier is switched to the difference channel coax to the antenna. The switch-amplifier amplifies the P2 ISLS pulse to a nominal peak power of 8 kW prior to its being sent to the antenna. This pulse is transmitted through the difference antenna pattern, which is two main lobes with their axes about 20 degrees on either side of the antenna’s boresight, with a minimum signal strength along the boresight. The reason for the two radiation patterns is to narrow the antenna beamwidth effectively in conjunction with the operation of the ISLS circuit at the transponder. The IFF transponder responds only to the P1 and P3 interrogation pukes, which are at least 9 dB above the P2 pulse. The transponder will not respond if the P2 pulse level exceeds the P1 level. If an IFF transponder is located more than a small angle off of the antenna’s boresight, the P2 pulse will be received at a much higher level than the P1 and P3 pulses, and no reply will be generated. MODE 4 TRANSMISSION.— Mode 4 opera- tion is very similar to that of modes 1,2, and 3/A. The major difference is that the KIR-1A computer generates the interrogation trigger, GTC trigger, and the ISLS control gate instead of the synchronizer. The synchronizer generates a mode 4 pretrigger that is sent to the computer. The computer then prepares the challenge video, which is sent back to the synchronizer for entry onto the interrogator trigger line to the RT. Mode 4 transmission is the same as in the other modes as far as the patterns are concerned. Both the sum antenna pattern and the difference antenna pattern are used. RECEPTION.— During reception, both the sum and difference antenna patterns are open for IFF transponder reply energy. The purpose of this is to effectively narrow the reply reception antenna bandwidth. This function is called RSLS. The reply RF energy is applied to the receiver-transmitter via the switch-amplifier’s sum and difference channels. If the energy is above maximum receiver sensitivity, and the sum channel energy is greater than the difference channel energy by a fixed factor, the receiver will develop video pulses for decoding. These video pulses are applied to the mode 4 decoding circuits within the receiver-transmitter and to modes 1, 2, and 3/A decoding circuits in the synchronizer. For modes 1, 2, and 3/A, the video pulses are checked for the presence of the standard IFF reply bracket pulses spaced 20.3 microseconds apart. If these pulses are there, a single video pulse is applied to the radar set for display. The coded pulses between the brackets are checked against the code selected at the IFF interrogator control. If the codes match, another single display video pulse is applied to the radar set for display. This second pulse is delayed 12 microseconds (or 28 microseconds if the long-range display is enabled) from the first pulse. When the correct code challenge is used to initiate the interrogation, the first pulse (bracket decode) is not sent to be displayed. 3-22