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