synchronizer, and a delayed radar trigger is sent back
to the radar set after the interrogation trigger is
generated. The synchronizer develops three basic
. 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
. 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
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 antennas 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
antennas 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 antennas 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-amplifiers 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
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.