MODE 4. Mode 4 operation is for military use
only and allows for secure identification of friendly
aircraft and surface vessels.
generates a reply code according to a preset crypto key
list. As shown in figure 3-2, mode 4 interrogations use
encoded, multipulse trains with 4 (sync) pulses and an
ISLS pulse, followed by up to 32 information pulses.
When the transponder receives and processes a
valid mode 4 interrogation, it sends out a time-coded,
three-pulse reply. The interrogator converts the valid
mode 4 reply back to one pulse. The reply is then time
decoded before it is presented on the indicator. There
are no emergency replies for mode 4 or mode C.
MODE C. Mode C replies used by civilian and
military aircraft indicate aircraft altitude and are taken
automatically from the aircrafts barometric altimeter.
Mode C interrogations are the same as those for SIF
modes. Replies are binary codes contained between
bracket pulses similar to those for SIF modes.
The reply, derived from an encoder linked to the
aircraft altimeter, may represent any altitude from
-1,000 feet to +126,700 feet in 100-foot increments.
Shipboard transponders are wired to reply to mode C
interrogations with bracket pulses only (code 0000).
Commercial aviation has implemented the Traffic
Alert and Collision Avoidance System (TCAS), which
uses a low-power mode C interrogator-processor.
Using mode C altitude reports, it computes the closest
point of approach (CPA) to other aircraft and displays
the information as an overlay on the weather radar
indicator. General aviation aircraft flying below 12,500
feet reply to mode C with empty brackets (code 0000),
the same code used by Navy ships.
TCAS cannot distinguish between replies sent by
your ship and those sent by small aircraft. It assumes
that a mode C target is at the same altitude as itself if no
altitude is reported. Therefore, your ships mode C reply
can set off a projected collision alarm in the cockpit of
an arriving or departing airliner, causing the pilot to
make unnecessary and dangerous maneuvers. Since
this situation is a great threat to air safety, your
transponders mode C should always be secured in or
near port, unless you are testing the unit, with the
As we mentioned earlier, the interrogator and
transponder sections of the AIMS Mark XII IFF operate
independently of each other.
In the following
paragraphs, well discuss each section, beginning with
the interrogator section.
INTERROGATOR SECTION. The major units
of the interrogator section (except the video decoder
group) are usually mounted in a rack located in the radar
equipment room, as shown in figure 3-3.
A simplified block diagram of the interrogator
section is shown in figure 3-4. The Interrogator Set
AN/UPX-23, provides rf challenges for the various
It also receives transponder replies and
processes them into proper video signals for application
to the decoders and indicators.
The pulse generator provides IFF system
pretriggers that initiate challenges for the enabled
modes. In a slaved IFF system, associated with a
specific radar, the pulse generator synchronizes the
interrogations with the radar. In a black IFF system,
not associated with a radar, it produces triggers
Figure 3-3.Mark XII IFF interrogator equipment.