CHAPTER 3
RADAR SYSTEM INTERFACING
In the previous chapters, we discussed a basic pulse
radar system, basic types of radar sets and specific radar
equipment used in the fleet. Most every radar we’ve
mentioned can interface with other systems. In this
chapter we’ll look at some of the systems that use that
radar information, such as Identification Friend or Foe
(IFF) systems, Direct Altitude and Identity Readout
(DAIR) systems, and Navy Tactical Data Systems
(NTDS). We will not teach you specific equipment, but
will help you identify and understand the interface of
radar information with the various systems used in the
Navy today.
Most of the equipment discussed in this chapter has
specific maintenance training available. However,
except for certain crypto equipment, you do not need
specific training to work on the gear. Remember, as an
ET, you can become an expert maintainer of ANY
electronic equipment.
The first system we’ll talk about is Identification
Friend or Foe (IFF) equipment, specifically, the AIMS
Mark XII IFF system, used by aircraft and surface
vessels.
IDENTIFICATION FRIEND OR FOE
(IFF) SYSTEMS
IFF equipment, used with search radars, permits
automatic identification of targets before they are near
enough to threaten the security of a friendly craft. In
addition to friendly identification, modern IFF systems
also provide other information such as type of craft,
squadron, side number, mission, and aircraft altitude.
GENERAL THEORY OF OPERATION
IFF completes the identification process in three
basic steps: (1) challenge, (2) reply, and (3) recognition.
Challenge
The IFF interrogator sends a coded challenge in the
form of pulse pairs. The selected mode of operation
determines the spacing between the pulses.
Reply
A friendly target’s IFF transponder will
automatically reply to the coded challenge with an
omnidirectional transmission. It sends a different set of
pulses at a slightly different frequency than the
interrogator frequency. A suppression (blanking) signal
keeps your ship’s transponder from replying to its own
interrogator.
Recognition
The IFF interrogator receives the coded reply and
processes it for display on an indicator. Recognition of
the target is based on the ppi display. The coded reply
from a friendly craft normally appears as a dashed line
just beyond the target blip, as shown in figure 3-1.
The identification process uses two sets of IFF
equipment, the interrogator set and the transponder set.
A ship may have one or more interrogator sets, but will
have only one transponder set. Normally, interrogators
and transponders aboard ships function independently.
Interrogator
The IFF interrogator operates like a radar
transmitter and receiver. It uses a small directional
antenna either attached to or rotated in synchronization
with the air search radar antenna. The modulator of the
search radar set provides synchronization triggers for
the IFF interrogate.
When processing replies for display, the IFF
interrogator uses the time lapse between the
transmission of a challenge and the reception of a reply
to determine range.
The synchronized antenna
information provides the correct bearing.
A high output power is not required for the one-way
trip to the target taken by the transmitted pulses, so the
IFF interrogator can operate at low peak power (1 to 2
kilowatts).
Transponder
The IFF transponder is a receiver-transmitter
combination that automatically replies to a coded
3-1
