Chapter 3 Radar System Interface

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