Equipment Components

MODE 4.— Mode 4 operation is for military use only and allows for secure identification of friendly aircraft and surface vessels. IFF automatically 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 aircraft’s 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 ship’s 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 transponder’s mode C should always be secured in or near port, unless you are testing the unit, with the antenna disconnected. internally. Equipment Components As we mentioned earlier, the interrogator and transponder sections of the AIMS Mark XII IFF operate independently of each other. In the following paragraphs, we’ll 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 modes. 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. 3-4