Displays call sign, pressure altitude or FL, ground speed, destination — Page 241, Lesson 232

We're now moving into the heart of Secondary Surveillance Radar — the part that actually puts the information on the controller's screen. Let me walk you through what the display shows, how the frequencies work, and then the modes and codes that make it all function. First, the display. When a controller looks at an SSR return, they see the aircraft's call sign, its pressure altitude or flight level, its ground speed, and its destination. So the display isn't just a blip — it's a data block with identity, altitude, speed, and where the aircraft is going. Now the frequencies. This is a critical distinction between primary and secondary radar. The ground station transmits a narrow beam at 1030 MHz and receives at 1090 MHz. The aircraft does the opposite: it receives at 1030 MHz and transmits omni-directionally at 1090 MHz. Both of these are in the UHF band. So the interrogation goes down from the ground at 1030, and the reply comes back up from the aircraft at 1090 — omni-directionally, meaning the aircraft broadcasts in all directions, not a narrow beam. Now let's get into the modes and replies. There are two main modes you need to know. Mode A is for identity. The interrogation pulse spacing is 8 microseconds. The reply consists of 12 pulses, and these give 4096 combinations. The framing pulses are spaced 20.3 microseconds apart. There's also an extra pulse called SPI — that's Special Position Identification — used for squawk Ident, and it lasts for 20 seconds. So when a pilot presses Ident, that SPI pulse is transmitted for 20 seconds so the controller can positively identify the aircraft. Mode C is for automatic pressure-altitude. The interrogation spacing here is 21 microseconds. The altitude is transmitted and displayed every 100 feet, with a tolerance of ±50 feet from the given level. And here's an important operational rule: you switch off Mode C if the difference is greater than 300 feet — though in the UK that threshold is 200 feet. So if the altitude readout disagrees with the assigned level by more than that, the transponder's altitude reporting is switched off. Now, special codes. These are squawk codes with specific meanings. 7700 is emergency. 7600 is radio failure. 7500 is unlawful interference — that's the hijack code. These are universal and the controller recognises them instantly. But SSR has its disadvantages, and you need to understand these because they're the reason Mode S was developed. First, garbling — that's overlapping replies when aircraft are less than 1.7 nautical miles apart. Second, fruiting — that's interference caused by replies to other interrogations, from other radars. And third, limited codes — only 4096 of them, which isn't enough in busy airspace. That brings us to Mode S — the S stands for selective. Its key feature is selective addressing: nearly 17 million codes from a 24-bit address. So each aircraft has a unique address. Mode S also provides data link — air-to-ground, ground-to-air, and air-to-air. And the height read-out is in increments of 25 feet, much finer than Mode C's 100 feet. There are several interrogation modes in Mode S. There's All Call — that's mode S. There's Broadcast, which gets no reply. There's Selective calling, using the unique aircraft address. And there's Intermode — that's an A/C/S All Call, which covers all three modes. Finally, the benefits of Mode S. It gives unambiguous aircraft identification. It gives improved surveillance — it eliminates garble, resolves over-interrogation and reflections. It gives improved situation awareness for the radar controller. There's no code shortage — with 17 million addresses, you're not running out. It gives reduced R/T — reduced radio telephony, less talking. And it gives improved short term conflict alert — the system can warn the controller of potential conflicts sooner. So to tie it together: SSR interrogates at 1030 MHz, the aircraft replies at 1090 MHz, Mode A gives identity, Mode C gives altitude, and Mode S gives selective addressing with data link and much finer altitude resolution. The special codes 7700, 7600, and 7500 are the emergency, radio failure, and unlawful interference squawks. And the disadvantages of conventional SSR — garbling, fruiting, and limited codes — are exactly what Mode S was designed to fix. Now, the excerpt ends with a set of practice questions on this material. Those are the book's practice questions — let's try them one at a time.