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

I want to walk you through the heart of the Secondary Surveillance Radar system — what it displays, how it talks to the aircraft, and the codes and modes that make it work. This is the system ATC uses to see you as a labelled target, not just a blip. First, what does the controller actually see on the screen? The display shows the aircraft's call sign — that's your registration or flight number. It shows pressure altitude or flight level — that's your altitude referenced to the standard pressure setting of 1013.25 hPa, which is what we call flight level. It shows ground speed — your speed over the ground, not through the air. And it shows destination — where you're headed. So the controller gets a full picture of who you are, how high, how fast, and where you're going. Now the frequencies. This is a two-way conversation. The ground station transmits a narrow beam at 1030 MHz and receives at 1090 MHz. The aircraft receives at 1030 MHz and transmits omni-directionally at 1090 MHz. Notice the asymmetry: the ground sends a narrow, focused beam — that's the interrogation — and the aircraft replies in all directions, omni-directionally. This whole exchange happens in the UHF band, the ultra-high frequency range. So the ground asks, the aircraft answers, and the ground listens on a different frequency than it transmits on. Now the modes — these define what the aircraft is being asked. Mode A is for identity. The interrogation pulses are spaced 8 microseconds apart. The reply consists of 12 pulses, and those 12 pulses give 4096 combinations — that's your four-digit squawk code. The framing pulses — the pulses that mark the start and end of the reply — are spaced 20.3 microseconds apart. There's also an extra pulse called SPI, which stands for Special Position Identification — that's the squawk Ident function. When you press Ident, that extra pulse is transmitted for 20 seconds, so the controller sees your target flash on the screen. Mode C is for automatic pressure-altitude. The interrogation pulse spacing here is 21 microseconds. The aircraft transmits its altitude, and it's displayed every 100 feet, with an accuracy of ±50 feet from the given level. So if you're cleared to FL 350, the readout should be within 50 feet of that. There's a critical rule: you must switch off Mode C if the difference exceeds 300 feet — and in the UK, the threshold is tighter at 200 feet. That's a safety limit to prevent a faulty altimeter from giving ATC a false altitude readout. Now the 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 standard across the system, and every pilot must know them cold. But SSR has disadvantages, and I want you to understand them because they explain why Mode S exists. First, garbling — that's when replies overlap because two aircraft are less than 1.7 nautical miles apart. Their replies collide and the ground station can't read either properly. Second, fruiting — that's interference caused by replies to other interrogations. Your transponder replies to every interrogation it hears, including ones from other radar stations, and those stray replies clutter the picture. Third, there's a limited number of codes — only 4096, which isn't enough in busy airspace. That brings us to Mode S — the S stands for Selective. Its key feature is selective addressing. Instead of 4096 codes, it uses a 24-bit address that gives nearly 17 million unique codes. Every aircraft gets its own permanent address, so there's no code shortage. Mode S also provides a data link — air-to-ground, ground-to-air, and even air-to-air. And the height readout is in increments of 25 feet, much finer than Mode C's 100 feet. Mode S has several interrogation modes. There's All Call — that's how the ground station finds out which aircraft are in range. There's Broadcast — which requires no reply. There's selective calling — using the aircraft's unique address to talk to one specific aircraft. And there's Intermode — which is an A/C/S All Call, meaning it can interrogate Mode A, Mode C, and Mode S aircraft all at once. Finally, the benefits of Mode S. It gives unambiguous aircraft identification — no more guessing which blip is which. It gives improved surveillance — it eliminates garble, and it resolves over-interrogation and reflections. It gives the radar controller improved situation awareness. There's no code shortage — 17 million addresses. There's reduced R/T — reduced radio telephony, because the data link carries information that used to require voice. And there's improved short term conflict alert — the system can warn the controller sooner when two aircraft are on a collision course. So the whole picture: SSR interrogates on 1030, the aircraft replies on 1090, Mode A gives identity, Mode C gives altitude, and Mode S gives selective, unambiguous, data-linked surveillance with far more codes and finer altitude resolution. That's the system that puts your call sign, altitude, ground speed, and destination on the controller's screen.