Secondary Surveillance Radar (SSR) — Page 235, Lesson 218

Let’s start with the big picture, because this chapter is all about how air traffic control actually "sees" you in the sky. We’ve got primary radar, which bounces a signal off the aircraft skin and shows a blip. But secondary radar is different — it’s a cooperative system. The ground station asks a question, and the aircraft’s transponder answers. That’s the whole idea behind Secondary Surveillance Radar, or SSR. Now, the frequencies. The ground station transmits — we call that interrogating — on 1030 MHz. It receives the reply on 1090 MHz. On the aircraft side, it’s the mirror image: the aircraft receives on 1030 MHz and transmits, or transponds, on 1090 MHz. And there’s a specific delay built in — the aircraft replies after a delay of 50 microseconds. That delay is deliberate, so the ground station can measure range cleanly. Let’s talk about the antenna patterns, because they’re opposite. The SSR ground antenna transmits a narrow beam in the horizontal plane — that’s how the controller knows your bearing, by which direction the beam is pointing when you reply. But the aircraft transmits omni-directionally, meaning the radiation pattern is circular around the aircraft. So your reply goes out in all directions, and the ground station just hears it when its narrow beam is pointed at you. Now, the modes. The ground station interrogates the aircraft with a predetermined series of pulses on the 1030 MHz carrier. The transponder then transmits a coded reply on 1090 MHz. There are two main modes you need to know. Mode A is an interrogation to identify an aircraft. Mode C is an interrogation to obtain an automatic height read-out of the aircraft — that’s your altitude, transmitted automatically. How does the transponder know which mode is being asked? That’s where the pulse spacing comes in. To differentiate between the interrogations, three pulses are always transmitted: P1, P2, and P3. The spacing between P1 and P2 is fixed at 2 microseconds — that never changes. The spacing between P1 and P3 is what determines the mode. For a Mode A interrogation, the P1-to-P3 spacing is 8 microseconds. For a Mode C interrogation, it’s 21 microseconds. So the transponder measures that gap and knows whether you’re being asked for identity or altitude. Now let’s look at the reply. The transponder will reply correctly to a Mode A or C interrogation provided the pilot has correctly selected the mode and code allocated by ATC. That’s a key point — the pilot has to set the right code on the transponder panel, or the reply won’t be what the controller expects. When the transponder receives a valid interrogation, it transmits two framing pulses, called F1 and F2, 20.3 microseconds apart. Those frame the reply. Between them, there are 12 usable information pulses. There’s also a pulse labelled X, but that’s for Mode B, which is at present unused — so you can ignore it operationally. Each of those 12 information pulses can be transmitted or not. That gives you 2 to the power of 12 — 4096 possible combinations of pulses, or codes. Those codes are numbered 0000 to 7777, and here’s the catch: the figures 8 and 9 are not available. So it’s an octal system — digits only go 0 through 7. There’s one more pulse worth knowing: the Special Position Identification pulse, or SPI. It may be transmitted together with the information pulses when the pilot presses the "Ident" button on the transponder, usually at ATC’s request. This pulse comes after the last framing pulse, and it’s automatically and continuously transmitted for about 20 seconds. It produces a distinctive display on the controller’s screen, so the controller can pick out a particular aircraft by asking the pilot to "Squawk Ident." So to tie it together: the ground station interrogates on 1030 MHz with a specific pulse spacing that defines the mode. The transponder replies on 1090 MHz with a 12-pulse code framed by F1 and F2, and optionally an SPI pulse when you press Ident. The whole system is cooperative — it only works if your transponder is set correctly and answering.