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

I want to walk you through Secondary Surveillance Radar — SSR — and I want to start by making sure you understand what makes it "secondary" in the first place. Primary radar works by bouncing a signal off the aircraft's skin and listening for the echo. SSR is different: the ground station sends out an interrogation, and the aircraft's transponder actively replies with a coded signal. That's the whole idea — the aircraft is a participant, not just a reflector. Let's look at the frequencies, because they're very specific and you'll be expected to know them cold. The ground station transmits — we say it interrogates — on 1030 megahertz, and it receives on 1090 megahertz. The aircraft is the mirror image of that: it receives on 1030 and transmits — we say it transponds — on 1090, and it does so after a delay of 50 microseconds. That delay is deliberate; it gives the system time to process the interrogation before the reply goes out. Now, the antenna patterns matter here. The SSR ground antenna transmits a narrow beam in the horizontal plane — that's how the controller knows the bearing of the aircraft. But the aircraft transmits omni-directionally, meaning its radiation pattern is circular around the aircraft. So the reply goes out in all directions, and the ground station uses the narrow beam it transmitted to determine where the aircraft is. Let me bring in a figure that shows this. Now, the modes. The aircraft is interrogated by a predetermined series of pulses on the 1030 MHz carrier, and its 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. So Mode A tells the controller who you are, and Mode C tells them how high you are. To differentiate between the interrogations, three pulses are always transmitted — they're labelled P1, P2, and P3. The spacing between P1 and P2 is fixed at 2 microseconds. The spacing between P1 and P3 is what determines the mode: it's 8 microseconds for a Mode A interrogation, and 21 microseconds for a Mode C interrogation. So the transponder looks at that P1-to-P3 spacing and knows whether it's being asked for identity or for altitude. Now let's talk about the reply. The aircraft transponder will reply correctly to a Mode A or C interrogation provided the pilot has correctly selected the mode and the code allocated by ATC. That's a critical operational point — the transponder only does what you've set it to do, so the mode and code selection is your responsibility. On receiving a valid interrogation, the transponder transmits two framing pulses, F1 and F2, which are 20.3 microseconds apart. Between those framing pulses there are 12 usable information pulses. There's one pulse labelled X, and that's reserved for Mode B, which is at present unused. So effectively you have 12 pulses that can each be transmitted or not transmitted. That gives you 2 to the power of 12 — 4096 possible combinations of pulses, or codes. Those codes are numbered 0000 to 7777, and the figures 8 and 9 are not available. So you're working in an octal system, essentially — digits 0 through 7 only. Let me show you the reply pulse pattern so you can see the framing and the information pulses laid out. There's one more pulse I want you to know about — the Special Position Identification pulse, abbreviated SPI. This pulse may be transmitted together with the information pulses when the pilot presses the "Ident" button on the transponder, usually at ATC's request. It comes after the last framing pulse, and it's automatically and continuously transmitted for about 20 seconds. What it does is produce 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 when you hear "squawk ident," that's the SPI pulse being activated. That's the core of SSR — the interrogation on 1030, the coded reply on 1090, the mode determined by pulse spacing, and the 4096 possible codes with the SPI pulse for identification.