I want you to think of primary radar first, because that’s the baseline — Page 227, Lesson 213

Let’s start with the big picture, because this chapter is all about a fundamental shift in how radar works. I want you to think of primary radar first, because that’s the baseline. Primary radar relies on the reception of a reflected pulse — that is, the echo of the transmitted pulse. The ground station sends out a pulse, it bounces off the aircraft, and a tiny fraction of that energy comes back. That’s the echo. Now, secondary radar is completely different in principle. Instead of relying on a reflection, secondary radar receives pulses that are transmitted by the target itself, in response to interrogation pulses. So the ground station doesn’t just listen for an echo — it actively asks a question, and the aircraft answers by transmitting its own pulses. Secondary surveillance radar, which we abbreviate SSR, is one type of secondary radar system. And I want you to note that DME is another such system, but that’s covered in Chapter 15, so we’ll leave it there for now. Both primary and secondary surveillance radars are used to track the progress of an aircraft. But they have very different strengths. Primary radar actually provides better bearing and range information of an aircraft than SSR does. That’s an important point — primary is more accurate on position. However, its biggest disadvantage is the lack of positive, individual aircraft identification. In other words, primary radar just shows you a blip; it doesn’t tell you which aircraft that blip is. And that positive identification is required for adequate safe control by ATC, particularly in crowded airspace. There’s also a practical drawback: primary radars require higher transmitter power outputs, because the single pulse has to make a two-way journey — out to the aircraft and back as an echo. So how does SSR solve this? It requires the aircraft to be fitted with a transmitter and receiver combined, and that unit is called a transponder. The pilot will set a four-figure code that has been allocated by ATC. Then, when the ground station — which we call the interrogator — sends out its interrogation, the transponder will transmit information automatically, in pulse coded form. So the transmissions are only one way from transmitter to receiver. That’s a key phrase: the reply is one-way, from the aircraft’s transmitter to the ground receiver, which is why it doesn’t suffer the two-way power loss of primary radar. Now let’s look at the advantages of SSR over primary radar, because this is the heart of the chapter. First, it requires much less transmitting power to provide coverage up to 200 to 250 nautical miles. That’s a specific range figure — 200 to 250 NM — and it’s achievable with far less power because you’re not fighting the echo loss. Second, SSR is not dependent on an aircraft’s echoing area or aspect. With primary radar, a small aircraft or one viewed from an awkward angle gives a weak echo. SSR doesn’t care about that, because the aircraft is actively transmitting. Third, it gives clutter free responses, because it does not rely on returning reflected pulses. There’s no ground clutter, no sea clutter — the reply is a clean, deliberate transmission. Fourth, and this is the big one for ATC: SSR positively identifies an aircraft’s primary response by displaying its code and call sign alongside. So the controller sees the blip and immediately knows who it is. Fifth, it indicates an aircraft’s track history, speed, altitude and destination. So not just position — the controller gets a richer picture of what the aircraft is doing. And sixth, it can indicate on a controller’s screen that an aircraft has an emergency, has lost radio communications, or is being hi-jacked. These are special conditions that the transponder can flag, and they show up directly on the screen. So the takeaway is this: when SSR is used in conjunction with primary radar, the advantages of both systems are realized. Primary gives you the accurate bearing and range; SSR gives you the identity and the extra data. That’s why the two radars are usually co-located — they sit together, as shown in Figures 14.1 and 14.2. Now, on the display itself. The SSR information is displayed in combination with the primary radar information on the same screen. So you see the primary blip, and overlaid on it you get the SSR data. That includes the call sign or flight number, the pressure altitude or flight level, the ground speed, and the destination. So the controller sees, for example, the flight number, what level it’s at, how fast it’s moving over the ground, and where it’s going — all on one screen. One more detail worth noting from the figures: the SSR aerial is mounted on top of a 23 cm primary radar aerial. That’s a specific physical arrangement — the secondary antenna sits physically on top of the primary antenna, which is why they’re co-located so neatly. So to tie it all together: primary radar is the accurate but anonymous echo system; SSR is the cooperative system where the aircraft’s transponder answers the interrogator with a coded reply. SSR gives you identity, altitude, speed, destination, and emergency flags, at far lower power and out to 200 to 250 NM, but primary gives you the better bearing and range. Together, they give ATC the complete picture.