Secondary Surveillance Radar (SSR) — Page 241, Lesson 228

We're starting fresh on Secondary Surveillance Radar, and I want to take you straight into the heart of the modern system: Mode S. Before we get to its benefits, let me anchor you on one crucial technical detail about how the interrogation actually works. In the older Modes A and C, when the ground station wants to talk to any aircraft in range, it sends what we call an "All Call" interrogation. Now, here's the specific pulse structure for that. The interrogation consists of four pulses: P1, P2, P3, and a short P4 pulse. The P4 pulse is the key—it's the marker that tells the transponder whether this is an All Call that it should respond to, or a selective call meant for a specific aircraft. In the intermode A/C only All Call, that P4 pulse is short, and it signals to Mode A/C transponders that they should reply. Now, let's talk about why Mode S is such a leap forward. The first major benefit is Unambiguous Aircraft Identification. Think about this: every aircraft gets assigned a unique address from a pool of almost 17 million possible addresses. That's not just a squawk code—that's a permanent, unique identity for that aircraft. Combined with automatic flight identity reporting, this gives us unambiguous identification. Because that unique address is embedded in every interrogation and reply, the system can also include data link messages targeted to a particular aircraft. That means we get selective calling—the ground can talk to one specific aircraft—in addition to the All Call messages. The second benefit is Improved Integrity of Surveillance Data. Mode S has superior resolution and uses selective interrogation. What does that do for us? It eliminates synchronous garble—that's when replies from two aircraft at the same range overlap and corrupt each other. It resolves the effects of over interrogation, which happens when too many aircraft are replying and overwhelming the system. And it simplifies aircraft identification even in the case of radar reflections, where a signal bounces off a building or terrain and creates a false target. Third, we get Improved Air Picture Tracking and Situation Awareness. The radar controller sees a better, more current air picture. Because we have unambiguous identification, enhanced tracking techniques, and increased downlink data from the aircraft, the horizontal and vertical tracking both improve. The controller knows exactly who each target is and where it's going. Fourth, and this is a big operational one: Alleviation of Modes A/C Code Shortage. Right now, in the EUR region, there's a shortage of usable SSR codes—there just aren't enough four-digit squawk codes to go around. Mode S eliminates that problem entirely because each aircraft has its own unique address. No more sharing or scrambling for codes. Fifth, we get a Reduction of R/T Workload. R/T means radiotelephony—the voice communication between controller and pilot. With the progressive introduction of Mode S, that voice workload drops. For example, code verification procedures—where the controller asks the pilot to confirm the squawk code—are no longer required, because the system already knows exactly who the aircraft is. Finally, there are Improvements to Short Term Conflict Alert, or STCA. This is the safety net that warns the controller when two aircraft are on a collision course. Mode S improves it in a few ways. It eliminates synchronous garbling, so the data is cleaner. It produces a more stable speed vector—that's the arrow showing the aircraft's direction and speed. And it can acquire aircraft altitude reporting in 25-foot increments, if the aircraft has compatible barometric avionics. That finer altitude resolution gives the ground system much more precise vertical data. Plus, access to the downlinked vertical rate—how fast the aircraft is climbing or descending—gives early, accurate knowledge of aircraft manoeuvres. That means STCA can predict conflicts sooner and more reliably. One important note here, and I want you to remember this for your exams: while the ground system benefits from altitude reporting in 25-foot intervals, there is no intention to change the existing practice of displaying altitude to the controller in 100-foot increments. So the system works in fine detail, but the controller's display stays at the standard 100-foot resolution. Let me show you how this all fits together visually. Here's the SSR operating in the UHF band—that's the frequency range it uses. And if you look at the radar display, you can see the positions of aircraft in the London TMA, which is exactly the kind of dense traffic environment where Mode S's benefits really shine. The primary and secondary radar comparison shows you how SSR complements primary radar for ATC. So, to tie it all together: Mode S gives us a unique address for every aircraft, cleaner surveillance data, better tracking, relief from the code shortage, less voice workload, and a stronger conflict alert system. That's the foundation we'll build on as we go deeper into how the transponder and ground station actually communicate.