Secondary Surveillance Radar (SSR) — Page 235, Lesson 225

Let’s start with the big picture. We’ve been talking about Secondary Surveillance Radar, and now we’re moving into Mode S. I want you to understand why it exists before we get into the technical details. Mode S is being introduced to overcome the limitations of the present Modes A and C. The letter ‘S’ stands for Selective addressing. So the whole point of this system is that the ground station can talk to one specific aircraft, not just broadcast to everyone. But here’s the critical constraint: the new system has to be compatible with the existing Modes A and C. That means it can’t just replace them overnight — it has to work alongside them, supplementing the present system. Now, let me walk you through the main features of Mode S, one by one. First, the availability of codes. The aircraft address code is made up of a 24 bit code. A bit is a binary digit — a 0 or a 1. With 24 of them, the system has over 16,700,000 discrete codes available. That’s over 16.7 million unique addresses. These are allocated to individual aircraft on a permanent basis. And here’s the key point: the code is incorporated into the aircraft at manufacture and remains with it throughout its life. So it’s like the aircraft’s permanent identity, not something assigned per flight. Second, the data link. Mode S is supported by a ground data network, and it has the ability to handle uplink and downlink data messages over the horizon. Uplink means ground to air, downlink means air to ground. And Mode S can provide ground-to-air, air-to-ground, and even air-to-air data exchange, using communications protocols. So it’s not just radar returns — it’s a proper data communication system. Third, reduction of voice communications. The intention is that the majority of the present RTF messages — that’s radio telephony, the voice communications — will be exchanged via the data link instead. Messages to and from an aircraft will be exchanged via the aircraft’s CDU. CDU stands for Control Display Unit — that’s the pilot’s interface in the cockpit. So instead of talking on the radio, the information goes through the CDU, which reduces voice communications significantly. Fourth, height read-out. This will be in 25 ft increments. So altitude is reported in steps of 25 feet, which is much finer than what we get with Mode C. And more data on an aircraft’s present and intended performance will be available to the ground controllers. So not just where you are now, but what you’re planning to do. Now, the interrogation modes. This is where we get into how Mode S actually operates. There are four modes, and I want you to remember each one carefully. The first is All Call. This is used to elicit replies for acquisition of Mode S transponders. Acquisition means finding and identifying aircraft that have Mode S transponders. So the ground station sends out an All Call to see who’s out there. The second is Broadcast. This transmits information to all Mode S transponders, but no replies are elicited. So it’s one-way — the ground station sends data out to everyone, but nobody answers. The third is Selective. This is for surveillance of, and communication with, individual Mode S transponders. For each interrogation, a reply is elicited only from the transponder uniquely addressed by the interrogation. So the ground station addresses one specific aircraft, and only that aircraft replies. That’s the selective addressing we talked about — that’s why it’s called Mode S. The fourth is Intermode, specifically Mode A/C/S All Call. This would be used to elicit replies for surveillance of Mode A/C transponders and for the acquisition of Mode S transponders. So this is the compatibility mode — it lets the ground station talk to the older Mode A and C transponders while also finding the new Mode S ones. Now, let’s talk about the pulses, because this is a key technical difference. Mode S does not transmit the P3 pulse, but it has an additional P4 pulse, which can be either long or short in duration. In the older systems, P3 is part of the interrogation pulse train. Mode S drops P3 and adds P4 instead. And here’s how it ties together for the Intermode A/C/S All Call. The interrogation will consist of P1, P2, P3, and the long P4 pulses. So in that specific intermode case, you do have P3 — but you also have the long P4. The length of P4 is what tells the transponder whether it’s being addressed as a Mode A/C transponder or a Mode S transponder. That’s the clever part — the pulse duration carries the meaning. Let me show you the display so you can see how this all fits together in the real ATC environment. So to summarise what we’ve covered: Mode S is selective addressing, built to overcome Mode A and C limitations while staying compatible. It has a permanent 24-bit address, over 16.7 million codes, a data link for over-the-horizon messages, reduced voice comms via the CDU, and 25 ft height read-out. It operates in four interrogation modes — All Call, Broadcast, Selective, and Intermode A/C/S All Call. And it uses a P4 pulse instead of P3, except in the intermode case where you get P1, P2, P3, and the long P4 together. That’s the foundation. Once you’ve got this, the next step is understanding how the transponder actually responds to each of those interrogations.