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

We're starting a new topic now: Mode S, which is the next step in Secondary Surveillance Radar. Let me set the stage first, because this builds directly on the modes A and C you already know. Mode S is being introduced to overcome the limitations of the present modes A and C. The 'S' stands for Selective addressing. That's the key concept — instead of interrogating every transponder in range, the system can address one specific aircraft. But here's the critical design constraint: the new system has to be compatible with the existing modes A and C, so it can supplement the present system rather than replace it overnight. Let me walk you through the main features of Mode S, one by one. First, the availability of codes. The aircraft address code will be made up of a 24-bit code. That's a binary number with 24 digits, and it gives the system over 16,700,000 discrete codes available for allocation to individual aircraft on a permanent basis. This is a huge jump from the limited codes of modes A and C. The code is incorporated into the aircraft at manufacture and remains with it throughout its life — it's like a permanent identity, not something assigned per flight. Second, the data link. The system is supported by a ground data network and has the ability to handle uplink and downlink data messages over the horizon. Let me unpack that. Uplink is ground-to-air, downlink is air-to-ground. Mode S can provide ground-to-air, air-to-ground, and air-to-air data exchange using communications protocols. So aircraft can talk to each other too, not just to the ground. Third, the reduction of voice communications. The intention is that the majority of the present RTF messages — that's radio telephony, your voice communications — will be exchanged via the data link instead. Messages to and from an aircraft will be exchanged via the aircraft's CDU, which is the Control Display Unit — the pilot's interface panel. This results in a reduction in voice communications, which reduces frequency congestion and pilot workload. Fourth, height read-out. This will be in 25-foot increments — much finer than the 100-foot increments you might be used to 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 where you plan to go. Now let's look at the interrogation modes. Mode S operates in four distinct modes. The first is All Call, used to elicit replies for acquisition of mode S transponders — that's how the ground station finds out which Mode S aircraft are in its coverage. The second is Broadcast, which transmits information to all mode S transponders, and note this carefully: no replies are elicited. It's one-way. The third is Selective, 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 — that's the selective addressing in action. The fourth is Intermode, and this is the mode A/C/S All Call, which would be used to elicit replies for surveillance of mode A/C transponders and for the acquisition of mode S transponders. So it's a hybrid that works with both old and new equipment. Finally, the pulses — this is where the physical signal differs. Mode S does not transmit the P3 pulse, but has an additional P4 pulse, which can be either long or short in duration. Let me explain what that means. In conventional modes A and C, you have P1, P2, and P3 pulses. Mode S drops the P3 and adds P4. For the Intermode A/C/S All Call, the interrogation will consist of P1, P2, P3, and the long P4 pulse. So in that intermode case, P3 is present, but the long P4 tells the transponder whether it's being addressed in Mode S or in the older mode. That's the core of Mode S — the 24-bit permanent address, the data link capability, the reduction of voice traffic, the 25-foot height increments, the four interrogation modes, and the P4 pulse structure.