SSR and ACAS — Page 249, Lesson 321

I want to walk you through the start of the SSR and ACAS chapter. This is where we separate the radio-navigation engineering from the air law side of things, and I want you to understand that distinction right away. The technical requirements of the EASA Theoretical Knowledge syllabus for SSR are covered in 062 Radio Navigation. That's the engineering course. But the operation of SSR in the ATC environment — how we actually use it in the air traffic control system — that is squarely within the remit of Air Law. So in the chapters on ATC, whenever we discuss the use of radar, it is implied that where required, the radar-derived information will be enhanced by the use of SSR. Radar gives the controller a blip; SSR gives that blip identity and data. Now, the background. The modern SSR systems were developed from equipment used in World War II as a means of identifying 'friend' from 'foe' — that's the IFF system, Identification Friend or Foe. The basic system was rather crude. It effectively gave the ground station a response from the aircraft indicating that the aircraft was fitted with the then highly secret equipment. Because only 'friendly' aircraft had the equipment, the radar operator could easily distinguish a friendly aircraft radar response from an enemy response. The airborne equipment was given the code name 'parrot' — and that explains some of the rather peculiar phraseology associated with the operation of a modern SSR system. You'll hear that word echo through the phraseology. Current SSR systems enable individual aircraft to be identified on a flight-by-flight basis. And they add facilities: to pass data via the SSR system, to indicate aircraft altitude, and — using the redundant capacity of the SSR system — to provide effective collision avoidance enhancement. That last point is the seed of ACAS, which we'll get to. Now, the carriage of transponders. The airborne equipment is called a transponder, and the name tells you exactly what it does: it transmits a response to an interrogation. PANS OPS — that's the Procedures for Air Navigation Services, Aircraft Operations — requires that where a serviceable transponder is carried in an aircraft, unless ATC instructs otherwise, it will be used at all times. Individual states may specify certain conditions where the carriage and operation of a transponder is mandatory. But in either case, the operation of the equipment will be regardless of the provision of an ATC service. That's an important point: you don't switch the transponder off just because you're not talking to a controller. The requirement stands on its own. Now the mode of operation. The output codes of the SSR system consist of groups of 4 numbers in the range 0 to 7. So each digit can be 0, 1, 2, 3, 4, 5, 6, or 7 — that's an octal system, four digits each in that range. The ground station — the interrogator — transmits a coded signal that prompts the aircraft transponder to reply. The overall SSR system has several different 'modes' of operation. The response will be mode Alpha with the addition of mode Charlie. In addition, the military have other modes of operation of SSR which overlap with civilian usage. And here's the key operational point: in order to correctly indicate the required response code, the mode as well as the code should always be specified. The pilot is to respond with the mode and code. Typically a radar controller will request the mode and code as follows — and that's where we'll pick up with the actual phraseology. Let me make sure you've got the core distinction locked in. The interrogator is the ground station; it sends the coded signal. The transponder is the airborne equipment; it replies. The reply carries a code — four digits, each 0 to 7 — and that code is only meaningful when paired with the mode. Mode Alpha is the basic identity response; mode Charlie adds altitude. And because the military modes overlap with civilian usage, you never just say a code — you always specify mode and code together. That's the discipline the controller expects, and the pilot responds with both. That's the foundation of SSR in the ATC environment. When you're ready, we'll move into the actual request phraseology and then on to ACAS.