Secondary Surveillance Radar (SSR) — Page 241, Lesson 230

I want to walk you through the Mode S communication protocols and transponder levels, because this is where the secondary surveillance radar system really comes alive. We've already seen how SSR uses a transponder in the aircraft and an interrogator on the ground — now let's look at how they actually talk to each other. First, the communication protocols. Mode S defines two categories of message length. Standard length communications are a single transaction, and they come in two flavours. Comm-A is the transfer of information from ground to air — it's initiated from the ground. Comm-B is the transfer of information from air to ground, and here's the key difference: it may be either ground or air initiated. So the ground can ask for it, or the aircraft can volunteer it. Then we have extended length communications, which can carry up to sixteen 80-bit messages. Comm-C is the uplink — ground to air. Comm-D is the downlink — air to ground. So remember the pattern: A and C go down to the aircraft, B and D come up from the aircraft. A and B are standard length, C and D are extended. Now, the transponder levels. ICAO Aeronautical Telecommunications, Vol. IV, Annex 10 stipulates that Mode S transponders shall conform to one of four levels of capability. Let me take you through them in order. Level 1 is the basic transponder. It permits surveillance based on Mode A/C as well as on Mode S. With a Mode S aircraft address, it comprises the minimum features for compatible operation with Mode S interrogators. But — and this is important — it has no data link capability, and it will not be used by international air traffic. Level 2 has the same capabilities as Level 1, but it permits standard length data link communication from ground to air and air to ground. It also includes automatic aircraft identification reporting. This is the minimum level permitted for international flights. So if you're flying internationally, you need at least a Level 2. Level 3 has the same capabilities as Level 2, but it permits extended data link communications from the ground to the aircraft. So that's the uplink side of the extended messages. Level 4 has the same capabilities as Level 3, but it allows extended data link communications from the aircraft to the ground. So Level 4 is the full package — both directions of extended data link. Now let's move on to Downlink Aircraft Parameters, or DAPS. This is what the aircraft can automatically report down to the ground. There's basic functionality and enhanced functionality. The basic functionality includes four things. Automatic reporting of Flight Identity — that's the call sign used in flight. A Transponder Capability Report. Altitude reporting in 25 ft intervals, subject to aircraft availability. And Flight Status — whether the aircraft is airborne or on the ground. The enhanced functionality adds more parameters. Magnetic Heading. Speed — and here we have IAS, TAS, or Mach number. Roll Angle, which the system acquires at the start and stop of a turn. Track Angle Rate, also acquired at the start and stop of a turn. Vertical Rate — that's the barometric rate of climb or descent, or preferably baro-inertial. And finally True Track Angle and Ground Speed. Then there's the future expansion of Mode S surveillance services. When technical and institutional issues have been resolved, the downlinking of an aircraft's intentions is recommended for inclusion. Those intentions are: Selected Flight Level or Altitude, Selected Magnetic Heading, Selected course, and Selected IAS or Mach number. So the idea is that the ground would eventually know not just where you are, but where you intend to go. Finally, let me give you the SSR summary. SSR requires a transponder in the aircraft and an interrogator at the ground station. It has advantages over primary radar — and the aerial is mounted on top of the primary radar. That's the physical setup you'll see at a ground station. So to tie it all together: the interrogator asks, the transponder answers, and the level of that transponder determines exactly how sophisticated the conversation can be. Level 1 is the bare minimum, Level 2 is what international traffic needs, and Levels 3 and 4 unlock the extended data link in each direction.