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

We’re now moving into the heart of Mode S, and I want to start with the communication protocols, because these define exactly how the ground and the aircraft talk to each other. First, we have what’s called Standard Length Communications, which is a single transaction. This is broken into two types. Comm-A is the transfer of information from ground to air, and it is always initiated from the ground. Comm-B is the transfer of information from air to ground, and this one may be initiated either by the ground or by the aircraft. Then we have Extended Length Communications, which can carry up to sixteen 80-bit messages. Within this extended format, Comm-C is the uplink, meaning the ground sends to the aircraft, and Comm-D is the downlink, meaning the aircraft sends to the ground. Now, the key document here is ICAO Aeronautical Telecommunications, Vol. IV, Annex 10. It stipulates that Mode S transponders shall conform to one of four levels of capability. Let me walk you through each level, because each one builds on the previous. 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. Critically, 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 adds standard length data link communication, both from ground to air and air to ground. It also includes automatic aircraft identification reporting. This is the minimum level permitted for international flights. Level 3 has the same capabilities as Level 2, but it permits extended data link communications from the ground to the aircraft. Level 4 has the same capabilities as Level 3, but it allows extended data link communications from the aircraft to the ground. So you can see the progression: Level 1 is basic surveillance, Level 2 adds standard data link and identification, Level 3 adds extended uplink, and Level 4 adds extended downlink. Now let’s look at what the aircraft actually sends down. This is called Downlink Aircraft Parameters, or DAPS. There are two categories here. Basic Functionality includes automatic reporting of Flight Identity, which is the call sign used in flight. It also includes a Transponder Capability Report. Then we have altitude reporting in 25 ft intervals, subject to aircraft availability. And finally, Flight Status, which tells whether the aircraft is airborne or on the ground. Enhanced Functionality adds more parameters. We have Magnetic Heading. We have Speed, which can be IAS, TAS, or Mach Number. We have Roll Angle, which the system uses to acquire the start and stop of a turn. We have Track Angle Rate, again for system acquisition of start and stop of turn. We have Vertical Rate, which is barometric rate of climb or descent, or preferably baro-inertial. And finally, True Track Angle and Ground Speed. Now, looking ahead, there’s a 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. That means Selected Flight Level or Altitude, Selected Magnetic Heading, Selected course, and Selected IAS or Mach Number. Finally, let me give you the SSR Summary. SSR requires a transponder in the aircraft and an interrogator at the ground station. It offers advantages over primary radar, and the aerial is mounted on top of the primary radar. Let me show you the setup. Here’s a figure of primary and secondary radar used for ATC. And here’s a radar display showing aircraft positions in the London TMA. And this figure shows that SSR operates in the UHF band. So to tie it all together: the interrogator on the ground sends a signal, the transponder in the aircraft responds, and the data link protocols—Comm-A through Comm-D—define how that information flows. The transponder level determines what capabilities are available, and the DAPS parameters tell us exactly what the aircraft is reporting.