Future Air Navigation Systems (FANS) — Page 330, Lesson 388

We’re starting a brand-new topic now: Future Air Navigation Systems, or FANS. This is the system that moves air-ground communication away from just voice and onto data links. Let me set the scene first, because the whole chapter hinges on one idea: an Air Traffic Service Unit, an ATSU, can use different combinations of data-link services, but not all combinations are allowed. Here’s the rule, and it’s a hard one. An ATSU may use AFN only, or ADS only, or CPDLC only. Or it may use AFN and CPDLC together. Or ADS and CPDLC together. But it may not use AFN and ADS together. Let me unpack those acronyms before we go further, because they’re the vocabulary of this whole chapter. AFN stands for Air Traffic Facilities Notification. That’s the message that tells the ground system that an aircraft is entering its airspace and is capable of data link. ADS is Automatic Dependent Surveillance — the aircraft automatically transmits its position and other data, dependent on the onboard navigation systems. CPDLC is Controller-Pilot Data Link Communications — text messages between the controller and the pilot instead of voice. So the rule is: you can pair either AFN or ADS with CPDLC, but you can never pair AFN with ADS. The typical architecture of this is shown in the next diagram, Figure 24.4. Now, there’s a distinction between two flavours of FANS: FANS A and FANS B. FANS B is very similar to FANS A, but it operates within High Density airspace having good VHF coverage. So FANS B is for busy airspace where you have solid Very High Frequency radio coverage. FANS B is operated over the Aeronautical Telecommunications Network, the ATN, which is operated by SITA. The ATN is an internetwork architecture that allows ground/ground, ground/air and avionic data subnetworks to inter-operate. So it’s a network that ties together the ground systems, the air-ground link, and the aircraft’s own avionics data buses, all into one interoperable whole. Let me also mention the DCDU, which appears in Figure 24.5 — that’s the Data Communication Display Unit, the screen in the cockpit where CPDLC messages are shown to the pilot. Now, the chapter gives us a long list of useful abbreviations, and I want to walk you through them in groups, because they’re the working vocabulary you’ll need. Starting with communications. ATIS is Automatic Terminal Information Service — the recorded broadcast of weather and airport information. D-ATIS is Data Link Air Terminal Information Service — the same information delivered over data link. HF is High Frequency, a portion of the RF spectrum. SATCOM is Satellite Communications. VHF is Very High Frequency, also a portion of the RF spectrum. FANS, of course, is Future Air Navigation System. ACARS is Aircraft Communications Addressing and Reporting System — the older data link system that FANS builds on. ACMS is Aircraft Condition Monitoring System — the system that monitors aircraft systems and reports their condition. Then we have the surveillance and addressing group. ADS is Automatic Dependent Surveillance. ADS-C is Automatic Dependent Surveillance - Contract, specifically for FANS 1/A — the "contract" meaning the ground system and aircraft agree on when and what data will be sent. AFN is Air Traffic Facilities Notification, for FANS. AMS is ACARS Message Security, as specified in ARINC 823 — that’s the security layer for ACARS messages. AOA is ACARS Over AVLC — AVLC being Aviation VHF Link Control, the data link protocol. AOC is Airline Operational Communication, or the Airline Operational Centre — the airline’s own ground operations. ARINC is Air Radio Incorporated, USA — the standards body. ATM is Air Traffic Management. ATN is Aeronautical Telecommunications Network — note the book says France, though it’s an international standard. ATSU is Air Traffic Service Unit. AVLC is Aviation VHF Link Control. CDU is Control and Display Unit — the pilot’s interface. CMF is Communications Management Function. And CMU is Communications Management Unit — the box that manages the data link communications. So the picture is: FANS is the future system, built on ACARS and the ATN, using AFN for notification, ADS for surveillance, and CPDLC for controller-pilot messaging, with the CMU and CDU in the cockpit and the DCDU displaying messages. And the key constraint to remember is the pairing rule — AFN and ADS never together.