Right, let's pick this up with the two global systems themselves — Page 330, Lesson 386

Right, let's pick this up with the two global systems themselves. We've established that separation is significantly reduced, and if position accuracy degrades for any reason, the aircraft still have TCAS to warn of any potential conflict. Now, to give that required global coverage, there are two systems established: FANS A and FANS B. Let's look at FANS A first. FANS A provides a communications, navigation and surveillance system — that's the CNS — and an automatic dependent surveillance system, which we call ADS. Communications utilize current frequency allocations in HF and VHF, and GNSS provides the navigation input for the surveillance. Currently, it's used by some AOCs — that's the airline operations centre — to monitor the progress of aircraft at all stages of flight. So, for example, the aircraft system will automatically inform the AOC of gate departure, take-off, landing and gate arrival. In-flight progress and the operation of on-board systems can also be monitored and, where necessary, messages can be passed to alert or assist the crew when potential problems are detected. FANS A is used over oceanic and remote airspace, and it's transmitted over the ACARS network, which is operated by ARINC. Now, the components of the FANS A system. There are three main ones: AFN, ADS, and CPDLC. AFN stands for ATS Facility Notification. This is a contact message initiated by aircrew, or by an automatic trigger within the aircraft. If acknowledgement has not been received within a pre-set time, or there is an erroneous reply, then an error message must be displayed to the aircrew. ADS stands for Automatic Dependent Surveillance. This is a controller set-up contract — we call it ADS-C — with an aircraft's FMS, without any pilot input, using automated and customized reports. The flight crew have no workload associated with this set-up. The types of contract available are: Periodic, On Demand, On Event, and Emergency Mode. Only the flight crew can declare and cancel ADS-C emergency reporting. Note this carefully: the flight crew can initiate the emergency mode, but the aircraft cannot initiate a contract. Then we have CPDLC — Controller Pilot Data Link Communications. CPDLC permits data link messages to be generated for all stages of flight. The messages have a fixed format defined within the FMS and ATSU computers — ATSU being the air traffic services unit — and they're activated by the ATC controller or pilot either as an instruction or as a response to a request. The messages are annotated according to whether a response is required or not. For example, if the pilot is instructed to report levelling at FL310, then the message will remain open until the aircraft reaches FL310, when the FMS will send the response. The confirmation that the aircraft is at FL310 does not require a response from the controller, so the message will automatically close once received at the ground station. And there's a figure here showing the elements of a CPDLC system — the ground earth station, the ATSU, the effective control, the SATCOM link, and the VHF COM link. That's the physical architecture that carries these messages. So to tie it together: FANS A gives you the CNS backbone, ADS-C gives the controller automated surveillance reports without pilot workload, and CPDLC gives the two-way data link messaging for all stages of flight. That's the core of FANS A.