Future Air Navigation Systems (FANS) — Page 322, Lesson 376

Let’s start at the very beginning of Future Air Navigation Systems — FANS for short. I want you to understand why this whole concept exists before we touch any hardware. Right now, today, aircraft are controlled using voice communications. That’s the baseline. Over and close to populated landmasses, ATC — Air Traffic Control — uses radar to provide what we call positive control of aircraft, and they talk to the pilot on VHF, which is Very High Frequency radio. VHF is line-of-sight, so it works fine when you’re near the ground stations. But here’s the problem. Over oceans, deserts, and polar regions, VHF and radar may simply not be available. There’s no ground station in range, and radar doesn’t reach that far. So in those areas, ATC has to provide what’s called a procedural control service. That generally requires HF — High Frequency — communications. HF bounces off the ionosphere, so it can travel huge distances, but it’s unreliable and hard to use. Now, the consequence of procedural control is severe: it results in high vertical, lateral, and longitudinal separation distances. In plain terms, aircraft have to be kept very far apart from each other — up, sideways, and along the track — because ATC simply doesn’t know exactly where each aircraft is. And that means a low traffic density. You can’t pack many aircraft into that airspace because you can’t safely get them close together. Let me give you a concrete picture of how bad this is. Position reports are passed by aircraft crossing the North Atlantic every 10° of longitude up to 70°N, and every 20° north of that. So as you fly west across the Atlantic, you call in your position at each of those longitude lines. The result is that ATC receives a position update only every 30 to 60 minutes. Think about that — for up to an hour, the controller has no fresh information on where you are. And it gets worse. Because of the difficulties with HF communications, in these remote areas the pilot is rarely communicating directly with the ATC controller. Instead, messages are relayed through a third party — a middleman who passes the message between the two. So you have delayed position reports, and you’re not even talking to the controller directly. Now contrast that with the populated areas. Over and around populated areas, radar allows positive identification and control of aircraft. The controller sees you on the screen, knows exactly where you are, and can issue tight clearances. So here’s the core problem the whole FANS concept is built to solve. The increase in international air traffic is posing major limitations in those remote areas. To increase capacity over the rest of the world — over the oceans, deserts, and poles — ATC needs real-time information on aircraft positions. That information must have improved accuracy, and there must be better communications between the air traffic controller and the pilot. Now, technology has caught up to give us the tools. We now have GNSS — Global Satellite Navigation Systems — providing position accuracy to better than one nautical mile. There are still some issues with the service GNSS provides, but in general, this high level of position accuracy is now globally achievable. And we have SATCOM — Satellite Communication systems — which give the potential for global communications. So that’s the foundation. FANS is the answer to the question: how do we get real-time, accurate position information and reliable communications in the parts of the world where radar and VHF simply don’t reach? The answer is satellite-based navigation and satellite-based communication, replacing the old procedural control with something far more precise. Let me show you the trade-offs. That figure lays out the advantages and disadvantages of data link communication — which is the heart of FANS. We’ll dig into that next.