Future Air Navigation Systems (FANS) — Page 322, Lesson 379

Let’s start at the very beginning of FANS — Future Air Navigation Systems — because the whole idea grows out of a problem. Before data link existed, pilots and controllers talked to each other by voice on the radio, and voice had real weaknesses. I want you to hold those weaknesses in your head, because every piece of data link equipment we’re about to name exists to fix one of them. Here are the disadvantages of voice communications, exactly as they’re listed. First, many aircraft share one frequency — so you’re all talking over each other. Second, transmission is synchronous — meaning you have to wait your turn, speak, then wait for the reply; it’s a strict back-and-forth, which is slow. Third, language confusion — pilots and controllers worldwide may not share a first language, and accents or phraseology can be misunderstood. Fourth, limited channels — there simply aren’t enough frequencies to go around. Fifth, VHF is line-of-sight only — if the aircraft is beyond the horizon from the ground station, the signal is blocked by the curvature of the Earth. And sixth, HF — high frequency — suffers interference and is tiring to listen to, because the signal bounces off the ionosphere and fades. So to overcome these problems, data link services were developed. Let me define the term precisely: a data link is a means of connecting one location to another, for the purpose of transmitting and receiving information. Notice it’s not tied to any one frequency — data links may be established on any frequency, but they require additional equipment on both the ground and the aircraft. That’s a key point: it’s not just a radio; both ends need the gear. Now, the three media used for data link are HF, VHF, and SATCOM — satellite communications. Let me walk you through their characteristics, because they’re contrasted directly. HF is long range, but it suffers interference, and it’s slow speed. VHF is short range — line of sight — but the quality is good, and it’s medium speed. SATCOM is global, except over the poles, and it’s also line of sight — but to the satellite, not the ground — and it gives good quality at high speed. So you can see the trade: HF reaches far but is poor and slow; VHF is clean but short; SATCOM is the best quality and speed but has that polar gap. Two more terms you must know, and they’re directional. The transmission of signals from an earth station to a satellite or airborne platform is called an Uplink. The transmission of signals from a satellite or airborne platform to an earth station is called a Downlink. So uplink goes up, downlink comes down — and note the airborne platform can be an aircraft, not just a satellite. Now, who actually provides these data link services to aviation? Currently there are two organizations. One is SITA — that stands for Société Internationale de Télécommunications Aéronautiques, based in France. The other is ARINC — Air Radio Incorporated, based in the USA. So one European, one American — those are the two service providers. Finally, the equipment fitted to the aircraft to operate a data link. There are three units you need to know. First, the CMU — Communications Management Unit. Its job is to select the frequencies required for all radio equipment. So it’s the brain that tunes the radios. Second, the DCDU — Data Communications Display Unit. This displays any messages received, or sent, via the data link — it’s your screen for the messages. And third, the MCDU — Multi Control and Display Unit. And here’s the neat part: the MCDU could be a combination of the CMU and the DCDU — so one unit that both manages the frequencies and displays the messages. So the whole picture is this: voice had six specific weaknesses; data link fixes them by sending information digitally over HF, VHF, or SATCOM; the service is provided by SITA or ARINC; and on the aircraft, the CMU picks frequencies, the DCDU shows messages, and the MCDU can do both jobs in one box. That’s the foundation of FANS — and it’s exactly the logic that drives everything else in this chapter.