By Bjorn Fehrm

08 July 2016, ©. Leeham Co: We have over the last Corners described the future Air Traffic Management systems as a combination of ADS-B and Controller-Pilot Data Link Communications, CPDLC.

What to do when there are no ground stations that can receive the ADS-B broadcast of the aircraft’s position and where it’s going? Or the aircraft’s VHF based CPDLC?

We now talk about crossing the large waters where there are no ground stations for neither ADS-B signals nor VHF communications, whether by voice or data.

Figure 1. North Atlantic Tracks (NAT) going eastbound. Source: Nav Canada

The solutions over these Oceanic areas have to be based on the aircraft following predetermined tracks, Figure 1, and continuously issuing position reports to ground controllers that keep the aircraft separated along the tracks based on the reports. We now cover how this has been done historically and the way forward.

Position Report-Based Navigation
When an aircraft flies outside coverage of primary or secondary (transponder based) radar the navigation has to be based on each aircraft delivering position reports combined with speed and heading. The position is generated from the aircraft’s inertial navigation or GPS and has traditionally been communicated to the controllers via HF radio. HF radio has varying voice quality and is normally conducted over relay stations. It’s therefore a slow medium which cannot control a large number of aircraft.

The slow and fault-prone HF communication meant that the separation between aircraft in, for example, the North Atlantic Tracks systems (Figure 1) had to be large. This meant the NAT system had low capacity. Given that 1,400 aircraft need to cross the Atlantic every day, a better system for the NAT and its equivalents over the Pacific Ocean was needed.

FANS gets developed
The result was FANS (Future Air Navigation System). It was developed over many years by ICAO, Boeing, Airbus and airlines with very long overwater routes (Qantas, Cathay Pacific, Air New Zeeland, Unite Airlines). The driving force was better routes over the Pacific Ocean due to more effective reporting and therefore lower separation requirements.

The FANS system has three parts. The previously described Controller Pilot Data Link Communication (CPDLC), a satellite based communication link and the ground control part of CPDLC.

We described the CPDLC in previous Corners as the future for Continental airspace communication. It actually had its origin in FANS, as the Oceanic reporting was a larger problem in the 1980s than the communication over Continental airspace at the time.

FANS speeds up the Controller and Pilot communication with many standard messages and it also deliver position reports with regular intervals. FANS therefore allows tighter spacing of aircraft over the Oceanic areas. It has therefore been required for the more attractive and efficient routes over the Pacific and Atlantic oceans.

This made Boeing and Airbus implement FANS in all long range airliners and the new long range business jet also started relying on FANS.

Why no universal use of FANS
Given that FANS was developed before the CPDLC used in Europe’s upper airspace and elsewhere, one can ask why FANS wasn’t made the standard instead. FANS was developed to provide both reliable Controller-Pilot communication and the relay of reliable position reports over a range of different long range communication links.

The need for a system that covers Continental air traffic is different. Only the data link communication part is needed but with higher speed and a broader range of messages. The aircraft’s position, speed and heading are continuously broadcast by the ADS-B to all that care to listen.

The more complicated and flexible FANS is therefore reserved for long range aircraft crossing the oceans. It can be made compatible with the Continental CPDLC by a software update, meaning FANS-equipped aircraft don’t have to invest in an extra Continental airspace CPDLC system.