Future Air Navigation Systems (FANS)

1. Introduction — The Need for FANS

Aircraft are currently controlled using voice communications. Over and close to populated landmasses, ATC uses radar with VHF communications. However, over oceans, deserts and polar regions, VHF and radar may not be available. ATC must provide a procedural control service using HF communications, resulting in high vertical, lateral and longitudinal separation distances and low traffic density.

Position reports are passed by aircraft crossing the North Atlantic every 10° of longitude up to 70°N and every 20° north thereof — meaning ATC receive a position update every 30–60 minutes.

Advances in technology now provide:

• GNSS — global satellite navigation providing position accuracy better than 1 NM
• SATCOM — satellite communications providing potential for global communications through a single medium

FANS integrates these into a Seamless Global Air Traffic Management System.

2. Communications Systems

Until the late 1980s, air-ground communications relied solely on VHF and HF voice. In the 1990s, voice was extended to SATCOM (UHF) via geostationary satellites. Coverage is limited to approximately 80° of latitude. Polar regions require satellites in lower-altitude inclined orbits.

3. Data Link

A data link is a means of connecting one location to another for transmitting and receiving information. Data links may be established on any frequency but require additional equipment on both ground and aircraft.

Uplink / Downlink Convention

Airborne Data Link Equipment

4. ACARS

ACARS (Aircraft Communications Addressing and Reporting System) uses data link format to pass messages between the aircraft and ATC or aircraft operating companies using VHF. Messages can be printed out in the cockpit.

• Extended to SATCOM in the early 1990s for oceanic flights
• Extended to HF in 2001 to close the polar gap
• FMS interfaces added in the 1990s — met info, alternate routes evaluated by FMS
• Maintenance monitoring via ATSU/DCDU

5. Air Traffic Service Unit (ATSU) & CNS/ATM

The ATSU (Air Traffic Service Unit) provides:

• Flight Information Service
• Alerting Service
• Air Traffic Advisory Service
• Air Traffic Control Service (Area / Approach / Aerodrome)

CNS/ATM uses automation, digital technology and satellite systems to give a Seamless Global Air Traffic Management System.

6. Data Link Message Types

ATM (ATC) Data Link Messages

AOC (Airline) Data Link Messages — OOOI

Additional AOC messages: Load Sheet, Passenger Information, Weather Reports (METAR/TAF/SIGMET/VOLMET), Maintenance Reports.

7. Logon Procedure

flowchart LR
  A["Pilot inputs 4-digit ICAO\nATSU address into FMS"] --> B["FMS sends Logon Message\n(aircraft address + capabilities)"]
  B --> C["ATSU acknowledges\nlogon message"]
  C --> D["ATSU sends Connection\nRequest to aircraft"]
  D --> E["Aircraft sends Connection\nConfirm message"]
  E --> F["AFN Complete\n(ATS Facilities Notification)"]

8. FANS A — Oceanic / Remote Airspace

FANS A provides a CNS system and ADS. Used over oceanic and remote airspace, transmitted over the ACARS network (ARINC). Communications use current HF/VHF frequencies; GNSS provides the navigation input for surveillance.

FANS A Components

AFN — ATS Facility Notification

A contact message initiated by aircrew or automatic aircraft trigger. If acknowledgement is not received within a pre-set time, or there is an erroneous reply, an error message is displayed to the aircrew.

ADS-C — Automatic Dependent Surveillance – Contract

Controller-set contract with the aircraft's FMS, without any pilot input. The flight crew have no workload associated with setup.

CPDLC — Controller Pilot Data Link Communications

CPDLC permits data link messages for all stages of flight. Fixed-format messages activated by ATC controller or pilot. Messages annotated whether a response is required. An unanswered message (e.g. "report levelling at FL310") remains open until the FMS sends the automatic response.

9. FANS B — High Density Airspace

FANS B is very similar to FANS A but operates within high density airspace with good VHF coverage. Operates over the Aeronautical Telecommunications Network (ATN), operated by SITA. The ATN allows ground/ground, ground/air and avionic data subnetworks to interoperate.

10. Useful Abbreviations

Master Reference Tables

Key Numerical Values — Chapter 24

Q&A Answer Key