1. Introduction to Aeronautical Data Links
Seminar on the Implementation of Data Link and Satcom Communications
Bangkok, Thailand, November 2003
Introduction to Aeronautical Data Links
This introduction provides an overview to the basics of aeronautical data links, with emphasis on air-ground data links that have been standardized by the International Civil Aviation Organization.
This introduction is intended to be updated from time to time and is available on the website of the ICAO Aeronautical Communications Panel (ACP) (
Prepared by -
Loftur Jónasson & Jennie Jónasson
Iceland Telecom

2. Introduction
This presentation is intended to be a basic introduction to Air/Ground datalinks, based on the work done in ACP (formerly AMCP) WG-M, as well as our operations experience with Iceland Radio in the North Atlantic, where FANS-1/A is currently being utilised.
This introduction highlights aspects on the following topics:
FANS 1A –ACARS and ATN messages
The use of ATN compatible data links:
SATCOM (also called AMSS)
HF data link (HFDL)
VHF Digital Links: VDL Mode 2
VDL Mode 3
VDL Mode 4
The authors of this presentation have borrowed the work of many others in the aviation industry for this presentation. The authors would like to take this time to thank ARINC, SITA, the FAA, the Swedish CAA, Nav Canada, UK NATS and many others. The authors would like to thank them all for their contributions.
This introduction is by no means comprehensive.
It is intended to provide some basic information that could be useful and supportive to reviewing further material related to aeronautical datalinks.
The information provided in this introduction is mainly related to the use of air-ground data link in civil aviation and addresses data links developed for short range (line-of-sight) usage and for long range (beyond line of sight) of the aircraft.

3. Free Flight - the end goal
From the US FAA´s Architecture - Version 4.0, Section 6 Free Flight Phase 1, Safe Flight 21, and Capstone -
“Free Flight will allow pilots to change routes, speeds, or altitudes as needed, while in en route and oceanic air space. Air Traffic Controllers will not impose restrictions on pilot-initiated changes, except when there is a potential conflict with other aircraft or special use airspace. This capability will allow pilots to fly optimized profiles , the most efficient cruise speeds, wind-aided routes, and arrival descent profiles. Any activity that removes operational restrictions is a move towards Free Flight.”
Free Flight is an ideal that may never be fully achieved. However many believe it is a goal to strive for. The use of data link in aviation, both for air-ground and air-air communication, and in particular for the purpose of automatic dependent surveillance (ADS) is considered as an important step towards free flight. ICAO is expecting that ADS-Broadcast, a data link application where an aircraft transmits its position and other relevant information automatically, both in an air-ground mode and air-air mode, will provide the necessary tools towards free flight. Early benefits of ADS (most likely in a “contract” mode, whereby a discrete link is established between an aircraft and a ground station before the relevant surveillance data is transmitted) are expected in remote and oceanic areas through SATCOM or HF data inks. The availability of the global navigation satellite system (GNSS) is seen as the major tool for the aircraft to derive its position.
These developments transfer some of the responsibilities, currently in the hands of air-traffic controllers, to the cockpit of an aircraft and can only be introduced after carefully organized trials and evaluation.
From the aircraft point of view, introduction of these systems as moves towards a free flight environment, the increased work load to the pilots as a result of these developments, and the need to rely on proper functioning of complex equipment also requires careful evaluation.
With the technological advances that are available today, and the further technological advances that are expected to be introduced in the near future, it may be realistic to assume that in particular in some en route air space, true free flight may be achieved.

4. ATN and FANS 1/A
The above picture (courtesy of ARINC) illustrates the difference between an ACARS aircraft and an ATN aircraft.
The development of FANS-1/A and ATN is probably the most controversial in the history of aeronautical data link.
FANS 1 was developed by Boeing because of uncertainties on the development and implementation of ATN. AIRBUS developed a different version (FANS A), which is compatible and interoperable with the Boeing system. The aviation community did not want to wait until ATN was developed and implemented.
FANS 1/A are non-ICAO data links; no ICAO SARPs have been developed for these systems. FANS 1/A has been used very successfully for a long time in the South Pacific, using SATCOM.
With the introduction of VDL Mode 2 and HFDL, both of which are ATN compatible data links and standardized in ICAO, aviation has the option to introduce air-ground data links that are fully compatible with the ATN.

5. ARINC´s Explanation of Difference Between a FANS-1/A ACARS Message and an ATN MessageV F 3 1 S a m e M s g B i t A N / C r c f 6 A C R S M U T V H F B I - P L N . 6 2 1 • D o u t e r d O 3 , 2 2 P r o t o c o l
The applications that can be run over FANS ACARS systems today, will also be useable on the ATN. However the ATN will improve on message integrity as well as being enabler for a far greater variety of applications.
In FANS-1/A today the applications exchange bit oriented messages whose content is converted into characters to go across ACARS networks. The Airline Electronic Engineering Committee (AEEC) which writes the ACARS standards has developed the AEEC specification 622 defining this conversion process.
Character oriented protocols use particular characters for the transport data and other characters are reserved for system functions. Therefore the types and size of messages that are sent using character oriented protocols are limited.
There are no such restrictions when using bit oriented protocols.
The ATN, using bit oriented protocols, is able to send messages and data more efficiently than ACARS. ( B i t - t o - h e x , C R C ) A T O 8 4 1 4 C 9 6 4 E 4 B C R C 1 1 1 1 1 1 1 1 1 1 A T O 8 4 1 4 C 9 6 4 E 4 B C R C 1 1 1 1 1 1 1 1 A C A R S P r o t o c o l A T N P r o t o c o l

6. Character- vs. Bit-Oriented Messages
Protocol Data Units (PDU)
Character-oriented protocol
Bit-oriented protocol
PDU n
8-bit ASCII character
The above illustration (courtesy of ARINC) demonstrates that the transmission of messages with character-oriented protocols takes more time than the transmission of the same messages with bit-oriented protocols. Both messages in the above illustration send the same message. It shows that the use of bit-oriented protocols, as in the ATN, is more efficient than the use of character oriented protocols as in FANS-1/A (ACARS)
PDU m
Arbitrary sized bit fields

7. Transition of FANS 1/A (ACARS) to ATN
This is an illustration (courtesy of ARINC) on how ARINC plans the transition from FANS 1/A to ATN.
Do not look too heavily at the networks illustrated above, some are internal networks only. This illustration is to give you an idea of the transition foreseen.

8. Transition from FANS 1/A to ATN using VDL Mode 2
The above illustration is how ARINC foresee’s a transition to an ATN environment.
Step 1: ACARS
Step 2a: Character Applications
over VDL
Step 2b: VDL
Step 3: VDL/ATN

9. ICAO data link systems that can be used during flightx i T a k e - O f f D e p a r t u r e E n R o u t e A p p r o a c h L a n d T a x i
This diagram demonstrates the various ICAO data link systems, potentially useable for the different phases of flight.
During the en-route phase of flight, when within the reach of a ground station, in principle, any one of the VDL Modes could be used. However, in remote or oceanic areas, or in areas where no ground-infrastructure with VDL Mode 2, 3 and 4 stations is available, either SATCOM or HF data link can be used.
In principle again, SATCOM or HFDL can also be used during taxiing, take-off, departure, approach and landing. However, the performance characteristics of SATCOM and HFDL are worse than those of the VDL in terms of link setup and transfer delays.
The typical messages to be exchanged during the different phases of flight are shown in slide 10
Within l.o.s.
VDL 2, 3, 4
Outside l.o.s.
SATCOM
HFDL
VDL 2, 3, 4
VDL 2, 3, 4
VDL 2, 3, 4
VDL 2, 3, 4
VDL 2, 3, 4
VDL 2, 3, 4
l.o.s. : line of sight

10. Different types of data link messages as a flight progressesx i T a k e - O f f D e p a r t u r e E n R o u t e A p p r o a c h L a n d T a x i
As you can see in the above picture, there are many types of data link messages.
VDL Mode 2 , VDL Mode 3 and VDL Mode 4 offer the capability for full ATN compliant data exchange and can be used during the taxi, take-off, departure, approach, landing and taxi phases of the flight, when within the reach of an appropriate ground station.
From Aircraft
Link Test/Clock Update
Fuel/Crew Information
Delay Reports
Out
To Aircraft
PDC
ATIS
Weight and Balance
Airport Analysis
V-Speeds
Flight Plan-Hard Copy
Load FMC
From Aircraft
Off
From Aircraft
Engine Start
To Aircraft
Flight Plan Update
Weather Reports
From Aircraft
Position Reports
Weather Reports
Delay Info/ETA
Voice Request
Engine Information
Maintenance Reports
To Aircraft
ATC Oceanic Clearances
Reclearance
Ground Voice Request
(SELCAL)
From Aircraft
Provisioning
Gate Requests
Estimate Time-of-Arrival
Special Requests
Engine Information
Maintenance Reports
To Aircraft
Gate Assignment
Connecting Gates
Passengers and Crew
ATIS
From Aircraft On
From Aircraft In
Fuel Information
Crew Information
Fault Data from Central Maintenance Computer

11. Overview of the VDL Modes
Name
VDL Mode-2
VDL Mode-3
VDL Mode-4
Access method
CSMA
Carrier Sense Multiple Access
TDMA
Time Division Multiple Access
STDMA
Self-Organising Time Division Multiple Access
Capability
Data Only
Data and Voice simultaneously
Modulation
D8PSK
GFSK
Channel band-width
25 kHz
ICAO has standardized three different modes of the VHF digital link (VDL), VDL Mode-2, Mode-3 and Mode-4.
The various modes of VDL are not interoperable with each other, i.e. a VDL Mode 2 aircraft station cannot communicate with a VDL Mode 3 or a Mode 4 ground station.
The numbers Mode 2, Mode 3 and Mode 4 do not mean that a higher number mode is an evolution of a lower number mode; the were assigned by ICAO in the order that they were brought to ICAO for standardization.
VDL Mode-2 is data only, seen as a successor to ACARS, can be used for ACARS or ATN. The combination of the D8PSK (Differentially Encoded 8-Phase Shift Keying) modulation scheme and its CSMA (Carrier Sense Multiple Access) Algorithm has been shown to provide a user data capacity, per 25 kHz channel of over 10 kilobits per second when it is fully loaded.
With VDL Mode-2, the same frequency can be shared with multiple ground stations. VDL Mode-2 was designed for a relatively easy transition and implementation.
VDL Mode 2 is currently being implemented in Europe, North America and the Asia-Pacific regions.

12. The VDL Modes
The numbers mean what order they entered ICAO for standardising – they are not in succession
VDL Mode-1
Taken out of Annex 10 before ever implemented
– no longer exists
VDL Mode-2
Data Only
Successor to ACARS
25 kHz
VDL Mode-3
Voice & Data together
US FAA Program
25 kHz
VDL Mode-4
Data Only
Primary purpose is ADS-B
Swedish design
25 kHz
VDL Mode-3 is a system designed to provide digitized voice and data on the same channel. VDL Mode-3 uses an algorithm called TDMA (Time Division Multiple Access) and can support both voice and data communications. VDL Mode 3 can carry up to 4 voice channels simultaneously on one frequency, or a mixture of voice and data channels, for instance 2 voice and 2 data or 0 voice and 4 data.
The FAA NEXCOM program is developing multi-mode radios that will maintain analog voice service at 25 kHz channel spacing and also have the capability to provide an analog voice service at 8.33 kHz channel spacing along with VDL Mode-3 digitized voice and data service. FAA intends to implement VDL Mode-3 digitized voice first and then data some time around 2011.
VDL Mode-4 uses an algorithm called STDMA (Self-Organizing Time Division Multiple Access) which allows for the use of the same frequency by multiple ground stations. It is “self-organizing” in the sense that each participant reserves the use of a future slot and does not rely on a centralized reservation system. VDL Mode-4 was originally designed for ADS-B but it has recently been enhanced to have the capability of providing a point-to-point communications link in an ATN subnetwork. It could not though handle ADS-B and CPDLC on the same channel. VDL Mode-4 has incorporated aspects of communications, navigation and surveillance in its avionics which makes it more ambitious than the other VDL’s. On the other hand, this also means that it may be unlikely that the same set of avionics can be used for VDL Mode 4 and voice, unlike the other VDLs.
VDL Mode 4 has been implemented in operational networks in Europe, Russia and Mongolia.

13. The VDL Modes and 25 kHz/8.33 kHz voice systems
Simultaneous Voice & Data
(4 channels voice or ATN A/G data)
Analog Voice
Data Only
(ATN A/G)
Data Only
(ATN A/G
and ADS-B)
Analog Voice
Analog Voice
Analog Voice
CSMA
CSMA
TDMA
TDMA
STDMA
STDMA
DSB AM
DSB AM
DSB AM
DSB AM
DSB AM
DSB AM
DSB AM
DSB AM
D8PSK
D8PSK
D8PSK
D8PSK
GFSK
GFSK
Together with developing SARPs for VDL, ICAO developed SARPs for a DSB/AM voice system, operating on 8.33 kHz channels. This was in response to the need to increase the voice channel capacity in the VHF band, particularly in the core Europe area. VDL Mode-3 could also be used to increase the voice capacity in the VHF band.
ICAO has noted that in some regions there is a shortage of available VHF channels while that is not an imminent problem in other regions. Therefore systems such as 8.33 kHz voice or VDL Mode-3 can be implemented on a regional or even subregional basis.
8.33 kHz channel spacing has been successfully implemented in the EUR Region. VDL-Mode 3 implementation is scheduled in the United States under the “NEXCOM” program. The NEXCOM program is building radios that can handle 8.33 kHz for voice as well as the VDL Mode-3 digitized voice and data service.
8.33
8.33
8.33
25kHz
25kHz
25kHz
25kHz
25kHz
Voice Channels
Voice Channels
MODE 2
MODE 2
MODE 3
MODE 3
MODE 4
MODE 4

14. Long range data link systems Propagation Paths of SATCOM and HFDL
(Satellite)
SCINTILLATION
“CLOUD”
Jónhvolf in Icelandic
IONOSPHERE HF
The above illustration provides a general idea of how HFDL and SATCOM work.
SATCOM works with satellites and ground earth stations. Through the ground Earth Sation, messages are sent to ATC and airlines, typically through a service provider, such as ARINC or SITA. When using satellites in a geostationary orbit, polar areas are not covered.
HFDL uses a network of ground stations which is operated by ARINC. This network has currently an almost global coverage. Global coverage is expected in the next few years. In the HFDL network, an aircraft can potentially communicate with any ground station, thus significantly increasing the probability of establishing a link, compared to the current voice system, where an aircraft, for soverignty reasons, is only allowed to communicate with one ground station at a time.
The Ionospheric conditions that apply to HF voice do not affect HFDL in the same fashion, so HFDL can be used when HF voice cannot. IF the HF voice conditions are poor, or in what is called a “Blackout” condition, then communication with an aircraft using HF voice can be impossible. If a flight is in an area where there is only HF voice coverage, that means that there will be no communication with that flight. This is where HFDL would be useful.
SATCOM
Propagation problems
affecting HF and SATCOM
are fairly independent
HFDL GS
GES
GS=Ground station
GES=Ground Earth Station

15. HFDL (HF Data Link) HFDL - High Frequency Data Link
With ground stations around the world
Iceland Radio houses one of the Ground Stations
Can accommodate ACARS or ATN
Developed to be used in areas where satellite cannot be used
Cheaper alternative to SATCOM
HFDL is based on the principle that as an aircraft is flying around the world, it can utilize any HFDL ground station on any active frequency to communicate.
The ICAO SARPs (Standards And Recommended Practices) for HFDL accommodate both FANS 1/A (ACARS) and ATN protocols.
HFDL will most likely never be a primary data link - but a good use would be Polar areas and remote places where it is not feasible to have VHF stations or where there is no reliable satellite coverage.
Unlike the VDL’s or current satellite technology which depend upon “Line Of Sight” – HFDL is not reliant on Line Of Sight.
HFDL provides for a cheaper communication system for airlines than SATCOM.
The availability numbers for HFDL are very similar to SATCOM. The propagation issues that affect HF Voice do not affect HFDL.

16. SATCOM (AMSS) Satellite Communications
A system available for ACARS and for ATN
Satellites can be used for Data Link and for voice - often referred to as SAT Voice
Inmarsat is the current provider for aeronautical communications 1
The current provider of SATCOM services is Inmarsat - Within the next few years it is expected that the Japanese MTSAT system will be operational. Both systems comply with the current AMSS SARPs. They both use satellites in a geostationary orbit and, as a consequence, have no coverage in polar areas.
The airlines contract mainly through ARINC or SITA who are allied with Inmarsat GES operators.
Developments for future AMSS systems may include regional systems using satellites in a geostationary orbit or, with true global coverage, using satellites in non-geostationary orbits.
1 - Inmarsat is also provider for maritime and land mobile satellite communications.

17. ADS-C Automatic Dependent Surveillance-Contract
The C stands for contract. An ADS-C message is only sent after a link “contract” between the aircraft and the ground has been established.
ADS-C is currently used using SATCOM or HF data link.
ADS-C is a system where the ADS message, which includes the position of an aircraft, is transmitted to a ground station and forwarded to an ATC centre. It can be used directly to be plotted on a display or read by an air traffic controller.
ADS-C is currently used through SATCOM, HF data link and VHF ACARS systems.
It provides surveillance in areas where no radar infrastructure is available, particularly in remote and oceanic areas
It operates only in an air-to-ground mode.
We are currently using ADS-C in the North Atlantic.
ADS-C is sometimes called “ADS-A” – the “A” means “Address”

18. ADS-B Automatic Dependent Surveillance-Broadcast
ADS-B is a broadcast of the aircraft’s position, mainly derived from the GNSS system. It provides the pilot of a properly equipped aircraft a display on his instrument panel of where other aircraft are in relation to his aircraft.
ADS-B is a system where an aircraft automatically and periodically transmits in a broadcast mode an ADS-B message that includes its position. All aircraft and ground stations that can receive these messages are informed on the position of the transmitting (broadcasting) aircraft.
Through this system, all aircraft in the vicinity of the transmitting aircraft can identify its position. Since all aircraft are transmitting their position periodically, each aircraft is aware of the positions of other aircraft. The ground station will be able to exercise surveillance over all aircraft (ADS-B equipped) within their airspace. ADS-B emulates a radar-like ATC service in areas without radar
The position of the aircraft is typically derived from the GNSS system, however, other means over determining the position of an aircraft in the air (VOR, DME, INS) may also be used to generate an ADS-B report.

19. ADS-Broadcast Concept
Situational Display
Aircraft emits signal
Aircraft 1
Aircraft 3
ATC Surveillance
When Aircraft 1 sends a signal, Aircraft 2 and Aircraft 3 and ATC can see Aircraft 1 on their displays. In an ADS-B environment all aircraft will be “broadcasting” signals to other aircraft and the ground.
The 11th Air Navigation Conference stated that ADS-B would serve as an important enabler or cornerstone of several of the ATM operational concept components, including traffic synchronization and conflict management.
All the aircraft in the area will see the other aircraft on a situational display.
ICAO standardized systems that can transmit and receive ADS-B messages are:
SSR-Extended Squitter (1090 MHz)
VDL Mode 4 (operating in the VHF band)
The FAA and Eurocontrol are evaluating the above technologies along with UAT (Universal Access Transceiver) - developed by the FAA and MITRE CAASD.
UAT is currently going through the standarisation process as a potential third alternative system or technology for ADS-B.

20. Mode-S Extended Squitter
Mode-S was standardised by ICAO several years ago
The ICAO 11th Air Navigation Conference has decided that all ADS-B implementations should support the use of Mode-S squitter
The Mode-S squitter message is sent by the Mode-S or ACAS (aka TCAS) transponder which is mandatory equipment on airplanes on long-haul commercial air-routes today.
The original purpose of Mode-S was for ground radar stations to identify aircraft and send them Mode-S interrogations – like a sqawk or transponder code.
It was found that the Mode-S/ACAS transponders could be modified to include in their squitter message the position of the aircraft as calculated by the aircraft navigation system.
The advantage of Mode-S is that it would use avionics that must be on all commercial aircraft anyway.
Mode-S has been standardised for many years so it is considered to be mature and stable.

21. ICAO Communications/Navigations Surveillance (CNS) Environment
Air Traffic Management Centre
Satellite Ground Earth Station
VHF Voice and Data
Mode-S – Secondary Surveillance Radar
Radar would be surveillance
Using satellites to determine your location would be navigation
Using satellites or VHF to talk with airplanes is Communication
This is an illustration of the “CNS” concept.
The idea for the future is that the aviation world move away from “Air Traffic Control” to “Air Traffic Management”
The idea is that pilots will eventually have more responsibility for self separation.

22. What is the ATN?
“The ATN concept emerged from a need to interchange bit-oriented digital data over dissimilar aeronautical data links, using, for interoperability purpose, the principles of the International Organization for Standardization (ISO) open systems interconnection (OSI) architecture.”
The A-T-N stands for Aeronautical Telecommunication(s) Network.
The original idea with the ATN is that an aircraft can fly anywhere in the world and use any type of data link with the pilot not knowing any difference – in other words, it is transparent to the user.
For example, if SATCOM is best in a certain area of the world - the “router” in the ATN software determines that SATCOM is the best data link medium and chooses that.
If the aircraft is in a part of the world where VHF Data (digital) Link or SATCOM would not work, the router senses that and chooses HF Data Link (HFDL).

23. Describe the ATN
“The ATN design supports the incorporation of different air-ground subnetworks and different ground-ground subnetworks, resulting in a common data transfer service. Furthermore, the ATN design is such that user communication services may be introduced in an evolutionary manner”
Because the ATN in concept is like “Windows” and there will be applications that can be run over it - the ATN may be implemented in stages or there may be such a thing as “ATN-Lite” or a limited ATN.
Some people in aviation say that the ATN as it was originally envisioned will not take place.

24. OSI 7 Layer Protocol Reference Model
System A System B
Layer 7
Application
Layer 6
Presentation
Layer 5
Session
Layer 4
Transport
Layer 3
Network
Layer 2
Data Link
Layer 1
Physical
Layer 7
Application
Layer 6
Presentation
Layer 5
Session
Layer 4
Transport
Layer 3
Network
Layer 2
Data Link
Layer 1
Physical
OSI – Open Systems Interconnection.
ISO – International Organization for Standarization.
The OSI stack was developed by the ISO.
The Application Layer is the OSI layer closest to the user.
The Presentation Layer ensurse that information sent by the Application Layer of one system is readable by the Application Layer of another system.
The Session Layer establishes, manages, and terminates sessions between applications.
The Transport Layer attemtps to provide a data transport service that shields the upper layers from transport implementation details.
The Network Layer is a complex layer that provides connectivity and path selection between two end systems that may be located on geographically diverse subnetworks.
The Data Link Layer provides reliable transit of data across a physical link.
The Physical Layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.
Some people say that the ATN should have been designed with TCP/IP protcols and not OSI. This has been a bit of a controversy.
There is talk of incorporating TCP/IP into the ATN. Some even point to this as a great controversy. However in reality, TCP/IP is only a potential replacement on the lower layers in Ground/Ground sub-networks and possibly in some future Air/Ground datalink sub-networks of the ATN. The aeronautical data link systems as standardised so far, will not need to be changed to incorporate TCP/IP in the forseeable future, even if parts of the ground elements of an ATN network carrying data from these aeronautical data-links use TCP/IP. The ATN Concept as such still holds.

25. First CPDLC Message in Miami area
This is a picture of the first Controller Pilot Data Link Communications message in the Miami (Florida, USA) airspace.
This was done using VDL Mode-2, which will eventually transtion to the ATN.
CPDLC has been very popular among the participating pilots and air traffic controllers.