Aeronautical Communication Technologies Simulator

A.C.T.S.

ACTS: A Generic Simulator
The need for simulation
Objectives
ACTS Design principles
Architecture
Validation
Scope of Applications for ACTS

VDL Mode 2 - ACTS
VDL Mode 2 Context
VDL2 Simulation goals
VDL2 Specific requirements
Architecture
External inputs and scenarios
What does ACTS look like ?
Some milestones

Contacts

ACTS: A Generic Simulator

The need for simulation

The aeronautical world expects air traffic to further grow significantly over the next decade, leading to more congestion and delays. Improvement in air/ground communications is required to support a more sophisticated air traffic management. Beside ATS needs, new services are being desired for airlines operation and passengers applications. By the nature of aircraft flying fast, high and far, the air/ground digital segment will thus remain the bottleneck of aeronautical communications systems.

Accurately simulating the digital air/ground segment is thus crucial. With simulation, it is feasible to test an infinite number of situations where operational conditions, traffic, and influent system parameters are varying in search for optimised performance and capacity.
As heavy simulations can nowadays be achieved on a single personal computer, this simulation-based strategy becomes affordable, especially when the design has been modular enough to limit to a few parts the adaptations required by every further application.

Eurocontrol position in regular contact with avionics manufacturers, service providers, airlines and CAAs is an advantage for such strategy. The Experimental Centre adds to that an expertise in air traffic simulation with all required data, next to the labs where validation of results can best be tackled.

Objectives in ACTS development

ACTS focuses on the air/ground segment, especially the layers named physical and data-link according to the OSI model. The segment is operated in an aeronautical context, and takes into account all types of communications (ATS,AOC...) present on it.

ACTS delivers an in-depth understanding of the capacity and its limitations factors, helping to validate the intended system deployment. The effect of adding new applications on the system can be studied.

Tuning system parameters for each operational context is also intended in order to optimise quality and reliability of the communications, primarily those ATS-oriented.

Future communications systems, especially those based on 3rd Generation -CDMA or satellite, typically derived from other communications sectors, should best also be validated through ACTS before heavy financial commitment are undertaken.

ACTS Design principles

ACTS design started from following prerequisites:

 A modular architecture, whose main parts are re-used across several systems

 A Session running in real-time or faster

 A Simple platform (typically PC under windows)

 Application of EEC –case tools (Visual C++)

ACTS architecture

ACTS simulates the air-ground segment, with the specific radio aspects and access mechanism of the system under study. The behaviour of ‘N’ stations (aircraft, ground station, satellite...) is integrated in a given crucial sector.

Inputs are standardised in 3 main parts:

Air traffic input: the geographical position of each station is computed and regularly refreshed, as this determines propagation delay and link power budget. Interface for typical traffic simulators or radar-plot streams are foreseen. Also, an access is envisaged to an external air traffic simulator available in EEC that on-request will deliver stations positions.

Messages profiles: Each station is given a profile for messages distribution in time and length. Options for random or events-based transmissions (depending on the applications) are available across the different flight phases.

System parameters:
Whatever the communication system under study, there always are:

 Propagation model
(specific to each frequency -band)

 Physical layer parameters
(Power, feeder loss, antenna gain, noise figure…)

 Medium access control- layer parameters

 Data-link layer parameters

Due to propagation delays and link power budget, each station has a specific view of the channel. A main characteristic of ACTS is to cope with this fact for each station, through the channel filter module. Each station is given also a ‘system’ module with receiver/ transmitter (physical layer) and a data-link layer part including the MAC layer.

ACTS Validation

Any communication system simulation requires a rigorous validation campaign. Five fields of validation are planned:

 Application of certification-oriented tests (typically those of system MOPS) to individual station model in order to confirm its correct behaviour.

 Theoretical mathematical models are used for validating the main trends in the results.

 Cross-check with independent simulation (when available)

 Field trials with defined simplest scenarios.

 Initial operation feedback.

Scope of application for ACTS

The first air/ground communication system currently simulated is VDL Mode 2. Other systems are considered with minimised additional developments. Among others, ACTS could soon be adapted to:

 1090 -extended squitter (Mode S)

 Next VDL Modes

 UAT (Universal Access Transceiver)

 3G system (Wide-band –CDMA, UMTS...)

 Satellite-based system

VDL2 - ACTS

VDL Mode 2 context

Airlines intensively operate their ‘AOC’ communications over ACARS: a VHF 2.4 kbits/s system with CSMA access mechanism, deployed in Europe on a few channels. Those are so often saturated that new applications were put on hold in major airlines, in expectation of more capacity. VDL Mode 2 in its minimum version (called AOA) is currently launched by pioneer airlines and its availability is expected to rapidly growth so as to take over ACARS on medium term.

ATS data-link will be supported by deployment of CPDLC and ATN functions on top of VDL Mode2. This is being progressed in the frame of Link2000+ project across Europe for operation from end 2004 onwards.

VDL 2 simulation goals

 Accurate evaluation of capacity and performance of any VDL Mode 2 channel in an operational environment:

• Net Throughput Vs Channel load (Capacity of each channel at a quality of service threshold).

• Collisions rate, retransmission rate.

• Transmission delays.

• Lost messages.

Identifying the critical aspects:

• The impact of the hidden and the exposed stations

• Critical capacity environment (airport, en-route domains...)

• Best frequency scenarios for current and future (2005-2010) air traffic demands both supporting ATS and AOC needs

• The required ground station density compared to the providers currently plan

 Optimising the VDL Mode 2 MAC sub-layer (CSMA) and data-link layer (AVLC) parameters:

• CSMA persistence (p)

• Different timers values for both CSMA and AVLC (TM1, T1)

 Evaluating the possible benefit of improvements to VDL Mode 2 standard: among others evaluating the impact of

• Different p-values given to avionics (static or dynamic value), ground stations in en-route or airport environment.

• New hand-off mechanism (improved SQP scale, use of position information)

 Giving input for future real time ATC simulations including data-link.To evaluate impact on operations of:

• delays

• Lost messages

Specific VDL 2 simulations requirements

The simulator should simulate:

 Propagation delay

 Accurate VHF propagation model

 BER degradation, FEC-function, and resulting Frame error rate

 The VDL 2 specific Mac and Data link layer aspects including hand-off

 The Hidden/Exposed terminal effects

 Realistic AOC and ATS/ATC Traffic Messages distribution in time

 AOC and ATS/ATC message sizes distribution in all alternatives
(CPDLC over ATN/VDL 2, AOC on AOA)

Inputs / outputs

System Parameters input table

Statistics output table

AVLC message traffic monitor

External Inputs and scenarios

Air Traffic

• The first step consists in using an air traffic simulator log file (as presented below) where aircraft positions and status are refreshed regularly.

• In second step, an external library running in parallel, using CFMU flight plan as an input, will deliver on request station's informations.

Message profile

A formal cooperation is setup with SITA and ARINC to provide under NDA AOC message profiles for representative airlines in Europe.

ATS message profiles are extracted from Link 200+ requirements.

Five data-link users categories have been defined from small to large user and applied to airlines present in traffic records.

A flight is segmented in ten phases, during each one a certain amount of data has to be exchanged with the ground depending on the data-link users category it belongs to. A random distribution is added to determine transmission times.

Ground station density

SITA VDL2 ground stations in Europe that shall be deployed by 2005

Scenarios

The first two scenarios are based on Charles de Gaule (CDG) critical sector.

The air traffic will be the peak hour of the peak day of 2002

2005 : 1 frequency in use, all AOC traffic assumed migrated to VDL2

2010 : Up to 4 frequencies available, increased AOC traffic and all ATS/Link 2000 applications considered

Aircraft equipment rate according to EUROCONTROL STATFOR inputs

What does ACTS look like ?

VDL 2-ACTS Milestones

 High Level Simulation Requirements Þ October 02

 Architecture design Þ December 02

 Software development

• First release including physical and MAC layer Þ January 03

• Data-link layer implementation Þ March 03

 Simulation Inputs Þ Feb/March 03

 Validation Þ April/May 03

 Simulations completion 3Q 03

Contacts:

Eurocontrol EEC Bretigny
Bertrand DESPERIER
ACTS project manager
Tel +33 (0)1 69 88 76 09
Fax +33 (0)1 69 88 78 90
bertrand.desperier@eurocontol.int

Yannick MONTULET
ACTS Software developer
Tel +33 (0)1 69 88 72 36
Fax +33 (0)1 69 88 78 90
yannick.montulet@eurocontol.int

Eurocontrol HQ Brussels
Patrick DELHAISE
VDL2 Implementation project manager
Tel +32 (0)2 729 34 78
Fax +32 (0)2 729 35 11
patrick.delhaise@eurocontol.int

Massimiliano ESPOSITO
VDL2 Implementation engineer
Tel +32 (0)2 729 34 90
Fax +32 (0)2 729 35 11
massimiliano.esposito@eurocontol.int