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Data Link

Latest update 29/07/2017: please see below for more details.

OVERVIEW

Regulation (EU) 2015/310 of 26 February 2015 amending Regulation (EC) No 29/2009 describes the data link compliance requirements for data link services within the single European sky. The original Regulation was amended due to implementation and deployment issues in addition to on-going service performance issues. The significant amendments are:

  • The date of implementation affecting aircraft with an individual CofA first issued before 01 January 2014 has been pushed back from 05 February 2015 to 05 February 2020,
  • Aircraft first issued an individual CofA before 31 December 2003 that shall cease operating before 31 December 2022 need not be equipped (previously 01 January 1998 by 31 December 2017),
  • New State aircraft certified after 01 January 2019 shall be suitably equipped, and
  • Article 15, Entry into force and application has been delayed from 05 February 2013 to 05 February 2018.

ERA’s Position

Despite the on-going issues with data link, the huge investment already made and the fact that data link is a major enabler for other projects within the Pilot Common Project means that the programme will certainly continue until a solution is found. Therefore ERA will continue to work closely with the SESAR Deployment Manager, the Commission and all stakeholders through the auspices of bodies such as the ICB, DLISG, AAB and ATM IG, and other Airspace User Associations to ensure members are part of the process and remain fully advised throughout.

UPDATE

Latest update 29/07/16: 

As the use of VDL2 for CPDLC using ATN became more widespread, certain performance issues were observed that raised concerns on the usability of the system, principally, the high incidence of involuntary Provider Aborts (PAs). Some of these issues were attributed to specific avionics installations for which solutions have been identified and corrected but other problems remained.

A summary of the findings of the report follows and the full report is available to download from this page.

1. Ground infrastructure

The summary of the report’s findings essentially show that the existing VHF ground network deployment, which has been initially and mainly deployed to support Airline Operational Control (AOC) communications (for example, placing a ground station at each terminal), was not deployed in an optimised way to support the Air Traffic Control (ATC) services. In some cases, there was excessive ground station coverage overlap, which increases collisions from hidden terminal transmissions. In other cases, there may have been arrears of limited RF coverage, resulting from the incomplete deployment of the VDL ground station infrastructure, which created unexpected aircraft behaviours for the selection of ground stations.

Assessment of the various MF deployment options have concluded that the best model for MF deployment in Europe is a model where all the ground stations operating on VDL frequencies in a given Service area work together under one unique frequency licensee responsible for managing the traffic on the RF network.

MF network topology and channel allocation assessment has shown that the use of reserved VDL2 alternate frequencies is preferred over the use of common VDL2 alternate frequencies, because they van be operated closer to the RF load threshold. The addition of a fifth VDL2 frequency is preferred over the current four VDL2 frequency allocation in any network management approach. The use of CSC as a common command and control frequency shall be continued, but should be utilised for AOC/ATS data only in areas with low traffic levels. In general, airport area and en-route datalink operations should be on separate frequencies in areas with high traffic levels, however in areas with low to medium traffic levels, both airport area and en-route operations may be supported on a single frequency. The deployment of VDL ground stations should be determined by the need to provide VDL2 en-route coverage, as requested by the DLS regulation. Additionally, airport area coverage for initial logon procedures and for AOC traffic must also be provided.

2. Level of RF interference for the core European area

The level of RF interference for the core European area was measured and it was confirmed that the CSC channel is saturated with a channel use above 50% during peak hours over the core European airspace. In addition to allocating further channels, gains on the retransmission rate will also improve the capacity usage situation.

The main contributor to the RF issues encountered during the project’s lifetime has been the uncoordinated ground transmitter activity due to overlapping intra- and inter-CSP’s coverage (uncontrolled hidden transmitter effect), with a mean loss of more than 30% of the frames due to this type of RF interference. Additionally, satellite signals and modulated voice transmissions can also create interference, albeit on a much smaller scale (<1%).

3. Management of 'hot spots'

The current single frequency VGS deployment leads to areas with ineffective avionics’ handover behaviour (impacting the overall performance). ELSA has shown that 'typical' avionics are using 18% of the ENR channel capacity only for HO, but monitoring of 'best in class' avionics has shown that these are managing the HO more efficiently

Avionics suppliers shared the main principles for handover management. A detailed study was conducted by avionics partners in order to analyse the main reasons behind aircraft handovers. In congested areas (covered by many VGSs), or areas where there was a lack of coverage, the number of handovers increased dramatically for all avionics configurations. In addition, the interoperability tests identified differences between avionics implementations which led to various levels of performance in maintaining connectivity. These differences of implementation are driven by the flexibility of the guidelines defined in the current standards.

The sharing of channel usage between AOC and ATC contributes to the overall congestion of the frequency (mostly at the airport level), even though it is not the main contributor to the ATS problem.

4. Concurrent management of AOC and ATN data traffic

The current CPDLC traffic represents only a low percentage of the overall ATN traffic observed: the link maintenance constitutes significant overhead on the capacity usage. This ratio is nevertheless not at all representative of the nominal use of the CPDLC service (as of today, more than 4500 aircraft VDL2 equipped establish ATN connectivity with ground systems, while very few of them actually make operational use of CPDLC).

5. Management of air/ground communication service provision (distributed versus centralised):

ELSA performed a technical analysis of the existing approaches to network management. Different ground RF network architectures can be described by a combination of three factors:

  • Number of different RF networks (operated by different providers) in the same Service area.
  • Type of frequency licensing (or allotment) used for the VDL2 channels.
  • Type of GSIF advertisement operated on each channel – with one-GSIF either ARINC or SITA is accepted, with two-GSIF both are accepted.

A trade-off analysis has been conducted by considering the behaviour of the options, identifying which of them guarantees the best answer to the technical issues. The model ELSA determined as the best option for the multi-frequency network implementation is a single RF network is providing all VDL2 data link services in a Service area (areas homogeneous in terms of operational and technical needs, identical with FABs or new similar entities);

  • Alternate frequencies are reserved frequencies, licensed to only one operator in a Service area;
  • Two-GSIF channels are used, meaning all users can be accepted on the same network.

In general, different network architectures, for example one RF network with two DSP IDs or two overlapping RF networks with one DSP ID per network, can be used during the transition phase to support VDL2 multi-frequency operations

6. Avionics/ground end systems

The project has identified 'best in class' avionics configurations, which are considered as the set of airborne equipment necessary and sufficient to comply with the ATN/VDL2 performance expectations.

The interoperability tests, with more than 500 variants of tests performed, showed that 'best in class' avionics configurations passed the MF tests. The following additional results have been noted:

  • A few minor MF functionality issues without operational impact have been found.
  • Some MF interoperability improvements and clarifications for the relevant standards have been identified.

Issues with ground sub-systems have been identified. Several of these issues have already been fixed during the project’s lifetime; others have been noted for a follow-up.

Some flight crew operating procedures have suggested reset of the on-board equipment when facing CPDLC issues. This reset has proven many times to increase the instability of the link instead of solving the temporary issue.

Several protocol optimisations were identified leading to capacity gain by reducing the number of messages exchanged

The implementation of the recommendations will lead, if addressed in a coordinated way by all stakeholders, to an ATN/VDL2 system that provides acceptable performance to support the full deployment of ATN B1 CPDLC service in the European airspace. As an example, best in class avionics have demonstrated that they can provide less than 4 PAs per 100h CPDLC usage (compared to an average rate of 20 PAs per 100h of CPDLC as measured during ELSA project), without other ELSA recommendations being deployed.

The implementation of the recommendations should be addressed as soon as possible (or have already been addressed within the ELSA project). A proposed scheduling of these recommendations is depicted in the report however the exact implementation will have to be elaborated in a second step.
 

Useful Links:

http://eur-lex.europa.eu/legal-content/EN/TXT/?uri=uriserv:OJ.L_.2015.056.01.0030.01.ENG
http://www.sesardeploymentmanager.eu/

CONTACTS

For further assistance please contact Russell.Dudley@eraa.org