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During the LINK 2000+ programme EUROCONTROL developed and operated the first generation of the ‘MOON’ network for monitoring general VDL MODE 2, CPDLC and VDL MODE 2 AOC traffic to support the initial deployment.

So far MOON has only been operated by EUROCONTROL but it has proven to be an efficient tool for performance monitoring and has been instrumental in the detection and investigation of CPDLC, VDL MODE 2 implementation issues affecting interoperability or channel occupancy. MOON also provides key information for further capacity planning and multi-frequency VDL 2 deployment.

By connecting additional nodes to MOON, ANSPs could expand the MOON network to gain an independent view of their own CPDLC, VDL MODE 2 environment. By connecting to the MOON Server at EUROCONTROL participating ANSPs would also help to create European overview of the Datalink system, thereby significantly improving overall performance monitoring, problem investigation and resolution.


The MOON network is designed to enable in a cost effective way to:

  • Monitor VDL mode 2 channels load for a capacity planning purpose: MOON automatically generates a set of key statistics including the load of the VDL mode channels in operation.
  • Consult live VDL mode 2 traffic: Using a specific interface an operator can consult the technical aspects of the traffic exchanged
  • Detect early and automatically VDL mode 2 issues: Thanks to a powerful and extensible query engine, it is possible to set up a routine detection covering a wide range of possible VDL mode 2 implementation issues.
  • Automatically monitor issues resolution and reoccurrence: While an issue is reported to be solved by the industry or the airspace users, MOON enables to automatically monitor and notify its operators through E-mail generation in case of re appearance of the issue.
  • Generate detailed statistics: a wide range of statistics can automatically be computed by the MOON system. Charts (eg 2D or 3D bars or Pie charts or plain tabular reports) can easily be Copy/pasted for regular reporting.


The MOON monitoring system is made of local receivers stations named “Remote Monitoring Units” (RMU) communicating with a Central Monitoring Server (CMS). MOON infrastructure is totally independent of Communication Service Providers operating their own equipment for commercial VDL 2 service purposes.

Overview of the Moon Monitoring System

An RMU is made of a VHF digital receiver operating VDL Mode 2 and an industrial computer on which the recordings are stored. The traffic characteristics captured and recorded at each RMU are then provided to the Central Monitoring Server (CMS) through the internet or possibly the PENS network. The data transmissions from the RMUs to the CMS can be done automatically (on a regular – configurable – basis) or manually (at user’s request). The CMS role is to gather data traffic from RMUs, to merge them and filter out any potential duplicates (two RMUs could receive the same traffic because of coverage overlap). A large picture of VDL2 data link communication is then available for analysis and statistics. RMUs are deployed at different locations depending on the target area to be monitored. Due to propagation issues and in order to get as much uplink data as possible, it is always best to locate them close to active and operational VDL Mode 2 Ground Stations at airports.

RMU coverage

Due to physical particularities of the VHF propagation, the line of sight of each RMUs looks like an inverted cone. See picture below.

RMU Coverage

Consequently, the radio coverage provided by a MOON RMUs is optimal for aircraft enroute at their cruise altitude. On the other hand, MOON will benefit from additional RMUs for the following reasons

  • Although the coverage of the upper area is provided, the traffic in the lower space and in the vicinity of airports is currently only captured around 5 airports.
  • As depicted in the figure above, to capture the Uplink traffic to aircraft, the RMU needs to be close to the ground stations.

The figures below show the radio coverage of the currently deployed MOON RMUs at various flight levels

These pictures illustrate the limited radio coverage for traffic at low altitude or aircraft on the ground.

For these reasons, MOON coverage could be usefully improved by a partnership with ANSPs and airports adding RMUS at or close to airports.

MOON Central Monitoring Server

The CMS is the focal point of all the data collected by the RMUs and is located at EUROCONTROL. Hosting the “brain” of the MOON architecture the CMS achieves all the MOON functionalities for the users. By joining the MOON cloud, the ANSPs would benefit from all the CMS functionalities applied on the traffic of their RMUs deployed locally.

MOON key figures

Operated since 2006, the MOON network already captured about 400 millions of VDL mode 2 transmissions over 5 RMUS (Paris CDG, Amsterdam, Brussels, Frankfort and Hamburg).

Around 30 VDL Mode 2 implementation issues affecting commercial avionics or ground-equipments have been spotted and monitored by EUROCONTROL during the pioneer phase of Link2000+ and the resolution of these issues is coordinated by the EUROCONTROL MOON and Link2000+ teams.

In addition MOON provided the statistics and the field experience to predict VDL mode 2 traffic evolutions and the need for additional frequencies.


MOON is designed to be operated and to deliver its benefits at two levels: commonly but also locally. EUROCONTROL is operating the CMS for monitoring the datalink implementation issues and their resolution, and for capacity planning and performance monitoring purposes, at European level. The RMUs initially deployed by EUROCONTROL targeted Link2000+ pioneer deployment. By getting more RMUS deployed and connected to MOON CMS by ANSPs, the shared monitoring of performance and implementation issues should progressively cover the whole datalink deployment area.

Common high level statistics will be shared among all ANSPs, such as the evolution of the VDL MODE 2 equipped aircraft population, or the occurrences of most important issues (eg CPDLC provider aborts occurrences seen in Europe)

In addition, ANSPs who deploy and connect RMUs to MOON will benefit of an improved local view on datalink service and performance, independently of their communication service providers. ANSPs are indeed invited to install RMUs in the vicinity of their airports and local Communication Service Providers stations, and to connect those RMUs to the EUROCONTROL CMS. In return EUROCONTROL will provide ANSPs with a free of charge access to the CMS function with a visibility on their local traffic. The figure below shows an example of an ANSP having access to its installed RMUs through the CMS.

Several ANSPs accessing their RMUs

RMUs don’t require complex maintenance operations and are centrally monitored through the CMS. On the ANSP side, a contact person for in situ occasional intervention will be sufficient for the maintenance.