scope:

The present document states part 2 of the minimum performance requirements for VHF Datalink Mode 4 (VDL Mode 4) ground station equipment intended to be used for air-ground data communications, operating in the VHF band (117,975 MHz to 136,975 MHz and optionally 112,000 MHz to 117,975 MHz) allocated to the aeronautical mobile service. The equipment is also intended to be interoperable with mobile equipment supporting air-air data communications. It is designed to ensure that equipment certified to it will be compatible with the relevant ICAO VHF Digital Link (VDL) Standards and Recommended Practices (SARPs) and VDL Mode 4 Technical Manual (TM) [1].

Manufacturers should note that all or part of the frequency band 108,000 MHz to 117,975 MHz might become available for aeronautical communications.

The present document applies to Gaussian-filtered Frequency Shift Keying (GFSK) systems, with channel separations of 25 kHz intended for air-ground communications. The scope of the present document is limited to ground base stations.

The VDL system provides data communication exchanges between aircraft and ground based systems supporting surveillance and communication applications. The supported modes of communication include:

• broadcast and point-to-point communication;

• broadcast services including Automatic Dependent Surveillance - Broadcast (ADS-B), Traffic Information Service - Broadcast (TIS-B) and Flight Information Service - Broadcast (FIS-B) capabilities;

• air-air and ground-air services;

• operation without ground infrastructure.

VDL Mode 4 is designed to be an Air/Ground subsystem of the Aeronautical Telecommunication Network (ATN) [8] using the AM(R)S band and it is organized according to the Open Systems Interconnection (OSI) model (defined by ISO). It provides reliable sub network services to the ATN system. Other networks can also be supported but these have not been focussed on in the present document.

The present document specifies functional specifications of VHF communication ground station equipment intended to be used for air-ground and air-air data communications. The present document is derived from the standards and specifications in:

• VDL Mode 4 standards produced under the auspices of the International Civil Aviation Organization (ICAO) [1].

• Other relevant standards as defined in clause 2.

It is envisaged that manufacturers may provide equipment supporting:

• broadcast services only;

• point-to-point services only;

• both broadcast and point-to-point services.

EN 301 842-1 [4] deals with tests of the physical layer. The present document defines the core link layer requirements for the VDL Mode 4 ground station necessary to support all types of equipment. This includes a simple position broadcast functionality.

The present document deals with tests of the link layer sufficient to support core link layer functionality, and it also includes requirements and tests sufficient to recognize and respond to transmissions associated with point-to-point communication. The present document does not address requirements for the full ADS-B message set, or for other broadcast applications that can be supported by the VDL Mode 4 equipment. These are covered by EN 301 842-3 [9]. Detailed requirements for point-to-point communication are beyond the scope of the present document, but can be found in EN 301 842-4 [10]. EN 301 842-4 [10] also include the interface to the Aeronautical Telecommunication Network (ATN) as defined in ATN SARPs [8].

As the measured values of equipment performance may be a function of the method of measurement, standard test conditions and methods of test are recommended in the present document.

The present document is organized as follows:

• references, definitions, abbreviations and symbols are provided in clauses 2 and 3;

• clause 4 describes the VDL Mode 4 ground station link layer;

• clause 5 performance specifications for the VDL Mode 4 ground station and ground station co-ordination;

• clause 6 provides general design requirements;

• clause 7 provides protocol tests for core link layer functions;

• a document history is contained in clause 8;

• clause A provides a detailed cross-reference to the relevant requirements contained in reference [1];

• annex B provides a description of the ISO/IEC 9646 [7] Test Methodology.

Note that the system can support a very wide range of functions. It is not practical to provide specific tests for all aspects of its functionality. The approach used is to provide detailed tests for the core link layer functionality and to provide tests of those remaining requirements which, if wrongly implemented, could cause a deterioration in the service offered by other VDL Mode 4 stations. Therefore:

• a detailed set of protocol tests are provided for the core link layer functionality necessary to support broadcast functions;

• a detailed test of position encoding and decoding is provided because of the importance of position in the management of the VDL Mode 4 link specifically and the need to support ADS-B applications in general.

Mandating and Recommendation Phrases

a) "Shall":

the use of the word "Shall" indicates a mandated criterion; i.e. compliance with the particular procedure or specification is mandatory and no alternative may be applied.

b) "Should":

the use of the word "Should" (and phrases such as "It is recommended that...", etc.) indicates that though the procedure or criterion is regarded as the preferred option, alternative procedures, specifications or criteria may be applied, provided that the manufacturer, installer or tester can provide information or data to adequately support and justify the alternative.