The GNSS transponder and the time synchronized self organizing TDMA data link-a key to the implementation of cost-effective GNSS-based CNS/ATM systems

The Swedish Civil Aviation Administration (SCAA) has arrived at the conclusion that the most promising technology to meet some of the fundamental requirements of GNSS-based CNS/ATM applications is the introduction of the GNSS transponder and its GNSS-time synchronized self-organising Time Division Multiple Access (TDMA) data link which is a Two Way Data Link (TWDL). The GNSS Transponder includes a high precision GNSS receivers, communication processors and transceivers which can be integrated with other ATC and aircraft systems. This article discusses some of the available alternatives. The principal SCAA concerns with the Carrier Sense Multiple Access (CSMA) VHF data link (VDL) concept studied by the TCAO Aeronautical Mobile Communications Panel (AMCP) are related to its inability to meet requirements for time-critical message transfer. Without the capability to meet such requirements, the CSMA VHF Data Link (VDL) will have to be supplemented with another data link concept. The most promising alternative is the VDL concept which could form the basis for a single VDL concept for all phases of flight with the capability to satisfy all user needs. This VDL is based on GNSS-time synchronized self-organising Time Division Multiple Access (TDMA) technology with very precisely defined timeslots. During the evaluation-programme 2,250 time-slots for complete airborne ADS reports within the 1 minute frame have been used