Mode S ground stations with overlapping coverage must be co-ordinated to ensure they work correctly from both a surveillance and a data-link point of view. This co-ordination is achieved by a combination of techniques which include using II and SI code allocation, coverage map design and providing ground stations with network connections for inter-station coordination. In order to reduce FRUIT and cater for the limited number of II codes available within Mode S, common use of a single II or SI Code within a cluster of ground stations is to be adopted in the European implementation of Mode S. Theseclustered stations will co-ordinate the handover of aircraft passing between adjacent ground station coverage such that the acquisition performance of the ground stations can be maintained. This will be achieved by means of a Surveillance Co-ordination Network (SCN). The network will enable lockout to be achieved following initial acquisition of an aircraft, thus significantly reducing FRUIT levels and associated garble in the SSR environment.
A Surveillance Co-ordination Function (SCF) provides the information interchange over the SCN to allow interrogators to operate in such a cluster. When operating as part of a cluster the station’s operation is termed ‘Network Aided’. This document describes the protocols necessary to support this operation in any Cluster Mode or State. These Modes include:
- Central Mode where the cluster operation is maintained by a Cluster Controller (CC);
- Distributed Mode where the cluster operation is maintained by processes operating at each of the ground stations.
In addition to network aided operation within a cluster, the SCF can also support ‘Standalone’ operation when the connection between ground station and the surveillance co-ordination network is disabled.
A separate specification exists for the European Mode S ground station and the Cluster Controller (CC). This Interface and Control Document defines the interfacing required between the co-ordination network nodes.