A Zero order network of permanent stations for positioning service in Italy: some hypotheses and tests

In these years, GNSS permanent networks finalized to real time and post processing positioning services are under development in Italy; for administrative reasons, they are designed, created and managed at a local scale, corresponding to Italian Regions. A permanent network materializes and distributes a reference frame to the user community; to guarantee that all the local net- works distribute the same reference frame, they should be adjusted and monitored in a common zero order permanent network, following a common adjustment protocol. From a theoretical point of view, the global IGS net- work and the IGS adjustment guidelines constitute the natural shared infrastructure. However, from a practical point of view, IGS permanent stations alone cannot constitute the Italian zero order network, because they are not homogeneously distributed in Italy; moreover IGS has not the scope of monitoring local subnetworks, whereas such a task could be performed by some kind of governing board taking care of the zero order net- work. Actually, the natural choice is to establish a zero order permanent network at a national scale; this network should be adjusted and monitored in the global IGS network in order to provide the link between global and local networks. Moreover the zero order network also guarantees the consistency between local neighbouring networks. In order to fulfil these scopes, the zero order network should satisfy several scientific and technical requirements, from the design to the network monitor- ing choices. The present paper focuses on these topics: the Italian situation is analysed and a numerical test is presented: a set of about 60 permanent stations has been chosen according to a good design criterion and their data for a three months period have been analyzed. Related re- sults are discussed, with particular regard to the criteria for the automatic outlier rejection and results check.