Reference frames for GNSS positioning services: Some problems and proposed solutions

Positioning services based on GNSS Per- manent Networks (PNs) disseminate the reference frame to users; hence the networks must be adjusted by constraining the ITRF coordinates of some per- manent stations (PS), with the satellite orbits and clocks constrained to the values published by the In- ternational GNSS Service (IGS). Furthermore, the positioning service provider must use analysis meth- ods, conventions and practices fully consistent with IGS standards. In this framework some problems arise. A first one is that long term solutions for the IGS PS coordinates are not always fully consistent with weekly IGS products, requiring an appropriate use of IGS PS weekly solutions. Moreover, the consis- tence of the reference frame implied by di¤erent posi- tioning services must be checked, to avoid disconti- nuities between neighbouring networks. Clearly, in order to monitor the PS coordinates, the adjustment of a network needs to be performed on a quasi real time continuous (daily or weekly) basis. However, from a practical point of view, a continuous update of the PS coordinates is neither appropriate nor pos- sible and alternative proposals will be presented. Fi- nally, most users of real-time positioning services need coordinates in another reference frame, perhaps a local one or the national one. In this case the real- time service provider must distribute the transforma- tion between the broadcasted frame and the user re- quired reference frame. In this work some possible solutions of the aforemen- tioned problems are presented, based on experiences obtained during data processing and adjustments of the Piemonte and Lombardia (Northern Italy) net- works.