The co-ordinated global system of telecommunication facilities and arrangements for the rapid collection, exchange and distribution of observations and processed information within the framework of the World Weather Watch.
Observers worldwide add their observations to this network at the synoptic times, and the global models of the world tap into this network to get data to seed their computations, and they in turn put their results back onto this network for others to see.
From the WMO: "WMO Global Telecommunication System (GTS) is composed of a dedicated network of surface and satellite-based telecommunication links and centres operated around the clock all year round. It interconnects all NMHSs for collection and distribution of all meteorological and related data, forecasts and alerts, including tsunami and seismic related information and warnings. More than 50,000 weather reports and several thousand charts and digital products are disseminated through the WMO GTS daily. WMO is building on its GTS to achieve an overarching WMO Information System (WIS), enabling systematic access, retrieval, and dissemination and exchange of data and information of all WMO and related international Programmes. WIS will also be able to provide critical data to other national agencies and users dealing with many sectors including disaster risk management"