The Global Telecommunication System (GTS)
Real-time data are normally being exchanged internationally via the Global Telecommunication System (GTS) of the WMO Information System (WIS). GTS data are exchanged according to WMO Resolution 40 (Cg. XII) and Resolution 25 (Cg XIII) for meteorological, and hydrological data repectively. Annex 1 to the Resolution 40 defines "Essential" Data and Products to support WMO Programmes and which Members shall exchange without charge and with no conditions on use. All available in situ observations from the marine environment as well as upper air observations (e.g. ASAP soundings) are regarded as "Essential" by the Resolution.
GTS data can be requested by contacting any Permanent Representative of a National Meteorological and Hydrological Service (NMHS) with WMO (see WMO Publication No. 5 for their references). GTS data will gradually be integrated in the WMO Information System (WIS) which will also eventually provide for Data Discovery, Access and Retrieval service (DAR).
More information on the GTS, including WMO Publication No. 386, Manual on the GTS, can be found here.
The WMO Publication No. 306, Manual on Codes can be found on the WMO web pages here.