per day over the WMO Global Telecommunications System. Most AMDAR observations are in the Northern Hemisphere, and programs like EUCOS (EUMETNET Composite Observing System) are working to optimize AMDAR ascent and descent data for use by EUCOS member countries. For example in 2006, EUMETNET-AMDAR provided approximately 750 soundings per day.²¹ The RBSN observing stations and conventional upper air network do not all report on a routine basis, with the performance varying greatly by WMO region.²² Reports from stations over the United States are very reliable.

In addition to the GOS, specialized observing networks such as the Global Atmospheric Watch for chemistry and the World Hydrological Cycle Observing System provide data that may or may not be in real time. Approximately one-fourth of the RBSN stations make up the Global Climate Observing System (GCOS) Surface Network,²³ and approximately 20 percent of the upper air sites make up the GCOS Upper Air Network. As with the GOS, performance of the GCOS sub-set of the GOS is not at 100 percent.

The space-based subsystem of the GOS embraces the concept of a composite observing system with research and operational satellite data used in synergy.²⁴ Data are provided by both operational satellites and low-Earth orbit research satellites. Examples of the research products are hyperspectral sounding data from AIRS, altimetry measurements from JASON, precipitation measurements from TRMM, and sea-surface winds from ENVISAT. Much of the satellite data flowing into the GOS are used for routine analysis, nowcasting, and forecasting applications at the National Meteorological and Hydrological Services (NMHS) across the globe. Global NWP centers use the data for a variety of forecast guidance products.

How the GOS is expected to evolve over the coming decades was recently discussed in WMO Technical Document No. 1267, “Implementation Plan for Evolution of Space and Ground-based Subsystems of the