Over the past few years, the importance of satellite data has progressively increased, to the extent that satellite systems now provide the main sources of information for NWP data assimilation systems. Its contribution to forecast skill in the Northern Hemisphere is now more important than that of radiosondes. In the Southern Hemisphere forecast skill has dramatically improved over the last five years, and is now at a similar level to that of the Northern Hemisphere. This is due to a combination of NWP model improvements and an enhanced use of satellite observations at large in data assimilation schemes. In particular, a robust and sustained positive impact of satellite data in modern data assimilation systems has been obtained from the direct use of radiance observations. Observing System Experiments (OSEs) are carried out at ECMWF on a regular basis, in support to Space Agencies and/or as a sanity check of the operational data assimilation system. This talk will report on the latest impact studies performed at the Centre, with a special emphasis on the impact of the recently available EUMETSAT's METOP data on the quality of the ECMWF model forecast skill. The ease with which METOP instruments have been integrated into the ECMWF operational suite highlights four points:

• The intrinsic high quality of all the instruments evaluated so far (AMSU-A, MHS, HIRS, ASCAT and IASI)

• The importance of the continuity in instrument availability (around 10 years for AMSU and much more for HIRS) and/or of the scientific and technical heritage (ERS-2 for ASCAT and AIRS for IASI)

• The vital importance of a good data coverage complementarity between the mid-morning (METOP-IASI) and afternoon (AQUA-AIRS) orbits

• The excellent dialogue between ECMWF and the EUMETSAT and CNES space agencies

Our preliminary experience with METOP data raises high expectations about the NPOESS programme and its risk reduction project NPP, which will carry similar class sounding instruments.

ECMWF has also recently introduced operationally the assimilation of rain-affected microwave radiances. The latest status of this activity will be highlighted, together with its implications in terms of requirements and expectations about the DMSP, NPOESS, and possibly Post-EPS programmes.

Last but not least, and after one year of routine operations, this talk will relate the ECMWF experience on the assimilation of GPS radio-occultation measurements (in particular from the COSMIC constellation), and again translate it in terms of requirements for future space missions.