Innovative Atmospheric Remote Sensing: GNSS Radio Occultation
Wickert, J.¹; Beyerle, G.¹; Heise, S.¹; Jakowski, N.²; Michalak, G.¹; Rothacher, M.¹; Schmidt, T.¹; Viehweg, C.^1
1GFZ Potsdam; ²DLR Neustrelitz

In 2006 a multi-satellite constellation for precise global atmospheric sounding using the innovative GNSS radio occultation (RO) technique became reality. In addition to the German CHAMP (CHAllenging Minisatellite Payload) satellite, which provides almost continuous data since 2001, several other LEO (Low Earth Orbiting) satellites started to operationally provide GNSS RO data. The six satellites of the U.S.-Taiwan COSMIC/Formosat-3 mission (Constellation Observing System for Meteorology, Ionosphere and Climate) were successfully launched on April 14 and the European METOP satellite on October 19, 2006. In addition the U.S.-German GRACE-A satellite (Gravity Recovery And Climate Experiment) provides continuously data since May 22 of the same year. Altogether these satellites are expected to provide about 3,500 occultation measurements per day in near-real time. The constellation, consisting of 9 satellites, multiplies the potential of the GNSS RO technique for several applications in atmospheric research, weather forecast and climate change related studies. A milestone for these applications was the beginning of operational assimilation of GNSS radio occultation data to improve global weather forecasts in late 2006.

We introduce the GNSS radio occultation technique and review status and recent results of the various occultation missions. The unique long-term data set from CHAMP, e.g., covers already more than 5 years. This allows for climatological investigations, e.g., of gravity wave parameters and tropopause characteristics on a global scale. The long-term set is complementary to the multi-satellite set, mainly based on the COSMIC/Formosat-3 data, which allows for GNSS RO applications with much higher spatial and temporal resolution. Currently only data of the U.S. American GPS are exploited for the application of radio occultation. The signals of GALILEO and also of the currently reanimated Russian GLONASS would allow for a significant increase in the importance of GNSS RO for global atmospheric remote sensing. The application of these Navigation Satellite Systems for GNSS RO not only would increase the number of possible measurements, the new signal structures also have the potential to increase the accuracy of GNSS RO and the range of its applications.