Radio Occultation with CHAMP -- GPS Radio Occultation with CHAMP and GRACE: Recent Results -- Sensitivity of Stratospheric Retrievals from Radio Occultations on Upper Boundary Conditions -- Error Characteristics of Refractivity Profiles Retrieved from CHAMP Radio Occultation Data -- Refractivity Biases in GNSS Occultation Data -- Stellar Occultation with GOMOS -- GOMOS Ozone Profiles at High Latitudes: Comparison with Marambio and Sodankylä Sonde Measurements -- Ozone and Temperature Retrieval Results from GOMOS Validated with CHAMP and ECMWF -- Modeling Errors of GOMOS Measurements: A Sensitivity Study -- Wave Optics Algorithms for the Processing of Radio Occultation Data -- Asymptotic Wave Optics Methods in Inversion and Direct Modeling of Radio Occultations: Recent Achievements -- Processing Radio Occultation Data by Full Spectrum Inversion Techniques: An Overview and Recent Developments -- Evaluation of the Processing of Radio Occultation Signals by Reconstruction of the Real Signals -- Radio Holographic Filtering of Noisy Radio Occultations -- Future GNSS Occultation Missions and the LEO-LEO Occultation Concept -- Preparing for COSMIC: Inversion and Analysis of Ionospheric Data Products -- The Operational EPS GRAS Measurement System -- ROSA: The Italian Space Agency GPS Radio Occultation Receiver. Signal Tracking Characteristics and Terrestrial Measurement Campaign -- Tropospheric Water Vapor from LEO-LEO Occultation: Estimation by Differential Attenuation Measurements near 20 GHz -- Processing X/K Band Radio Occultation Data in Presence of Turbulence: An Overview -- Use of GNSS Occultation Data in Numerical Weather Prediction and in Atmospheric Studies -- Assimilation of GNSS Radio Occultation Data into Numerical Weather Prediction -- Observation Operators for the Assimilation of Occultation Data Into Atmospheric Models: A Review -- Analysis of Atmospheric and Ionospheric Wave Structures Using the CHAMP and GPS/MET Radio Occultation Database -- Are we Observing Mountain Waves Above the Andes Range from GPS Occultation Profiles? -- Analysis of Seasonal and Daily Mid-Latitude Tropopause Pressure Using GPS Radio Occultation Data and NCEP-NCAR Reanalyses -- CHAMP Radio Occultation Detection of the Planetary Boundary Layer Top -- Use of GNSS Occultation Data for Climate Monitoring and Climate Change Studies -- Monitoring Climate Variability and Change by Means of GNSS Data -- Climate Benchmarking Using GNSS Occultation -- Global Climatologies Based on Radio Occultation Data: The CHAMPCLIM Project -- Pre-Operational Retrieval of Radio Occultation Based Climatologies -- Assimilation of GNSS Radio Occultation Profiles into GCM Fields for Global Climate Analysis. Since the early use of the occultation measurement principle for sounding pla- tary atmospheres and ionospheres, its exploitation in atmospheric remote sensing has seen tremendous advances. In this book we focus on sensors on Low Earth Orbit (LEO) satellites, which exploit solar, lunar, stellar, GNSS (Global Navi- tion Satellite Systems), and LEO-crosslink signals for observing the Earth's - mosphere and climate. The methods all share the key properties of self-calibration, high accuracy and vertical resolution, global coverage, and (if using radio signals) all-weather ca- bility. The atmospheric parameters obtained extend from the fundamental va- ables temperature, density, pressure and water vapor via trace gases, aerosols and cloud liquid water to ionospheric electron density. Occultation data are therefore of high value in a wide range of fields including climate monitoring and research, atmospheric physics and chemistry, operational meteorology, and ionospheric physics. nd The 2 International Workshop on Occultations for Probing Atmosphere and Climate - OPAC-2 - was held September 13-17, 2004, in Graz, Austria. OPAC-2 aimed at providing a casual forum and stimulating atmosphere fertilizing scientific discourse, co-operation initiatives, and mutual learning and support amongst members of all different occultation communities. The workshop was attended by 40 participants from 12 different countries who actively contributed to a scientific programme of high quality and to an excellent workshop atmosphere, which was judged by the participants to have fully met the aims expressed.