Use of occultation methodology for observing the Earth's atmosphere and climate has become so broad as to comprise solar, lunar, stellar, navigation and satellite crosslink occultation methods. The atmospheric parameters obtained extend from the fundamental variables temperature, density, pressure, water vapor, and ozone via a multitude of trace gas species to particulate species such as aerosols and cloud liquid water. Ionospheric electron density is sensed as well. The methods all share the key properties of self-calibration, high accuracy and vertical resolution, global coverage, and (if using radio signals) all-weather capability. Occultation data are thus of high value in a wide range of fields including climate monitoring and research, atmospheric physics and chemistry, operational meteorology, and other fields such as space weather and planetary science. This wide area of variants and uses of the occultation method has led to a diversi fication of the occultation-related scientific community into a range of different sub-communities, however. The 1st International Workshop on Occultations for Probing Atmosphere and Cli mate-OPAC-1- held September 16-20, 2002, in Graz, Austria, has set in ex actly at this point. OPAC-1 aimed at providing a casual forum and stimulating at mosphere fertilizing scientific discourse, co-operation initiatives, and mutual learning and support amongst members of all the different sub-communities. The workshop was attended by about 80 participants from 17 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.

This book presents carefully edited and peer-reviewed papers from the 2nd International Workshop on Occultations for Probing Atmosphere and Climate (OPAC-2), held in Graz, Austria. It starts with a general introductory paper and proceeds to address the full range from methodology in general via specific occultation methods (GNSS-LEO, LEO-LEO, stellar and solar) to the use of occultation data, with focus on atmospheric physics, meteorology and climate.

Building on its heritage in planetary science, remote sensing of the Earth's at- sphere and ionosphere with occultation methods has undergone remarkable dev- opments since the rst GPS/Met 'proof of concept' mission in 1995. Signals of Global Navigation Satellite Systems (GNSS) satellites are exploited by radio occ- tation while natural signal sources are used in solar, lunar, and stellar occultations. A range of atmospheric variables is provided reaching from fundamental atmospheric parameters such as density, pressure, and temperature to water vapor, ozone, and othertracegasspecies. Theutilityforatmosphereandclimatearisesfromtheunique properties of self-calibration, high accuracy and vertical resolution, global coverage, and (if using radio signals) all-weather capability. Occultations have become a va- able data source for atmospheric physics and chemistry, operational meteorology, climate research as well as for space weather and planetary science. The 3rd International Workshop on Occultations for Probing Atmosphere and Climate (OPAC-3) was held September 17-21, 2007, in Graz, Austria. OPAC-3 aimed at providing a casual forum and stimulating atmosphere for scienti c disc- sion, co-operation initiatives, and mutual learning and support amongst members of alldifferentoccultationcommunities. Theworkshopwasattendedby40participants from 14 different countries who actively contributed to a scienti c programme of high quality and to an excellent workshop atmosphere. The programme included 6 invited keynote presentations and 16 invited pres- tations, complemented by about 20 contributed ones including 8 posters.