Use of the occultation measurement principle for observing the Earth's atmosphere and climate has become so broad as to exploit solar, lunar, stellar, navigation and satellite-crosslink signals, to employ the whole electromagnetic spectrum from EUV/UV via VIS/IR to MW and radio, and to utilize all kinds of atmosphere-radiation interaction such as refraction, absorption, and scattering.

Occultation methods share the unique properties of self-calibration, high accuracy and vertical resolution, global coverage, and, if using radio signals, all-weather capability. They thus bear great utility for meteorology and climate science, and other fields.

The many different uses and variants of the occultation method have led to a diversification of the occultation-related scientific community into several sub-communities, however. OPAC-3, as OPAC-2 three years ago, and OPAC-1 five years ago, sets in exactly at this point and aims at providing a casual forum and stimulating atmosphere fertilizing scientific discourse, co-operation initiatives, and mutual learning & support amongst members of all the different sub-communities.

Since the early use of the occultation measurement principle for sounding planetary atmospheres and ionospheres, its exploitation in atmospheric remote sensing has seen tremendous advances and extensions. A particular boost was felt from the late eighties onwards when a variety of intriguing opportunities for applications to the atmosphere of our home planet Earth were increasingly recognized, such as the use of new signal sources like Global Navigation Satellite System (GNSS) signals.

Today we deal with and plan sensors on Low Earth Orbit (LEO) platforms, which exploit solar, lunar, stellar, GNSS, and LEO-crosslink signals. Also airborne and "mountain-top" platforms are explored. The sensors, together, smartly utilize the full range of the electromagnetic spectrum from EUV/UV via VIS/IR to MW and radio, as needed, and exploit different types of atmosphere-radiation interaction such as absorption and scattering, both from molecules and aerosols, as well as refraction. The geophysical parameters obtained this way - from the Earth's surface up through the complete atmosphere - extend from the fundamental atmospheric mass field variables temperature, pressure, and geopotential via the fundamental variable trace gases water vapor and ozone (and many further trace species relevant in atmospheric chemistry and radiation) to key particulate species such as tropospheric and stratospheric aerosols and cloud liquid water. Furthermore, ionospheric electron density is sensed.

All these measurements rest on one and the same occultation principle with its unique properties of providing self-calibration (via normalized intensities or time-standard reference), high accuracy and vertical resolution, global coverage, and in case of using radio signals all-weather capability. Occultation data thus bear enormous utility for applications in weather and climate system studies, operational meteorology and climatology, and other fields such as space weather and planetary science. The self-calibration property is particularly crucial for climate research and climate change monitoring applications, as it enables unique long-term stability in climatological datasets. The latter can be built from occultation data of different satellites and times without inter-calibration efforts. In fact, a controversy such as the recent one on the tropospheric temperature record over the last two decades, involving the heavily calibration-dependent Microwave Sounding Unit (MSU) data, could have been presumably saved had suitable occultation data been available.

The many different uses, promises, and variants of the occultation method have led to a diversification of the scientific community into more or less separate communities for solar and stellar occultation on the one hand and radio occultation on the other hand. And even within these broad community classes, several sub-communities are visible. In addition there exists a somewhat fragmented diversity of groups of occultation data users.

This situation is a pity as it leaves a myriad of opportunities for synergy, cross-fertilization, co-operation, and mutual support unexplored and thus unnecessarily impedes full prosperity and acceptance of the value of occultation-related science.

The OPAC-3 Workshop, as its predecessors OPAC-2 and OPAC-1, sets in exactly at this point and serves to:

The workshop's focus is on occultation sounding of Earth's (neutral) atmosphere and climate and related exploitation and application of the data. The areas of ionospheric and planetary occultations and applications are foreseen to be covered by invited papers.

Scientific sessions will cover all topical areas and associated topics noted. Contributions on results from very recent occultation sensors (e.g., FORMOSAT-3/COSMIC, MetOp/GRAS, CHAMP&GRACE&SAC-C/GPS, GOMOS&SCIAMACHY, SAGE-III, ACE) are particularly encouraged. Should the notifications of interest indicate a need to allow also for contributions on ionospheric and planetary occultations, a dedicated poster session will be allocated to these topics.