During the last years the Agency developed a sound scientific and industrial expertise in the field of limb sounding in Europe, which lead to flight instruments like GOMOS or GRAS. To further consolidate this position we expand our expertise to the SWIR spectral region.

Here we focus on the use of dedicated artificial signal sources in contrast to signals of opportunity (e.g. GPS) or natural signal sources (e.g. direct or back-scattered sunlight) to probe Earth's atmosphere.

The observational conditions in the SWIR spectral range are excellent due to an atmospheric window between the visible radiation from the Sun and the thermal infrared irradiation from the Earth. All major greenhouse gases comprise spectral features (basically vibrational and rotational-vibrational structures) in that band which can be exploited for quantitative observations. In contrast to passive methods, which use natural signals sources like backscattered light or sun, moon and stars irradiation directly, a dedicated signal source offers many advantages. The signal to probe the atmosphere can be tuned to best suit the requirements to observe specific atmospheric constituents concerning wavelength and necessary signal to noise ratio (SNR).

Scientific Requirements:

As a starting point the concept of ACCURATE was chosen as a baseline. ACCURATE as an innovative mission concept based on a limb sounding geometry using SWIR lasers as signal sources was recently proposed to ESA in response to a call for ideas for possible Core Earth Explorer Missions in March 2005 and again as Earth Explorer Opportunity mission in 2010. Accurate was presented to the review process and received, in a stringent scientific and technical peer assessment process, a positive evaluation in both submissions but was not selected, in the latter case (2010) for programmatic reasons.

Nevertheless, the evaluation recognized the potential of this novel observational approach, which resulted in ESAC recommendations for further studies and research.

The IRDAS study was implemented as a response to the ESAC recommendation. The main objective was to better assess the performance and impact of such a mission, the spectroscopic properties of selected lines and broadband atmospheric effects. All those before mentioned areas required a better characterization. Based on those findings line selection can be optimized and simulation tools updated and refined to generate reliable results. The results of this activity are vital to further promote active occultation observations in the SWIR spectral region.

Results:

The main objective of the Study was to establish a sound scientific baseline for active occultation in the SWIR spectral region by application of the differential absorption method in limb sounding geometry in the 2.0 - 2.5 µm spectral band.

The main results are summarized in the Final Report, which can be downloaded from the Results Section below.

If you are interested in more detailed information concerning the different aspects and results of this study the Science Objectives and Observational Requirements of the ACCURATE Mission Concept is available for download from the Results Section below.

Scientific Impact of an ACCURATE Mission and Synergies and Complementarities with other Missions and GHG Observations is available for download from the Results Section below.

Spectroscopic Properties of Greenhouse Gases From 2.0 to 2.5 µm is contained in WP6 Report-v9-final. This is available for download from the Results Section below.

Derivation of an Error Model for the 2 to 2.5 µm Region is contained in WP7-Error-Report-final. This is available for download from the Results Section below.

In the course of this study the software package ALPS (ACCURATE LIO Performance Simulator) was developed.

ALPS is a small, self-contained software package written in IDL (Interactive data language). It is developed under lead of the Wegener Center for Climate and Global Chang, University of Graz (WEGC), Graz, Austria. Current developers are WEGC and the Department of Chemistry of the University of York (UoY), York, UK.

The software enables simplified LIO (LEO-LEO Infrared-laser Occultation) end-to-end performance estimations under different atmospheric conditions. In particular, LIO trace species retrieval errors and wind retrieval errors can be estimated under consideration of various atmospheric losses/gains and observing system/instrumental errors. The estimations are based on limb transmission profiles, influenced by atmospheric absorption, that are computed using the Reference Forward Model (RFM) that is mainly developed by A. Dudhia at the University of Oxford. RFM is operated using the HITRAN molecular spectroscopic database and FASCODE standard atmospheres.

The software and the respective documentation are available upon request; inquirers should be made by e-mail: Gottfried.Kirchengast@uni-graz.at

Outlook:

It is recognized that theoretical studies and simulations will not be able to represent reality to 100% accuracy. Especially since simulations cannot account for effects which are potentially not known or expected.

Based on the insight gained during the IRDAS study a follow on project is being implemented to validate/test the findings. IRDAS-EXP is implementing a field experiment to gather a valuable data set, useful not only in the immediate context but for atmospheric sciences in general.