| Titre : |
Atmospheric water vapour sensing by means of differntial absorption spectrometry using solar and lunar radiation |
| Type de document : |
Thèse/HDR |
| Auteurs : |
Stefan Walter Münch, Auteur |
| Editeur : |
Zurich : Schweizerischen Geodatischen Kommission / Commission Géodésique Suisse |
| Année de publication : |
2014 |
| Collection : |
Geodätisch-Geophysikalische Arbeiten in der Schweiz, ISSN 0257-1722 num. 92  |
| Importance : |
210 p. |
| Format : |
21 x 30 cm |
| ISBN/ISSN/EAN : |
978-3-908440-35-2 |
| Note générale : |
bibliographie, thèse publiée
Diss., Eidgenössische Technische Hochschule ETH Zürich, Nr. 21491, 2013 |
| Langues : |
Anglais (eng) |
| Descripteur : |
[Vedettes matières IGN] Atmosphère
[Termes descripteurs IGN] correction troposphérique
[Termes descripteurs IGN] détecteur à transfert de charge
[Termes descripteurs IGN] pouvoir de résolution spectrale
[Termes descripteurs IGN] rayonnement solaire
[Termes descripteurs IGN] spectromètre
[Termes descripteurs IGN] spectrométrie
[Termes descripteurs IGN] teneur en vapeur d'eau
[Termes descripteurs IGN] troposphère
|
| Index. décimale : |
47.50 Atmosphère |
| Résumé : |
(auteur) Tropospheric water vapour plays a crucial role in the understanding of a variety of different atmospheric processes, ranging from local weather phenomena to global climate change. Regarding satellite geodesy, water vapour acts as disturbing factor for various measurement methods, causing path delays of radio signals and consequently leads to considerable biases in the measurement results. The spatial and temporal concentration distribution can hardly be modelled and therefore has to be determined instrumentally to correct the influence computationally. The application of the principle of “Differential Optical Absorption Spectroscopy” (DOAS) using the sun as radiation source to locally determine integrated water vapour concentrations (PW), has proven itself as a very potent methodology, with good relative and absolute accuracy, high temporal resolution and comparably low calibration efforts. It also seems especially well suited for validation purposes for independent measurement methods.
The intention of the presented project is to implement the findings from the development of earlier prototype instruments and to contemplate several further aspects related with the DOAS approach to determine atmospheric water vapour: Improved temporal coverage of the measurement method through inclusion of night time measurements with the help of moonlight, which are enabled by means of a massively increased system sensitivity to deal with the up to six orders of magnitude lower intensity of the background radiation with respect to solar measurements. Further it is to investigate, how reliable acquisitions of sun transmission spectra can be achieved, when deploying the system on moving platforms, for possible future validation measurements of satellite-borne radiometer data on satellite ground tracks on the open sea. The prototype instruments developed should particularly feature high field versatility, requiring eased transportability, resistance to weather and not least the possibility of a fully automatic measurement procedure, including instrument self-calibration.
For this purpose two identically constructed compact measurement systems were built. The instruments dispose of a custom-built telescope (heliostat principle) and an optically directly coupled spectrometer unit. The whole system is enclosed in a rugged aluminium hull, including most of the steering electronics. The motorized telescope is able to follow the moving light source fully autonomously, also with the platform moderately moving. A quasi-monochromator with an echelle grating as main dispersion element is used as spectrometer unit, allowing a compact architecture, a great spectral resolution and efficiency at the same time. Together with a back-thinned CCD detector highly resolved images of water vapour absorption lines can be obtained. The primary wavelength range lies between 789 nm and 802 nm, the reciprocal linear dispersion amounts to 7.3 pm/px at a focal length of just 400 mm. A motorized deflection mirror in the spectrometer allows the observation of adjacent spectral windows and serves for the highly precise position stabilization of the spectrum on the CCD sensor.
Test measurements with the sun and the moon as background radiation source show the extraordinarily high system light throughput and the high spectral resolving power of the apparatus. However illumination dependent interference structures on the detector (etaloning) prevent the deduction of usable transmission spectra from the measured raw data. For that reason a variety of optical measures to homogenize the radiation entering the spectrometer with respect to field and aperture are examined. As a feasible solution, with sufficient radiation distribution and a still acceptable intensity attenuation, a short quartz light guiding fibre with a hexagonal cross section has been found and implemented.
For instrumental control a software package has been developed, which autonomously handles the measurement process including the various calibration processes and the interaction of the various sensors and actuators. Additionally a variety of algorithms have been provided, helping to eliminate various defective influences in the raw data, as the correction of stray and false light portions or the elimination of interspersed beat structures in lunar spectra. Furthermore procedures which serve in spectrum processing have been supplied, as for the computation of a holistic intensity baseline or the dynamic determination of apparatus profiles. For the determination of the water vapour concentrations from the measured spectroscopic data established computational procedures could be used mostly.
Various available spectroscopic databases have been analysed regarding the suitability of the listed absorption line parameters for the deduction of reliable water vapour concentrations. Comparison between the two identically built solar spectrometers yield considerable deviations of up to 1.5 kg/m 2 in the zenith integral concentrations which are presumably ascribable to systematic influences likely caused by unrecognized stray light influence. The mean value of the stochastic deviations amounts to about 1.1 % of the slant PW concentration. The cross-comparison with an independent measurement method, in this case GPS meteorology, however also shows significant divergences and thus point to the various further systematic effects which have to be examined more closely, as e.g. the uncertainty of the published spectroscopic parameters regarding line strength and line broadening as well as the baseline determination in the measured spectra.
Despite the considerable sensitivity losses with respect to the originally planned design due to beam homogenization, the methodology has been successfully applied to lunar measurements, albeit with reduced accuracy as stated in stochastic appraisals and with considerably lower temporal resolution.
The project presented here confirms and clarifies the possibilities of the application of DOAS for ground- based remote sensing of integral water vapour concentrations, but also gives clear indication on the different systematic biases which have to be examined more closely, e.g. regarding the accuracy of the spectroscopic parameters (both, for line strength and line broadening), as well as the determination procedure of the baseline in the measured spectra. This work also points out the chances as well as the serious difficulties which arise from the design, construction and deployment of highly integrated DOAS spectrometers of high performance. Regarding the application of various optical components and detectors for the development of similar instruments valuable insights have been gathered. |
| Numéro de notice : |
14913 |
| Thématique : |
POSITIONNEMENT |
| Nature : |
Thèse étrangère |
| Note de thèse : |
PhD : Sciences : ETH Zurich : 2013 |
| DOI : |
10.3929/ethz-a-010006561 |
| En ligne : |
http://e-collection.library.ethz.ch/view/eth:7591 |
| Format de la ressource électronique : |
URL |
| Permalink : |
https://documentation.ensg.eu/index.php?lvl=notice_display&id=76813 |
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