QDOAS Projects Properties : Instrumental page
Except the observation Site, the Calibration File and the Transmission file, the fields that appear in this page depend strictly on the format of spectra files to process. Calibration file and instrumental corrections A preliminary wavelength calibration is necessary to browse spectra. If the wavelength calibration is coded in the spectra file, it is recommended to use it by keeping the field Calibration File empty. For the analysis of spectra, the wavelength calibration of the reference spectrum always has the priority over the spectra ones. It is important to select this file very carefully mainly when browsing spectra to save a new reference spectrum. Some instrumental corrections can be applied on spectra. For example, a transmission function previously determined in laboratory with calibrated sources can be specified in the Transmission file field. A two columns ASCII file is expected (wavelength calibration and transmission function) and spectra will be divided by this curve before the wavelength calibration procedure. Dark current correction is also possible according to the file format. The selected file format determines how dark currents are provided and how spectra are corrected. For the MFC STD file format, spectra can be corrected by dark current and offset. For the MFC BIRA-IASB binary format, spectra are corrected after averaging the dark currents and the offsets present in the file. The dark current should be a spectrum measured with a large integration time (typically 30 sec) and the offset should be the average of a large number of spectra measured with a very small integration time (typically 1000 x 3 ms). Note that QDOAS corrects dark current by offset. See details in the Description of Algorithms section of the QDOAS Software User Manual. Spectra measured by the following satellite instruments can be processed by QDOAS :
The wavelength calibration of earthshine spectra and irradiances is provided in the files and no transmission file is needed. Both fields should then be kept empty. The only information to provide should be the spectral region to process : the band type (for GOME and GOME2) or the channel and the clusters (for SCIAMACHY). The most popular file formats supported by QDOAS for ground-based measurements are :
Other formats are specific to the different Institutes that developed them. If a format is not supported by QDOAS, it should be possible to convert the files in ASCII. When the ASCII format is selected, spectra can be provided in the file one record per line (line format), one spectral value per line (column format) or several spectra in columns (from version 2.107). According to the checked flags in the Read from file group box, the following information are expected strictly in the given order :
In the column format, angles had to be given on the same line until version 2.106. From version 2.107, the column format accepts also matrices of spectra and angles have to be given in separate lines. With column format, a wavelength calibration can be provided with spectra. It should be given in first column of each data set. Some examples of ASCII files supported by QDOAS are given in annex of the QDOAS Software User Manual. The information on the observation Site is used to (re‑)calculate solar zenith angle from geolocation coordinates given for this site in the Sites sheet. This can be useful for example if the solar zenith angles saved in the files are not reliable enough. The abbreviation of the observation site is also used to build automatic output file names (see the Output Page). The longitude is particularly useful to select reference spectra of the day using the local time instead of Universal Time in case fractional days and times given in UT are distributed in two days (for example, measurements in China). Requested for some spectra files format, this option allows subtracting from a spectrum the signal averaged on the specified range of wavelengths. This is useful for example, to retrieve SO2 in the UV region where the signal is very poor and where the straylight is problematic. Even if the straylight can be attenuated by subtracting from the spectra, the average of the signal measured in the UV region (below 300 nm if possible), it is recommended to characterize it in laboratory.
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