Jos H.G.M. van Geffen and Roeland F. van Oss
Applied Optics 42, 2739-2753 (2003).
The result of the calibration, that is the change of the wavelength of a point in a given window with respect to an initial guess, appears to vary along an orbit and from orbit to orbit. This obviously limits the number of spectra that can be averaged before the wavelength calibration to improve the signal-to-noise ratio of the spectrum to be calibrated. This orbital variation is discussed in a forthcoming paper, which also investigates the change in the calibration results of solar spectra over a period of six years.
Once daily GOME measures a solar spectrum, when it flies over the North Pole into the sunlight along one of its orbits. GOME then flies towards the Sun, hence there is a Doppler-shift in the wavelength of lambda v/c, where v is the satellite's velocity (about 8.3 km/s) and c the speed of light. This shift is between about 0.008 nm at lambda=300 nm and 0.022 nm at lambda=800 nm. The calibration method's fitting of a shift and a squeeze against the solar reference spectrum, automatically corrects for this Doppler-shift.
There are a few improvements possible on the calibration method. The accuracy of the reference spectrum below 300 nm, for example, could be improved upon. The reference spectrum is based on ground-based and balloon-based measurements of the solar spectrum, and it thus contains some atmospheric absorption features, over and above the distinct solar Fraunhofer lines. It is very desirable to have a pure high-resolution solar spectrum, e.g. measured from space, as reference for the calibration of measured solar spectra. A high-resolution earthshine spectrum as reference for measured earthshine spectra could also be useful. The use of earthshine reference spectrum, however, would introduce problems in determining how good the method works, because of the large variation in atmospheric absorption features in measured spectra. Alternatively, one could use a true solar reference spectrum and also use ozone cross sections in the fit when optimising for ozone retrieval, though the subsequent ozone retrieval itself may then no longer be independent of the wavelength calibration.
The accuracy of the method may further be improved by extending the calibration to also fit the resolution of the slit function used to convolve the high-resolution reference spectrum. This will make the method computationally slower, which is not acceptable within the constraint of near-real time delivery of the data of the Fast Delivery Service and was therefore not done. Furthermore, fitting the resolution is perhaps only useful if the slit function itself is known well, which is not the case for GOME.
The Fast Delivery Service is based on a set of nine wavelength windows plus the instrument health parameters, together known as the EGOI-data. The wavelength calibration method has been designed for these nine EGOI-windows. The method is, however, not restricted to these windows: it can be used with any set of wavelength windows within the available reference spectrum. The method is thus suitable for use with any high-resolution (ir)radiance spectrometer, such as the satellite instruments SCIAMACHY (aboard ENVISAT; launched in 2002), OMI (aboard EOS-Aura; to be launched in 2004), and GOME-2 (aboard METOP; to be launched in 2005).
The GOME level-1 spectra produced with the GOME Data Processor (GDP) have
wavelength calibrations based on the spectrum of an onboard calibration lamp
and a shift is applied to the spectra (no squeeze). The GDP-provided
wavelength grid of the level-1 spectra appears to be insufficiently accurate
for some level 1-to-2 retrievals, such as for ozone profiles. The accuracy
of the DOAS-retrieval of e.g. ozone, BrO and NO2 will also benefit
from a more accurate wavelength calibration. It is therefore useful to have
the possibility to improve the wavelength calibration of GDP level-1 spectra
within user specified wavelength windows. For the purpose of such a
re-calibration, the GomeCal software package has been written and is made
available on-line via
The GomeCal package also provides an improved polarisation correction as
well as an additional correction for the degradation of the GOME instrument
and a radiometric calibration.
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