Wavelength calibration of spectra measured by
the Global Ozone Monitoring Experiment:
variations along orbits and in time

J.H.G.M. van Geffen
Applied Optics  43, 695-706 (2004).

7. Concluding remarks

The present paper investigated the variation of the wavelength calibration of GOME earthshine spectra, in casu the wavelength change Delta lambda with respect to an initial guess in selected wavelength windows (listed in Table 2), along an orbit and between successive orbits on three days, and a possible correlation with some instrument temperatures. The temperature of some of the elements of the optical system namely show an increase along an orbit due to warming of the instrument. The temperature T of the predisperser prism, the dispersive characteristics of which are expected to be temperature depended, appears to be a good proxy for the warming of the optical system.

For the wavelength window around 306 nm there is a significant correlation between Delta lambda and T along a given orbit, though this correlation is less obvious when considering several orbits at a time. The Delta lambda of the other selected windows shows no significant correlation with the predisperser prism temperature and the orbital variation is different for the different windows. What the cause is of these orbital variations in the wavelength calibration is therefore unknown.

The calibration results for the window with the lowest wavelengths, around 274 nm, show distinct peaks when the satellite is above the South Atlantic Anomaly (SAA), a region of the inner Van Allen radiation belt where much more charged particles impact on the instrument detector than elsewhere along the orbit. This increase in charged particle impacts on the detector causes spikes in the radiance levels and these spikes in turn hamper the wavelength calibration if the signal-to-noise ratio is not to good, which is the case for the shorter wavelengths of windows below about 300 nm. It is therefore advisable to omit the SAA-region (roughly between latitudes -5° and -40°, and between longitudes -5° and -80°) from the wavelength calibration and adopt, for example, the average results of the calibration between, say, +5° and +30° latitude of the orbit for the SAA-region.

The wavelength calibration of solar spectra measured over a period of nearly six years shows fluctuations in the change in wavelength Delta lambda with time, but without an apparent pattern. The degradation of the GOME instrument, clearly visible in the irradiance levels, has no effect on the wavelength calibration of the solar spectra.

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created: 16 October 2003
last modified: 3 February 2008