OMI in-flight wavelength calibration and the solar reference spectrum

The Ozone Monitoring Instrument (OMI) was launched in July 2004 and is one of four instruments onboard NASA's EOS-Aura satellite. OMI is a nadir-viewing UV-VIS spectrometer ranging from 270 to 500 nm, with a spectral resolution of roughly 0.5 nm. OMI obtains daily global coverage at the equator with a nominal sampling at nadir of 13×24 km². This paper discusses the in-flight wavelength calibration and the solar reference spectrum. Wavelength calibration is performed by means of fitting Fraunhofer structure in the radiance and irradiance spectra. It was found that when observing rapidly changing radiance signals, the wavelength scale changed in tune with this. We describe the details of this effect, explain the underlying optical mechanism and show that we can (and do) correct for it with a high degree of accuracy. This effect will be observable in any spectrometer with similar optics as that of OMI. A prerequisite for any in-flight wavelength calibration method that uses Fraunhofer lines in the observed spectra is a good quality high resolution solar reference spectrum. We describe how we calculate such a spectrum, based on combining high resolution ground based data, and medium resolution satellite measurements.