Research on the irradiance calibration of a VUV dual-grating spectrometer based on synchrotron radiation

A high accuracy instrument calibration is the guarantee of obtaining high precision atmospheric inversion data from a wavelength-dispersive space remote sensing instrument in the orbit. To improve the relative calibration uncertainty of a dual-grating spectrometer in the vacuum-ultraviolet (VUV) band, conventional secondary standard light sources, such as deuterium lamps, used in laboratory calibrating stations are not suited to meet high precision. In this paper we present the calibration of the space instrument based on the Metrology Light Source (MLS) of the Physikalisch-Technische Bundesanstalt (PTB) operated as a primary synchrotron radiation (SR) source with calculable spectral radiant intensity to overcome the limit imposed by secondary transfer standards. For taking into account the difference between the highly linearly polarized SR source and the un-polarized solar radiation, which will be observed by the dual-grating spectrometer in orbit, the calibration was performed at two orientations of the instrument, which are perpendicular to each other with respect to the incident SR. The factors contributing to the uncertainty budget were analyzed. It could be shown that a total relative uncertainty of 2.52% can be achieved, which has greatly improved the calibrating accuracy compared with that obtainable with the conventional standard light source calibrating stations in the VUV spectral range.