Research On Spaceborne SI Traceable Solar Spectrometer

The solar irradiance is the main source of energy for the earth, and is closely related to the human survival and development. Continuous and high accuracy measurement of solar irradiance has important significance for the solar physics research and studies on the earth's climate changes. Thus, for the purpose of high-precision measurement of solar spectral irradiance in orbit, research on spaceborne SI traceable solar spectrometer is suggested. The research contents include1. Design and analysis of the solar spectrometer.In order to improve the absolute accuracy of the measurement of spectral solar irradiance, the spaceborne SI tracable solar spectrometer is designed. According to the distribution and variation of solar spectral irradiance at top of the atmosphere, requirement analysis for the spectral coverage and spectral resolution of the solar spectrometer is performed and the system's total design scheme is determined. The whole spectral coverage of the solar spectrometer is 300 nm-2500 nm, and is divided into three channels. The three channels have a spectral coverage of 300 nm-1000 nm?900 nm-1700 nm and 1600 nm-2500 nm respectively and with spectral resolution of 2nm?4 nm and 10 nm. For each channel, the optical designs and tolerance analysis are made with plane grating and convex grating as dispersive elements respectively. The result shows that the spectrometer with plane grating has a better tolerance and has advantages in fabrication and alignment process while the spectrometer with convex grating has a better image quality. Then simulations of spectral response function and spectral resolution of the designed spectrometer is performed with three methods. The three methods are calculation with grating equation, geometrical ray tracing and convolution with Line Spread Function respectively. The results show that the spectral resolutions meet the requirement for the whole spectral range.2. Developing of the prototypeFor the convenience of fabrication and alignment, the design based on plane grating is chosen and the prototype of the solar spectrometer based on plane grating for the 300 nm-1000 nm channel is made. In the process of developing the prototype, the selections of the detectors for the three channels are made by considering the requirement of signal-to-noise ratio and spectral resolution together with the distribution and variation of solar spectral irradiance at top of the atmosphere. Then signal-to-noise ratio of the solar spectrometer when working in orbit is analyzed and estimated according to the performance of the chosen detectors. A sub-pixel shift device is added in the mechanical structure design process. A data acquisition and control system based on the linear array detector is designed. The PC program is designed for data communication and processing with Lab VIEW. Then the noise level, the non-uniformity of the responsivity between all detector pixels and the nonlinear response of the detector pixels are tested by experiment. The result of fabrication and alignment show that the spectral resolution is less than 2nm which meets the design requirement.3. Calibration of the prototype and spectral superresolutionThe wavelength calibration experiments are carried out with the characteristic lines of a mercury lamp and wavelength scanning by high-resolution grating monochromator. The calibration uncertainties are estimated to be better than 0.11 nm and 0.09 nm respectively with the two methods. The spectral response function of the prototype is measured by wavelength scanning method and the spectral resolution versus wavelength is obtained. A 1000 W standard irradiance lamp is used to carry out the laboratory irradiance calibration, with a calibration uncertainty evaluated to be better than 2.84%. The irradiance calibration for the SI traceable solar spectrometer in orbit is analyzed and the uncertainty for the whole spectral range is better than 1%.. At last, the theory and devices of spectral superresolution of the solar spectrometer is analyzed and a simulation of the superresolution process based on geometric ray trace is performed. Analysis of the uncertainty is also given when the spectral superresolution is performed.