A method for calibration of the measuring instrument of solar direct, diffuse and aureole for estimation of refractive index and size distribution of aerosol is proposed. The method is based on Improved Langley Method : ILM that allows a calibration of the instrument without stable atmospheric conditions. ILM estimates size distribution, volume spectrum of aerosol by using measured solar direct (optical depth), diffuse and aureole (volume spectrum) together with extraterrestrial solar radiant flux for the reference wavelength. Then calibrate the instrument with optical depth, air mass and extraterrestrial solar radiant flux through extrapolate by plotting logarithmic function of extraterrestrial solar radiant flux subtracted by air mass multiplied by optical depth to air mass multiplied by optical depth is equal to 0 (at the top of the atmosphere) . By adjusting air mass, reconstructed volume spectrum and phase function using extraterrestrial solar radiant flux, solar direct, diffuse and aureole, the volume spectrum is reanalyzed thus accurate volume spectrum, phase function optical depth and solar diffuse and aureole can be re-estimated.
The method proposed here allows estimation of extraterrestrial solar radiant flux at any wavelength by using recalibrated solar direct, diffuse and aureole, optical depth, air mass by ILM. It can be done by extrapolate by plotting logarithmic function of extraterrestrial solar radiant flux subtracted by air mass multiplied by optical depth to air mass multiplied by optical depth is equal to 0 at the reference wavelength then extrapolate by plotting that at the different wavelength based on Rate Langley Method : RLM.
The numerical simulation based on measured data of solar direct, diffuse and aureole with ±3% and ±5% additive measurement noise shows that the proposed method is superior to the existing ILM method. Also the actual experimental data of the different 15 days of fine weather condition shows approximately 47.7% of improvement of the calibration accuracy can be achieved by the proposed method in comparison to the existing ILM method in terms of Root Mean Square error of the linear regression analysis of the relation between days and estimated extraterrestrial solar radiant flux.
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