Calculation of HIRS/2 Channel Transmittances by the Exponential Sum Fitting of Transmission Model

The absorption coefficient, transmittance and weighting function of the channels of HIRS/2 depend on the temperature profile which varies with the location and/or time in turn. An improved ESFT (the Exponential Sum Fitting of Transmission) model suitable for calculating the HIRS/2 channel transmittance has been obtained by adding the instrument weighting function to the ESFT model proposed by Shi Guang-Yu. With this new ESFT model, the variations of transmittance and weighting function with the temperature can be calculated efficiently. Comparing with the exact line-by-line calculations, the results obtained show that the maximum absolute error in transmittance for CO2 channels smaller than 0.0036, and that for the H2O channel is smaller than 0.0035.