The test 549 data consists of Bruker SRF measurements with around 5 torr of room temperature CO2 in the transfer optics. This results in a quasi-transmittance spectra of CO2 being superimposed upon the SRFs. The CO2 features thus provide an absolute frequency standard which we can use to determine the true measured frequency points.
Only 10 of the channels in module 12 showed distinct CO2 lines in the measured SRFs; these were the only channels that we fit. The data was modeled for each channel by multiplying a calculated AIRS SRF (based upon our best fit of the test 386 average SRF) by a monochromatic CO2 spectra, and then convolving with the Bruker self apodization response for a 3.5023 cm OPD with a 1.94 degree field of view. The modeled self apodization handles both the broadening of the SRF and the frequency shift to smaller wavenumbers. The fit determined the best channel center frequency as well as the residual frequency shift needed to match the frequency calculated directly from the Bruker data (using a laser frequency of 15798.3064 cm-1). We also let the SRF width and a CO2 transmittance depth scale factor float.
Note: the Bruker interferogram data was zero-filled from 3.5 to 9.3 cm. This results in a point spacing of 0.053 cm, which is around 2.6 times more fine than the true resolution.
The figure below is an example of the fit of one of the 10 channels. The CO2 line is clearly visible as a notch in the right side of the SRF.
The residual frequency shift needed to match the the frequency calculated using the Bruker laser frequency is shown below. The Bruker frequency is around 0.017 +- 0.009 cm-1 too big. We are unsure how to explain this discrepancy. It is posssible our fits are not as good as we think; 0.017 cm is only around 12% of the true measured point spacing. Another possiblility is that the field of view angle was smaller than 1.94 degrees. However, the angle would have to be 1.58 degrees to match our fit, and that is perhaps well outside the estimated uncertainty in the angle. One other possiblity is a small error in the Bruker laser frequency. A laser freqency of 15797.929 cm-1 would match our fit.
