AIRS L1C ProcessingTue, Aug 1, 2017
- Clean and Fill
- Frequency Shift Correction
2.1 Clean and Fill: */home/chepplew/projects/airs/L1C/caf-6.1.0/ *
2.2 Frequency Shift Correction & Doppler: */home/chepplew/projects/airs/freqCal/ *
L. Strow has previously computed a correction matrix for the AIRS channels (actually modules) based on minimizing the bias between observed and RTA computed high-contrast water lines. The reference date for this correction is 2010-Jan-22. From this matrix a set of coefficients for a fitting model has been pre-computed (by H. Motteler) to correct the brightness temperatures for the AIRS channels so that the shift-corrected AIRS observations are referenced to the nominal channel centers for 2010-Jan-22.
4 Method: Frequency Correction
The function script airs_freq_shift_l1c.m is used to apply the frequency shift correction to a day of L1C granules. In this function the correction is applied to the brightness temperatures so these have to be converted back to radiances before saving the data. The principle independent variables required are the date and orbit phase, in 2-degree intervals from equator south-bound. Interpolation is used to improve the fitting to actual orbit position. function  = airs_freq_shift_l1c(sdate) % Method: % 1. Get the nominal L1b and L1c channel centers  &  % 2. Sort channel sets, find common and synthetic channels in L1C set. % 3. Load Observation data on L1c grid  and compute orbit phase % 4. Calculate frequency drift correction of channel centers  % 5. Resample radiances onto nominal grid and translate to L1C grid. % 6. Save data to new L1C MAT files.
5 Method: Doppler Correction
The function script airs_doppler_shift.m is used to compute the Doppler frequency shift to a day of L1C granules. The Doppler shift is a single value in parts per million which would be applied to all channels in the final application. Thus, one value per FOV (90 per cross-track scan) for all scans for the day. function [shift_ppm, asc, l1c_list] = airs_doppler_shift_l1c(sdate) % 1. the doppler shift calculation requires three adjacent satellite geolocation % values for fitting, therefore the first and last values for the day are sacrificed. % 2. Calls: calc_doppler_clh.m % 3. Dependencies: read_airs_l1c.m %
6 Results (Sample)
A day with a full complement of AIRS L1c granules is chosen to demonstrate the effect of the frequency shift calculations.
Firstly - the orbit phase is plotted in Figure 1.
Next the Doppler shift is evaulated for two fields of view at either extreme of the scans, shown separated by ascending and desconding parts of the orbit. In Figure 2 all data for the day are plotted.
The frequency drift correction is shown in Figure 3 as a difference in frequency for channel 600 (on the L1c grid) for the first four orbits of the day. This for illustration purposes of the intermediate computation that gives the shift.
The final computation of the frequency drift correction is done in brightness temperature space with the standard channel centre frequencies specified on 2010-Jan-20. The effect on the CO2 Q0brnach channel at 725.725 cm1 is shown in figure 4 separated into ascending and descending parts of the orbit for the all observations for the day. The color scale shows the actual scene brightness temperature from about 180 K at the blue, to 260 K at the yellow.
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