# SARTA for AIRS L1C

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## 1 Purpose, Context and Scope

To build a stand alone fast radiative transfer algorithm (SARTA) for the AIRS level 1C radiance data (referred to as AIRICRAD).

The AIRS L1C radiance data consist of 2645 sensing channels, most of which are from the L1B channel set, with some additional simulated channels to fill spectral gaps and some overlap channels ommitted. The AIRS L1C channels set provides the means to compare with the CrIS radiance data, in the bands that are common to both sensors.

The current approach to computation of TOA radiances for the RTP production for AIRICRAD data is to use an old version of SARTA that was built with the 2834 channel set devised before L1C was established. Channels matching the L1B are selected and fake channels identified.

Currently there is a growing need for an updated version of SARTA that is designed to work directly with the AIRICRAD and that is the purpose of this work and summary document.

## 2 Sources

### 2.1 Spectral channel specification

The spectral response functions and widths are available from the reference data base for AIRS determined before launch. The database consists of four HDF files for four different instrument conditions designated m130, m135, m140 and m150. These files are found in: /asl/data/airs/srf/ with files such as: srftables_m130f_withfake_mar08.hdf

There are at least three definitions of channel centers. i) This in the original reference HDF files discussed previously, ii) The actual AIRICRAD granules, iii) A look-up file available at: /home/chepplew/projects/airs/airs_f.mat or /home/strow/Work/Airs/L1c/l1c_chans.mat iv) A frequency drift corrected set which is nominally referenced to 20-January-2010. Each of these is different. Logically, the centers to match the current AIRICRAD observation set would be used for the SARTA.

The designation of the simulated L1C channels, that fill spectral gaps of the L1b set, are defined in a JPL cross-reference matrix available at: /home/chepplew/projects/airs/L1C/caf-6.1.0/src/map2378to2645.c So, all simulated channels take the identity of an L1b channel, so the SRF and width are taken from the original reference data base files.

A simple channel matching code has been written to output the channel specifications for the L1C set and is available at: /home/chepplew/projects/airs/L1C/get_l1c_grid.m There are a couple of checks available therein. A preliminary L1C set, using AIRICRAD to define channel centers, is available at: /home/chepplew/projects/airs/L1C/airs_l1c_srf_tables.hdf

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