Improvement in calibration algorithm of the AOS (acousto-optical spectrometer) using in-orbit measurement data

The instrument function and calibration are absolutely necessary to retrieve various physical quantities from the raw data. One of the most important for scientific results in atmospheric research is precise calibration of the frequency. For retrieving stratospheric winds with Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES), the current frequency accuracy, which is in the order of 100 kHz, should be improved to be less than 10 kHz. In order to find the suitable method for fulfilling this requirement, four different methods are applied to determine the center position of each comb picket. The procedure of the frequency calibration consists of two main steps. The first step is the determination of the center positions and the second step is the establishment of the relation between RF input frequency and channel number. It is found that the accuracy of the frequency calibration is primarily affected by that of the center position determination. Finally, it was realized that the precise-background method is the most suitable for our purpose.