Tangent height registration for the solar occultation satellite sensor ILAS: A new technique for Version 5.20 products

Abstract

The Improved Limb Atmospheric Spectrometer (ILAS) was a solar occultation satellite sensor that was developed by the Environment Agency of Japan to monitor the stratospheric ozone layer. This paper describes methods of registering tangent heights for ILAS vertical profiles of gas mixing ratio and aerosol extinction coefficient. Accurate tangent height registration is crucial for the retrieval of accurate gas mixing ratios from atmospheric absorption spectra. Three methods for tangent height registration have been applied to retrieved ILAS data. The first method is the transmittance spectrum method (TS‐M), which uses absorption spectra of oxygen molecules at around 760 nm (O2A band) measured by the ILAS visible channel and compares the average transmittance with that calculated theoretically from temperature and pressure using meteorological data. A tangent height is derived from these data. Version 3.10 ILAS data products use the TS‐M. A second method is the Sun‐edge sensor method (SES‐M). This method for registering tangent heights was in mind when ILAS was originally designed. The SES‐M geometrically determines the direction of the instantaneous field‐of‐view (IFOV) of the spectrometer from the angular difference between the top edge of the Sun determined with the SES and the spectrometer's IFOV. Information on the satellite's orbital position and solar position relative to the center of the Earth is used to register the tangent height. Version 4.20 ILAS data products use SES‐M. The third method is a hybrid method (Hybrid‐M) that was developed to correct for seasonal differences in tangent heights computed by the SES‐M. The Hybrid‐M assumes that the TS‐M can correctly determine the tangent height at 30 km. Version 5.20 ILAS data products (the latest version) use Hybrid‐M. Random and systematic errors in the Hybrid‐M tangent height registration were estimated. The root‐sum‐square (RSS) total random error is 30 m, while the total systematic error is +300 ± 360 m in tangent height. Actual errors in tangent height registration in Version 5.20 algorithm are considered to be small judging from the results of comparisons with independent validation data sets. The Hybrid‐M gives good estimates of tangent heights in Version 5.20 of the ILAS data processing algorithms.