Studying Envisat Attitude with SCIAMACHY Limb-scatter Measurements

Abstract

High pointing accuracy is a prime prerequisite of high quality remote sensing of atmospheric consitutents using limb viewing instruments. SCIAMACHY limb measurements were affected by limb tangent height errors of up to 3 km during the initial phase of the Envisat mission. These errors were caused by inaccurate satellite attitude information and possibly an instrument misalignment. Retrieval of pointing information from the limb spectra of scattered sunlight themselves is of crucial importance for all limb sensors. Here we present limb pointing retrieval results from SCIAMACHY based on two different algorithms: a Rayleigh-knee technique employing limb-radiance profiles at around 350 nm and an O3-knee technique based on limb radiance profiles between 295 and 305 nm. Both techniques use the shape of the limb-radiance profiles at the corresponding wavelengths as a signature for pointing retrievals. The retrievals allow to investigate seasonal, latitudinal and orbit-to-orbit variations in pointing errors. Moreover, indications for horizontally tilted azimuthal scanning of SCIAMACHY are found in azimuthally resolved pointing retrievals as well as comparisons of retrieved stratospheric O3 profiles with ground-based LIDAR measurements. The cause of this effect is likely a SCIAMACHY roll misalignment with respect to the satellite platform or an Envisat roll error.