MIPAS Level 1B Version 8 Radiometric Accuracy Assessment

The MIPAS mission is in its post-operational phase F. A final algorithm improvement will be done along with a full reprocessing of the entire data set covering a period of ten years from 2002 to 2012. The goal is to improve as much as possible the MIPAS data quality. The new data set will be referred as version 8. The MIPAS instrument has measured Infra-Red (IR) emission spectra at the Earth's limb in the middle and upper atmosphere along the orbit during day and night. The spectral range covered is 685 to 2410 cm-1 with a spectral sampling of 0.025 cm-1 during Full Resolution (FR) period and 0.0625 cm-1 during Optimised Resolution (OR) period. The MIPAS level 1B processor transforms instrument raw data into spectra calibrated spectrally and radiometrically with geo-location determined over the earth geoid of the line of sight tangent point. This paper presents an assessment of the radiometric accuracy budget. The main contributors to the error are the detector non-linearity, the gain calibration noise and variation along time. The detector non-linearity was re-characterized in-flight and related error estimated. A validation of the new characterization was done looking at overlaps with linear channels. This has led to an improvement of level 2 trending analyses and validation. The error due to the gain calibration noise was controlled and limited through the calibration scenario. The ice accumulation in the output optics and instrument temperature variation lead to a gain calibration variation along time. The effect of ice was monitored and controlled through periodic decontamination. Compared to the requirements and the previous version, the MIPAS level 1B version 8 will have an improved radiometric accuracy.