MIPAS Level 1B Overview and Improvements

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument is a Fourier Transform Spectrometer (FTS) on-board the Envisat polar orbiting satellite launched by the European Space Agency (ESA) in early 2002. It measures high-resolution emission Infra-Red (IR) spectra at the Earth's limb in the middle and upper atmosphere along the orbit. The spectral range covered is 685 to 2410 cm-1. The MIPAS level 1B algorithm goal is to transform raw data into spectra calibrated spectrally, radiometrically and to determine the limb geo-location over the earth ellipsoid. Its development started in the mid-nineties and the main concern at that time was the computer power needed to process in Near Real Time (NRT) the huge amount of data coming from the instrument. Special care to minimise computer operations was taken within the limits dictated by the instrument mission accuracy requirement. At launch, the processor was ready and exceptionally few minor corrections were made during the commissioning phase. Since then, the level 1B algorithm went through a series of evolutions. In 2004, the instrument encountered slide mechanism difficulties. Scientists and engineers developed a new measurement scenario allowing the recovery of the instrument operations. The new scenario still fulfils mission goals. It has a reduced spectral sampling from 0.025 to 0.065 cm-1 but has a finer spatial sampling of the atmosphere. In 2010, the Envisat orbit was modified allowing saving of fuel in order to extent the mission. This presentation gives an overview of the level 1B algorithm for spectral calibration, radiometric calibration and pointing determination. Instrumental aspects such as detector non-linearity and spikes are also covered. Finally, modifications to cope with operational changes and improvements since the beginning of the mission are presented.