MIPAS Level 1B algorithms overview: operational processing and characterization

This paper gives an overview of the MIPAS Level 1B (L1B) processor whose main objective is to cali- brate atmospheric measurements radiometrically, spectrally and geo-located. It presents also the results of instrument characterization done on ground and during the first years in- flight. An accurate calibration is mandatory for high quality atmospheric retrievals. MIPAS has shown very good perfor- mance and stability. The noise equivalent spectral radiance ranges from 3 to 50 nW/(cm 2 sr cm 1 ) and is well within the requirements over nearly the whole spetral range. The sys- tematic radiometric error is estimated to be within 1 or 2% in most situations. The MIPAS interferometer provides two-sided interfero- grams with a maximum optical path difference of 20 cm. It has a dual port configuration with two input ports and two output ports. Only one input port is needed to acquire data from a given scene. The second input port is designed to look at a cold target in order to minimize its contribution to the signal. The cold target is a cold plate of high emissivity cooled at 70 K. Each output port is equipped with four detec- tors covering different spectral bands. The signals detected at both output ports are similar and they are combined in some frequency bands to improve signal-to-noise ratio. The eight detectors are split into five bands, each band being covered by one or two specific detectors. Due to the limited data rate, the measured interferograms are filtered and decimated. The filtering before decimation prevents noise from out-of-band spectral regions to be aliased into the spectrum. The spectral coverage of the eight detectors and the five bands, together with the decimation factor, is given in Table 1. A nominal measurement sequence consists in a series of 17 high resolution atmospheric scene measurements made at different tangent heights starting at 68 km tangent height and descending to 3 km. The observation geometry is displayed in Fig. 1. The elevation steps go from 8 km step at high alti- tude to 3 km step at low altitude. The sweep duration at high resolution is 4.45 s. Before being transmitted to the ground the detected signal is amplified and analog filtered, digitized, numerically fil- tered and decimated, equalized and combined, bit truncated and finally packetized. The Level 0 (L0) product is then processed by the L1B ground segment processor to gener- ate geo-located spectra that are radiometrically and spectrally calibrated.