C. E. Blom, T. Gulde, C. Keim, W. Kimmig, C. Piesch, C. Sartorius, H. Fischer

1. Forschungszentrum KarlsruheTechnik und Umwelt Fig. 1: Artist’s view of the optic module. To reduce size of the instrument the optics is divided in two levels: the upper level contains the scan unit and the telescope, the lower level the interferometer. C. E. Blom, T. Gulde, C. Keim, W. Kimmig, C. Piesch, C. Sartorius, H. Fischer Institut für Meteorologie und Klimaforschung Forschungszentrum Karlsruhe GmbH / Universität Karlsruhe MIPAS-STR: a new instrument for stratospheric aircraft MIPAS-STR (Michelson Interferometer for Passive Atmospheric Sounding - STRatospheric aircraft) is a new instrument developed for remote sensing of a large number of atmospheric trace compounds (e.g. ClONO2, N2O5, NO, NO2 and HNO3) from high-altitude aircraft. It will be operated from the Russian M-55 Geophysica in the framework of the APE-GAIA (Airborne Polar Experiment - Geophysica Aircraft In Antarctica) in September/October 1999. We used modules of the Giessen diode laser system to test Brault’s approach of time-equidistant sampling which was implemented in the electronics of the interferometer of MIPAS-STR. Fig. 2:Linear representation of the optical path. The IR-radiation propagates from the scan mirror via the telescope and the interferometer to the detector unit. The instrumental FOV is defined by the lHe cooled apertures FS3 and AS3. The apertures AS, FS1 and shields reduce the radiation from outside the FOV as well as scattered radiation reaching the front optics. The radiation diffracted at the edges of the front optics is suppressed by the Lyot and aperture FS2. Fig. 3:The detector system. The entire focal plane with dichroic beam splitters, optical filters, Si:As-detectors etc. is cooled to 4 K. The division into 4 channels is necessary for NESR improvement to facilitate the detection of NO2 and NO (channels 3 and 4) and allows an efficient data reduction. Fig. 4:Scheme of the onboard electronics. The electronics is structured hierarchically. A transputer network connects the central computer with the independent subsystems. The state of the systems is defined by housekeeping and status data. Access from the main computer enables full control during operation of the instrument. Table 1:Characteristic instrument data. Fig. 5: Results from blackbody measurements with scan velocities between 1.58 cm/s and 4.30 cm/s. The mean of the phase spectra for forward and backward scans shows the electrical contributions to the phase. Fig. 6a: Spectrum of the diode laser. Low current was applied to the TDL to obtain almost monochromatic radiation. Fig. 6b: Same as fig. 6b but with expanded vertical scale. Since no monochromator was used, weak secondary lines (below 1%) can be observed. Fig 6c: Spectrum obtained by sine modulating the scan velocity. The modulation frequency was set to 150 Hz, the amplitude (p-p) was 40% of the nominal value of 3 cm/s. A time delay of 0 s was used. Fig 6d: Same as fig. 6c but with time delay of 22 s derived from the phase spectra shown in fig. 5. Note that the ghosts are reduced to about 20%. The non-vanishing part of the ghosts may be due to simultaneous amplitude modulation at the same frequency. Fig. 5: The aircraft M-55 Geophysica. Left: in Pratica di Mare (November 1996). Right: drawing of the M-55 with the ‘dorsal bay’ for MIPAS on top. August 1999