The Millimetre-wave Acquisitions for Stratosphere-Troposphere Exchange Research (MASTER) instrument is intended to sound the gaseous composition of the upper troposphere and lower stratosphere (UTLS) in a future ESA space mission. A significant and inherent advantage of operation at millimetre and sub-millimetre wavelengths in comparison to limb-sounders operating at infra-red and shorter wavelengths is low sensitivity to cirrus clouds. MASTER will employ relatively small vertical and horizontal spacings between limb views, in order to over-sample the atmosphere in the orbit plane. By viewing each air mass from different directions, and including this information in the retrieval, horizontal as well as vertical structure of atmospheric fields may be captured. In order to examine this tomographic limb-sounding approach for MASTER, a state-of-the-art 2-D radiative transfer model and retrieval model have been developed and used in simulation experiments. A linear analysis has been performed to establish achievable horizontal and vertical retrieval resolution for target species and to identify additional parameters to include in the state vector in order to reduce error sensitivities. A realistic mid-latitude scenario and appropriate instrument and model errors have been considered. By accurately modelling radiative transfer in two dimensions within the orbit plane, and using multiple limb-sequences simultaneously in a 2-D retrieval, a horizontal resolution better than 200 km can be achieved, together with ~2 km vertical resolution for retrievals of water vapour, ozone and other trace gases in the UTLS.
MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) is operating on board of the ENVISAT satellite and is acquiring for the first time high spectral resolution middle infrared emission limb sounding spectra of the Earth atmosphere from space. The measurement capabilities make it possible to determine every 75 sec. the vertical profile of several atmospheric trace constituents, during both day and night with an almost full coverage of the globe. This leads to a three dimensional measurement of the atmospheric composition. In order to handle the large data flow, an optimized code for the Level 2 near real time analysis of MIPAS data was developed by an international consortium of scientists under an ESA contract and was implemented in the ENVISAT Ground Segment. The code is designed to provide, in an automated and continuous mode, atmospheric vertical profiles of temperature and pressure, as well as of concentrations of O<sub>3</sub>, H<sub>2</sub>O, CH<sub>4</sub>, HNO<sub>3</sub>, N<sub>2</sub>O and NO<sub>2</sub>, in the altitude range from 6 to 60 km. The "commissioning phase," in which verification and validation of the instrument and of the analysis code are performed, is still in progress, but some preliminary results have been obtained. The first examples of the MIPAS near real time Level 2 data products, consisting of retrieved profiles and auxiliary data that characterize the measurement accuracy and resolution, are shown.