The Optimised Retrieval Model (ORM) performs in near real time the retrieval of pressure, temperature and volume mixing ratio of six high-priority species, namely O₃, H₂O, HNO₃, CH₄, N₂O and NO₂.
  

The retrieval strategy of ORM is based upon the following three choices:

The redundancy of information coming from MIPAS measurements makes it possible to select a set of narrow (less than 3 cm-1 width) spectral intervals containing the best information on the target parameters, while the intervals containing little or no information can be ignored. The use of selected spectral intervals, called  ‘microwindows’, allows the size of analyzed spectral elements to be limited and avoids the analysis of spectral regions more affected by systematic errors.

The unknowns are retrieved using the following sequence of operations. First temperature and ‘tangent pressures’ are retrieved simultaneously, then the target species volume mixing ratio profiles are retrieved individually in sequence. The reason of this approach is that a simultaneous retrieval of all the species would require a huge amount of computer memory, because the size of the matrices to be handled by the retrieval is proportional to the product between the unknown parameters and the number of observations. The sequence of the target species retrieval has been determined according to the degree of their reciprocal spectral interference and is: H₂O, O₃, HNO₃, CH₄, N₂O and NO₂.

A global fit approach is adopted for the retrieval of each vertical profile. This means that spectral data relating to a complete limb scan sequence are fitted simultaneously. The global fit provides a comprehensive exploitation of the information and a rigorous determination of the correlations between atmospheric parameters at the different altitudes. Besides, it permits the full exploitation of hydrostatic equilibrium condition and is better compatible with the modelling of the finite field of view of the instrument.
 

The ORM code has been developed by the Earth Observation group of IFAC-CNR (Firenze - Italy) in collaboration with the Department of Physical and Inorganic Chemistry - University of Bologna (Bologna - Italy), the ISAC-CNR (Bologna - Italy), the IMK (Karlsruhe - Germany), LPMA (Paris - France), University of Oxford and University of Leicester. The ESA study for the development of the ORM covered the years 1995 - 2002.