Odin is unique in its double mission to support both astronomical and atmospheric research. The main instrument, a submillimetre radiometer (SMR) is used for both disciplines and is for atmospheric measurements complemented by an optical spectrograph and infrared imager (OSIRIS).

The SMR is based on actively cooled tuneable heterodyne receivers with solid state local oscilllators and Schottky mixers. The receivers are fed by a highly accurate 1.1 metre Gregorian all CFRP telescope.

The fact that the satellite and its instruments still work fine is interesting, in view of the use of a large amount of novel technology such as indium phosphide MMIC amplifiers at 119 GHz in combination with commercial components such as FPGA's.

In some cases also commercial production methods were used. An extensive pre-launch test programme was in place to ensure a reliable payload and spacecraft. In total Odin has 14 fully operational active mechanisms including cooler, tuners for the interferometers, switch and calibration devices.

Odin is in a 600 km sun-synchronous polar orbit passing the equator north-bound around 18:00 local time. Its ACS system allows either a celestial staring mode with an absolute pointing accuracy better than 10 arc seconds real-time, or a limb-scanning mode with the reconstructed accuracy better than one arc minute - corresponding to a one kilometre altitude resolution.

The combination of the SMR and OSIRIS instruments makes Odin well adapted for atmospheric studies of higher altitudes. For instance water can be measured as high as 90 km. Odin has already provided a long-term data set, now to be continued.

Key trace species related to the ozone destruction process in the stratosphere are studied, among those ClO. Studies are also made of the global dynamics throughout the stratosphere and mesosphere, noctilucent clouds, and high-altitude sub-visual clouds.

Today, SMR is the most sensitive astronomy submillimetre heterodyne receiver in orbit, especially regarding water vapour and molecular oxygen. The study of star forming processes and chemistry in interstellar molecular clouds is a main topic here.

Other notable observations concern the outgassing of different isotopes of water from comets, evaporation of icy bodies - planets or comets - by expanding old stars, and the measurement of water in the atmosphere of Mars.

The low limit for the amount of molecular oxygen with this sensitive instrument is crucial, placing it 1000 times lower compared to what was expected from chemical models. Presently on-going are a spectral line survey, presently with 200 lines from 35 different molecules, and a search for specific molecules, like LiH, from the very early universe.

Odin was developed by Swedish Space Corporation, also responsible for the operations, on behalf of Swedish National Space Board and the space agencies of Canada, France and Finland. It serves astronomers and aeronomers of all four partner countries.

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