IASI (the Infrared Atmospheric Sounding Interferometer) is one of the most complex instruments that we have ever had to develop. Indeed, it would be true to say that it gave us a really hard time. Its principle is simple...
A beam of light coming from the atmosphere is divided into two beams by a semi-reflecting blade. The two beams are recombined after being reflected by mirrors, in the form of ' cube-corners '. By shifting one beam relative to the other one, it is possible to quantify the energy of each wavelength.
However, this was the first time we had ever developed this kind of instrument, which in addition is intended to be used in space, where everything is a lot more complicated.- During the mission, which is due to last 5 years, one beam will be moved relative to the other one a total of 350 million times.
- The data has to be transmitted to the ground at a faster rate than can be processed by a satellite, even an advanced one. So before sending the signal to Earth, it has to be processed on board and considerably compressed. That gave us lots of fun messing around with microprocessors in parallel.
- Working in the infrared makes it necessary to cool the instrument's 12 detectors to –180°C.
- The satellite is moving at high speed, so even though each measurement only lasts 160 milliseconds, a very precise and reliable scanning mechanism is required in order to compensate for this motion.
- The structure must be very rigid, both mechanically and thermally.
- Lastly, the control fixtures need to be even more precise than the instrument itself. Given such specifications, IASI's ground tests were quite an adventure. Even the slightest little anomaly in the assembly had repercussions. For example, a wedge pushed in a little too far meant endless headaches. In order to check the alignment of the blade and the mirrors with a precision of a tenth of a micron we used a verification system that uses a laser beam, which explains our odd-looking goggles.
Still, in the end, we can feel pretty satisfied: on board the MetOp-A satellite since 2006, IASI has been providing precise measurements ever since (above, the first image taken on November 27th, 2006). So scientists who study the climate now have a reliable source of data. IASI has rapidly become the reference for calibrating other instruments that observe the atmosphere. All that makes the hours we spent with our goggles on tracking down interferometer lines well worthwhile!
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