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ATMOS is an infrared spectrometer (a Fourier transform interferometer) that is designed to study the chemical composition of the atmosphere. In this section you will be able to read both general and detailed information as to why and how the instrument works.

Since the molecules of interest to the ATMOS investigation must be measured remotely (i.e., from outside the atmosphere itself), solar spectroscopy was the method of choice for making the measurements, using those periods during each orbit of the spacecraft when the atmosphere is between the Sun and the instrument (i. e., at sunrise and sunset as seen from the spacecraft). During sunset, for example, the tangent point of the ray path to the instrument penetrates deeper and deeper into the atmosphere until it is blocked by the surface of the Earth (or clouds); as seen from a typical shuttle orbit, the height of the tangent point changes at about 2 kilometers per second so that, to be able to distinguish changes in the composition with altitude, successive measurements of the spectrum must be made very rapidly. By analyzing the absorptions due to a given molecule in each successive spectrum, the variations in its concentration with altitude can be determined.

The rapidity at which the measurements must be made precludes the use of conventional scanning spectrometers which record the spectrum wavelength by wavelength, requiring minutes or even hours to cover relatively modest wavelength intervals. Furthermore, such instruments by their nature are limited to collecting radiation through very narrow entrance apertures. Thus, only a small amount of light enters the instrument, and only a very small fraction of that light is measured at any given time.

A technique which overcomes these limitations is based on the interference properties of light waves, and was first used in an analytical instrument by Albert Michelson almost a century ago. This technique, called interferometry, does not involve dispersion of the radiation being analyzed, and hence it can be admitted to the system through much larger apertures than the slits required for scanning instruments. Once inside, the radiation is split into two beams which travel separate paths through the instrument and are then recombined. If the length of one path is varied with respect to the other, the various wavelengths of light contained in the recombined beam will go in and out of phase as a function of the wavelengths themselves and the path difference. If the recombined beam is then focussed on a detector and sampled at the proper intervals, a signal called an interferogram is produced which is extremely complex but can be transformed to yield the amount of radiation contained in the original beam as a function of wavelength. Since information about all wavelengths of light in the beam is contained in every sample recorded, the technique is not subject to the second limitation of scanning spectrometers mentioned above.

Although the concepts involved in interferometry are straightforward, the instrument itself must work in the realm of fractions of wavelengths of the light being measured and thus severe demands are placed on the fidelity of the optics and the precision and accuracy of the sampling intervals. But for the advent of high stability single-frequency lasers, ultra-fast electronic components, and modern optical polishing techniques, the ATMOS sensor could not have been built. The instrument uses optical components polished flat to within a twentieth of the shortest wavelength of the light being measured, and generates an interferogram containing 400,000 sample points (the intervals between which are precisely controlled using a reference laser) every second! The corresponding rate at which the data are transmitted to the ground is 16 million bits per second.

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