The OSIRIS instrument makes observations of the atmospheric limb. This measurement technique is a “side-view” observation of the radiance of sunlight scattered from the atmosphere. The spectrum of light from the sun, in the process of traveling through the atmosphere and scattering off molecules and particles, possibly several times, is imprinted with the signatures of the atmospheric composition. Molecular emissions, resulting from atmospheric photo-chemistry, is also an important component of the OSIRIS measurements. The observation of the limb, or side-view, radiance of the atmosphere from a satellite provides the benefits of both an occultation and a nadir sounding experiment: vertical resolution combined with global coverage.
The Canadian Optical Spectrograph and InfraRed Imaging System (OSIRIS) is one of a new generation of satellite instruments designed to measure the atmospheric limb radiance of scattered sunlight (Llewellyn et al., 2004). The instrument is onboard the Swedish satellite Odin (Murtagh et al., 2002), which was launched on February 20, 2001, and continues full operation to date. The OSIRIS instrument is essentially two optical subsystems, suggested by its name: an optical spectrograph (OS) module and an infrared imager (IRI) module. The OS is grating spectrometer with a CCD detector, and measures spectra of the limb radiance from 280-810nm with a spectral resolution of approximately 1 nm. The field of view of the spectrograph when mapped on to the atmospheric limb at the tangent point is approximately 1km vertically and 40km horizontally. Vertical profiles of the limb radiance are obtained by taking OS exposures while performing a repetitive vertical scan of the single line of sight through selected tangent altitude ranges, nominally from 10 to 100 km. Successive exposures are spaced by approximately 2km in tangent altitude. The time required for a single altitude scan is near 1.5 minutes and so allows for nearly 60 scans per orbit. To satisfy the accurate three axis astronomical pointing requirements of the other instrument on Odin, the Sub-Millimetre Radiometer, the attitude control system of the satellite equipped with two star trackers, a sun tracker, magnetometers and gyros, and is activated by magnetic torquers and momentum wheels. The reconstructed knowledge of the limb pointing is approximately ±15 arcseconds, or ±200m in the vertical at the tangent point. The following figure is a plot of typical OSIRIS limb radiance spectra at selected tangent altitudes during limb scan 06432012. A transition in optical material is used to filter higher order light diffracted from the grating. This transition region, referred to as the order sorter, contaminates the measurements at wavelengths from 475 to 535 nm.
The IRI is composed of three vertical near infrared co-aligned linear array imagers that capture one dimensional images of the limb radiance at 1260, 1270, and 1530nm with a tangent altitude resolution of approximately 1 km. The imagers have parallel bore-sights; each one consists of an identical baffling system, lens, narrow-band interference filter, and a one-dimensional linear array of 128 thermo-electrically cooled InGaAs photodetectors placed in the focal plane of the lens. Approximately 30 photodetector elements at one end of each array are covered with a mask in order to provide a continuous measure of the dark signal in the array. Therefore there are approximately 100 lines of sight from each imager channel that measure simultaneously over 100 vertical kilometres in tangent altitude. The 1530nm channel is designed to measure a Meinel band hydroxyl vibrational rotational emission in the mesosphere. As this emission is extremely weak during the daytime, this channel also provides a measure of the limb scattered sunlight from the neutral atmosphere.