Canada launches TPA Mars probe, OSIRIS ozone sensor

17 Jun 2002

The Canadian Space Agency launched its ACTIVE rocket mission April 28 which supports two space science projects: the TPA (Thermal Plasma Analyser) Mars Probe and the OSIRIS (Optical Spectrograph and Infra-Red Imaging System) ozone measuring instrument.

The ACTIVE mission launched from the SpacePort Canada range in Manitoba will test the Thermal Plasma Analyzer. The Japanese spacecraft Planet-B will use the TPA, which measures energetic particles similar to those which cause Canada's Northern Lights, to sample the Martian atmosphere when it arrives in late 1999. Scientists believe that data will provide better understanding of the earth's own atmospheric systems as well as prepare for future trips to Mars. The ACTIVE mission contains a prototype of the TPA instrument; the principal investigators of both the ACTIVE mission and the TPA are University of Calgary professors.

The OSIRIS instrument will work in conjunction with the POSSEX/MOZE suite of instruments. POSSEX will measure the polarization of scattered sunlight in the atmosphere using four photometers, and the results will provide more thorough analysis of the OSIRIS data. MOZE is a student project to measure ozone concentration as a function of altitude; it will use two photometers of the same type as those for the POSSEX experiment, although with different filters.

OSIRIS, designed for Sweden's ODIN satellite, will detect several molecular species and aerosols. Its imaging spectrograph continuously covers the visible/near ultraviolet wavelengths from 280 to 800 nm, which yields a spectral resolution of 1-2 nm and a spatial resolution of 1 km. A CCD is the imaging element for the imaging spectrograph. Three InGaAs array detectors are the imaging elements for OSIRIS' three near-infrared telescopes which operate at 1.27 microns with each contiguous band being 10 nm in bandwidth.

OSIRIS will collect absorption lines and diffuse bands in the 280-800 nm spectrum of scattered sunlight as well as emission features (airglow) from the middle atmosphere in that wavelength and in the 1270 nm NIR wavelength. The broadband scattered light also contains information on the temperature of the background atmosphere and the possible presence of aerosols.

OSIRIS is a 12 kg instrument divided into three parts: an optics unit, an electronics unit, and a power unit. The electronics unit and the power unit are in a common enclosure, and all units are bolted together to form an assembly which is mounted to the satellite's honeycomb panel. Two penetrations through this panel provide paths for cooling straps to connect to the radiator on the exterior of the spacecraft that reject heat from OSIRIS's thermo-electric coolers. The fields-of-view of the UV/VIS and IR channels are aligned to produce simultaneous spectra of sources at varying altitudes above the surface of the earth. OSIRIS is mounted so that its optical axis is at a known angle with respect to the calibrated axis of the SMR, permitting collaborative measurements of atmospheric constituents that are significant to the understanding of ozone depletion.

The optics unit is a light tight box which contains the optical elements and the detectors for the VIS/UV and NIR parts of OSIRIS. All optical components and interior baffles are mounted to an optical bench surface. The optical unit is designed to permit independent testing of the UV/VIS instrument and the infrared instrument. The optics unit is kinematically mounted at three mounting points relative to the rest of OSIRIS. The entire instrument is designed to have +/-0.5 degrees of adjustment of its optical axis, The interior is finished with appropriate optical black coatings.

The slit of the UV/VIS spectrograph is oriented horizontally (perpendicular to the orbit plane). The image generated as the satellite nods displays the spectra of sources at various altitudes above the surface of the earth. The NIR part of OSIRIS uses filters to provide wavelength separation, while height information is obtained through the vertically oriented InGaAs linear detector array.

The spectral response of the CCD determines the overall passband of UV/Visible part of OSIRIS. An EEV Model CCD26 was chosen in part because of its relatively high quantum efficiency at the UV end of the spectrum. The IR portion uses InGaAs linear photodiode arrays from Sensors Unlimited with hybrid multiplexers. The passbands of the three NIR sub-channels are determined by narrow band interference filters. Both detectors must be operated at temperatures slightly lower than the ambient electronics box temperature.

For the UV/VIS channel, reflective optical elements are used wherever possible since they are achromatic and tend to introduce less stray light than refractive elements. An aspheric reflective grating provides the spectral separation. The objective mirror has a focal length of 250 mm. Thin film technology is used for the various optical coatings such as reflective mirror coatings and anti-reflection coatings. Magnetron sputtering or ion plating deposition will be used for the flight IR bandpass filters.

The principal investigator for the OSIRIS experiment is Dr. Ted Llewellyn of the University of Saskatchewan.