INSTRUMENT OVERVIEW
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    The Orbiter Infrared Radiometer (OIR) was used for vertical temperature sounding of the atmosphere from the cloud tops (60 km, 
    250 mb pressure) to roughly 150 km (on the order of 1 to 10 nanobar), and for investigations of cloud morphology, including the 
    identification of possible multiple layers and water vapor mapping. The instrument was based on the selective chopper radiometer 
    and the pressure modulator radiometer designs flown on Nimbus satellites. The instrument failed to operate after 72 orbits, on 
    February 14, 1979.
 
    The OIR is an 8 channel radiometer mounted on the instrument platform on the orbiter bus. It is fixed at an angle of 45 degrees, 
    boresighted in the same plane as the photopolarimeter and ultraviolet spectrometer, to be able to cover both polar and equatorial 
    regions of Venus. Channel 1 uses a CO2 pressure-modulation technique to measure temperatures anywhere from about 0.01 mb to the 
    beginning of the exosphere. Channels 2 through 5 covered wavelengths near the absorption band of carbon dioxide at 15 microns. 
    Channel 6 was centered at the 2 micron carbon dioxide band to study cloud structure. Channel 7 was a wide-band (0.2 to 4.5 microns) 
    to measure total reflected solar intensity. Channel 8 was centered at 40 to 50 microns, the strongest part of the pure rotational 
    band of water, to map the distribution of water vapor in the upper atmosphere. OIR is 24.8 x 17.5 x 25.1 cm in size with a mass of 
    5.9 kg, and uses 5.2 W power.
 
    The infrared light enters the instrument through a 48 mm aperture off-axis parabolic telescope. The 1.25 degree field of view 
    translates to a resolution of a few km near periapsis (the FOV is 5 degrees for channel 1). The nominal sample, or dwell, time 
    is 30 ms to reduce smearing by the spacecraft rotation. Temperature sensitivity is better than 0.5 K at 240 K. When the atmosphere 
    is being viewed tangentially at the limb during a limb crossing, the dwell time is reduced to 12 ms for about 150 ms.
 
    The instrument operated for 72 orbits, returning about 800,000 vertical profiles of upper atmospheric temperature. It failed on 
    14 February 1979.