Longwave Infrared Camera onboard the Venus Climate Orbiter

Abstract

The Longwave Infrared Camera (LIR) onboard the first Japanese Venus mission, PLANET-C, or the Venus Climate Orbiter, operates in the middle infrared region with a single bandpass filter of 8–12 μm, measuring thermal radiation emitted from the cloud tops of the Venusian atmosphere. A horizontal wind vector field at the cloud-top height will be retrieved by means of a cloud tracking method. In addition, absolute temperature will be determined with an accuracy of 3 K. Since solar irradiation scattered by the atmosphere is much weaker than the atmospheric thermal radiation, LIR can continuously monitor a hemispheric wind field independent of the local time of the apocenter throughout the mission life. Wind and temperature fields obtained by LIR will provide key parameters to solve climatological issues related to the Venusian atmosphere. The use of an uncooled micro-bolometer array (UMBA), which requires no cryogenic apparatus, as an image sensor contributes to the reduction of power consumption and the weight of the LIR imager. An instrumental field-of-view of 12° is equal to the angle subtended by Venus when observed from a height of 9.5 Rv. The pixel field-of-view corresponds to a spatial resolution of 70 km viewed from the apocenter. A mechanical shutter functions not only as an optical shutter but also as a reference blackbody. The temperature stability of the sensor is especially important, because fluctuation of thermal radiation from the internal environment of the sensor itself causes background noise. Therefore, the temperature of the UMBA package is stabilized at 313 ± 0.1 K with a feedback controlled Peltier cooler/heater, and a NETD of 0.3 K, which is required for this infrared imager, will be achieved. Flat field images are taken with the shutter closed several seconds before and after 1 s exposure for a Venus thermal image. After a Venus image is taken, the LIR imager takes a cold calibration image of deep space. This measurement sequence is repeated every two hours when the spacecraft is orbiting at apocenter. Image data are transmitted down to the Earth after onboard calibration and data compression by common digital electronics.