Ice Cloud Imager with polarimetric capabilities

This paper presents and proposes an Advanced Ice Cloud Imager, a low earth orbit radiometer insuring a polarimetric imagery of the Earth atmosphere from millimeter waves up to nearly one THz. It is focused on the characterization of ice clouds, convection and snowfall. It will extend the capabilities of the Ice Cloud Imager (ICI) thanks to enhanced polarimetry by: Measuring both orthogonal polarizations in all `window´ channels, measuring the full polarization vector for the lowest window channel, at 157GHz, including a 874 GHz channel to help bridge the gap between the millimetric and the far infrared measurements. The observation is based on a conical scan sub-millimeter wave instrument. The ground resolution is 12Km from 800Km altitude, on a “METOP-like” polar orbit. The full satellite is in the 500Kg/500W range. The channels frequencies range from 157GHz (with polarimetric capabilities) to 874GHz. The paper puts the emphasis on the impact of the polarization measurement capability onto the instrument architecture, in particular the antenna: in a first approach the offset antenna configuration has been explored, with one or two 157 GHz horns. Then a Cassegrain antenna has been preferred in order to meet the stringent cross polarization requirement. Finally, for accommodation and antenna performance reasons, the antenna has been split in a Cassegrain antenna for low frequencies and a separate small offset antenna for the two upper ones. The calibration system, also split in two parts, is depicted and the overall instrument implementation is shown. Finally the paper identifies the remaining technology steps needed to open the way to this highly accurate instrument: Proof of actual low cross-pol horn (design, manufacturing, verification); wide band Stoke parameters acquisition, aiming at a “157GHz radiometer with digital backend”: refinement of full Stoke parameters calibration.