The Passive Aerosol Cloud Suite (PACS) is an imaging polarimeter that employs wide field-of-view (FOV) optics to obtain a highly accurate polarimetric signal across an image with hyperangle, multiangle views. PACS is designed to measure the array of parameters necessary to retrieve aerosol and cloud microphysical characteristics. The final goal of PACS is a flight model able to compete for a position on the Aerosol-Cloud-Ecology (ACE) Decadal Survey mission or other space-based platforms, but adaptations of the basic technology to airborne and ground-based applications are anticipated.

The key problem is to measure sufficient wavelengths, angular range, angular density, sampling statistics, global coverage, and fine spatial resolution, and still maintain very high polarimetric accuracy. The heritage of polarimeters in space has included an imaging polarimeter that provides multi-angle, multi-wavelength polarized images of the Earth’s reflectance, but does not include wavelengths into the mid-infrared spectrum, and most importantly, does not measure the three polarization states simultaneously. High polarimetric accuracy cannot be achieved without simultaneous measurements of the three polarization states. The problem is to provide both high accuracy in the polarimetric measurements and imagery at the same time.

The purpose of the PACS design is to measure polarization with a high degree of accuracy over a broad spectral range, and also provide sufficient angular views, sampling statistics, moderate spatial resolution, images, and near global coverage from the ACE mission. PACS uses wide field of view optics to obtain a wide angular range, splits the incoming image into three identical images, passes each of the three streams through a different polarizer to obtain the three different polarization states simultaneously, then resolves the three images on three different detector arrays.

Spectral information is obtained from stripe filters placed directly on the detector arrays. Multi-angle views are obtained from the forward movement of the instrument observing the same target at different locations on the array, corresponding to different view angles. An important component of the PACS innovation is the design, theoretical development, and construction of a calibration system to assure high accuracy polarization.

Unique features include no moving parts, simultaneous measurements of three polarization states, broad spectral and angular ranges, and hyperangular capabilities. These features result in high-accuracy polarization measurements with high angular density across full images of ±50° view. Cloud microphysics can be retrieved for local clouds, something that was not possible before this reporting.

This work was done by Lorraine Remer and Vanderlei Martins of Goddard Space Flight Center. GSC-16216-1

Photonics Tech Briefs Magazine

This article first appeared in the January, 2015 issue of Photonics Tech Briefs Magazine.

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