A family of fully automated digital Whole Sky Imagers (WSIs) has been developed at the Marine Physical Lab over many years, for a variety of research and military applications. The most advanced of these, the Day/Night Whole Sky Imagers (D/N WSIs), acquire digital imagery of the full sky down to the horizon under all conditions from full sunlight to starlight.  Cloud algorithms process the imagery to automatically detect the locations of cloud for both day and night.  The instruments can provide absolute radiance distribution over the full radiance range from starlight through daylight.  The first of the WSIs was fielded in 1984, followed by the D/N WSIs in 1992.  These many years of experience and development have resulted in very capable instruments and algorithms that remain unique.  This article discusses the history of the development of the D/N WSIs, system design, algorithms and data products.  The paper cites many reports with more detailed technical documentation.  Further details of calibration, day and night algorithms, and cloud free line of sight results will be discussed in future articles. 

The primary goals of this project for JTO and ONR (Grant N00014-07-1-1060) were to further develop Extinction Imagers for use in the ocean environment, and to extend the capabilities into the Short Wave IR (SWIR).  Extinction Imaging is a method for determining the effective extinction coefficient over an extended path using a sensor at one end of the path.  It uses calibrated imagers to acquire the relative radiance of a dark target near the other the end of the path and the horizon sky in the direction of the dark target.  It is completely passive and thus covert, and the hardware is robust and relatively inexpensive.  It uses rigorous equations, which determine the extinction coefficient from the measured apparent contrast of the radiance of the dark target with respect to the horizon sky. 

The project was very successful.  We found that the ocean surface could readily be used as a dark target in red and SWIR wavelengths.  Both the red and the SWIR measurement results were excellent for daytime.  Comparisons with standard instruments, as well as uncertainty analysis, indicated that extinction imagers provide better measurements of the atmospheric extinction losses over extended paths than other methods of which we are aware.

Our secondary goals were to address the night regime, and to address slanted paths above the horizontal.  Regarding night, we found that the visible sensor acquired excellent data, but the ocean surface was not a good dark target in our wavelengths.  Recommendations on the handling of night are given in the report.  Regarding the lines of sight above the horizon, we developed a slant path algorithm that determines beam transmittance.  It performed very well.  Recommendations are made regarding integration of these techniques for military applications.