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Primary Quantities Measured with System Typical |
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There are three primary types of output generated by the WSI:1) calibrated radiance distributions,2) quantities such as cloud cover derived from the cloud decision images, and 3) visual imagery intended for visual assessment. Each of these outputs are discussed below.
The primary source of uncertainty in the radiometric data is the uncertainty of the calibration lamps. For a visible system requiring knowledge of the spectral lamp output and not just total lamp output, lamps are typically accurate to about 3%. With these lamps, net accuracies of 5% or better are normally achievable. Positioning accuracy is generally 1Ì when fielded by experienced teams, with a precision of 1/3Ì. Positioning accuracy can be checked and corrected for if necessary using the measured sun positions.
Quantities such as cloud cover depend on the accuracy of the cloud algorithm. The algorithm works quite well for opaque clouds. Thin clouds are more difficult to characterize due to the difficulty in assessing a very thin cloud with respect to a fairly thick haze. In-house processing has normally been quite accurate, but for the near-real timecode being fielded with the instruments, only the opaque results will be presented until the thin cloud algorithm can be verified or improved for use in totally autonomous mode.
When used for visual assessment, the red images are processed for easier viewing. (See windowing in the Theory of Operations section.)While this process introduces no errors, it is possible to misinterpret the resulting images. Specifically, data which appear offscale bright or dark are usually only outside the displayed range, and not actually offscale. Similarly, if features appear to be too dim to see clearly, they can usually be brought out by selection of a narrower display range. It is important for the user to check the image time, and know whether the image was taken under sunlight, moonlight, or starlight. It is often a user responsibility to be sure the instshadowed from extraneous bright lights so that the night sky will be imaged properly.
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