Measurements on pointing error and field of view ofCimel-318 Sun photometers in the scope of AERONET

[摘要] Sensitivity studies indicate that among the diverse error sources ofground-based sky radiometer observations, the pointing error plays animportant role in the correct retrieval of aerosol properties. The accuratepointing is specially critical for the characterization of desert dustaerosol. The present work relies on the analysis of two new measurementprocedures (cross and matrix) specifically designed for the evaluation of thepointing error in the standard instrument of the Aerosol Robotic Network (AERONET), theCimel CE-318 Sun photometer. The first part of the analysis contains apreliminary study whose results conclude on the need of a Sun movementcorrection for an accurate evaluation of the pointing error from both newmeasurements. Once this correction is applied, both measurements showequivalent results with differences under 0.01° in the pointing errorestimations. The second part of the analysis includes the incorporation ofthe cross procedure in the AERONET routine measurement protocol in order tomonitor the pointing error in field instruments. The pointing error wasevaluated using the data collected for more than a year, in 7 Sun photometersbelonging to AERONET sites. The registered pointing error values weregenerally smaller than 0.1°, though in some instruments values up to0.3° have been observed. Moreover, the pointing error analysis showsthat this measurement can be useful to detect mechanical problems in therobots or dirtiness in the 4-quadrant detector used to track the Sun.Specifically, these mechanical faults can be detected due to the stablebehavior of the values over time and vs. the solar zenith angle. Finally, thematrix procedure can be used to derive the value of the solid view angle ofthe instruments. The methodology has been implemented and applied for thecharacterization of 5 Sun photometers. To validate the method, a comparisonwith solid angles obtained from the vicarious calibration method wasdeveloped. The differences between both techniques are below 3%.

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