A Review of Protocols for Fiducial Reference Measurements of Downwelling Irradiance for the Validation of Satellite Remote Sensing Data over Water.

This paper reviews the state of the art of protocols for the measurement of downwelling irradiance in the context of Fiducial Reference Measurements (FRM) of water reflectance for satellite validation. The measurement of water reflectance requires the measurement of water-leaving radiance and downwelling irradiance just above water. For the latter, there are four generic families of method, using: (1) an above-water upward-pointing irradiance sensor; (2) an above-water downward-pointing radiance sensor and a reflective plaque; (3) a Sun-pointing radiance sensor (sunphotometer); or (4) an underwater upward-pointing irradiance sensor deployed at different depths. Each method-except for the fourth, which is considered obsolete for the measurement of above-water downwelling irradiance-is described generically in the FRM context with reference to the measurement equation, documented implementations, and the intra-method diversity of deployment platform and practice. Ideal measurement conditions are stated, practical recommendations are provided on best practice, and guidelines for estimating the measurement uncertainty are provided for each protocol-related component of the measurement uncertainty budget. The state of the art for the measurement of downwelling irradiance is summarized, future perspectives are outlined, and key debates such as the use of reflectance plaques with calibrated or uncalibrated radiometers are presented. This review is based on the practice and studies of the aquatic optics community and the validation of water reflectance, but is also relevant to land radiation monitoring and the validation of satellite-derived land surface reflectance.
Competing Interests: Conflicts of Interest: The first author is involved in the design of systems based on the “irradiance sensor” method and in the acquisition of data using the “irradiance sensor” and “sunphotometer” methods. Other co-authors are involved in the acquisition of data using the “irradiance sensor”, the “reflectance plaque” and the “sunphotometer” methods. The funders had no role in the design of the study other than the drafting of the FRM4SOC project Statement of Work, which states that such a protocol review should be performed. The funders had no role in the collection, analyses, or interpretation of data or in the writing of the manuscript other than general project monitoring and encouragement. The funders did encourage publication of this review in order to ensure that high quality measurements of known uncertainty are available for the validation of the satellite missions they design and operate. None of these interests is considered to be conflictual or to inappropriately influence the recommendations expressed here.