A review of protocols for Fiducial Reference Measurements of downwelling irradiance for validation of satellite remote sensing data over water
Kevin Ruddick, Andrew Banks, Emmanuel Boss, Alexandre Castagna, Robert Frouin, Martin Hieronymi, Cedric Jamet, Bettye C. Johnson, Zhongping Lee, Michael Ondrusek, Viktor Vabson, Riho Vendt, Kenneth Voss, Giuseppe Zibordi
This paper reviews the state of the art of protocols for measurement of downwelling irradiance in the context of Fiducial Reference Measurements (FRM) of water reflectance for satellite validation. 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; or 2) an above water downward pointing radiance sensor and a reflective plaque; or 3) a sun-pointing radiance sensor (sunphotometer); or 4) an underwater upward pointing irradiance sensor deployed at different depths. Each method, except the fourth, which is considered obsolete, is described generically in the FRM context with reference to the measurement nist-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 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 practice and studies of the aquatic optics community and the validation of water surface reflectance but can be relevant also to the much wider field of applications of the measurement of downwelling irradiance in land radiation monitoring and validation of satellite-derived land surface reflectance.