A review of protocols for Fiducial Reference Measurements of water-leaving radiance for validation of satellite remote sensing data over water
Kevin Ruddick, Emmanuel Boss, Alexandre Castagna, Robert Frouin, Alex Gilerson, Martin Hieronymi, Bettye C. Johnson, Joel Kuusk, 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 water-leaving radiance 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 former there are four generic families of method, based on: 1) underwater radiometry at fixed depths; or 2) underwater radiometry with vertical profiling; or 3) above water radiometry with skyglint correction; or 4) skylight blocked radiometry just above water. Each method 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 water-leaving radiance is summarized, future perspectives are outlined, and the question of which method is best adapted to various circumstances (water type, wavelength) is discussed. This review is based on practice and papers of the aquatic optics community for the validation of water surface reflectance from satellites but can be relevant also to the measurement of water-leaving radiance or water reflectance for other aquatic applications such as the development or validation of algorithms for remote sensing estimation of water constituents such as chlorophyll a or Inherent Optical Properties or related products.