The cosine error of in situ seven-channel radiometers designed to measure the in-air downward irradiance for ocean color applications was investigated in the spectral range with a sample of three instruments. The interchannel variability of cosine errors showed values generally lower than below 50 degrees incidence angle with extreme values of approximately 4–20% (absolute) at 50–80 degrees for the channels at 412 and . The intrachannel variability, estimated from the standard deviation of the cosine errors of different sensors for each center wavelength, displayed values generally lower than 2% for incidence angles up to 50 degrees and occasionally increasing up to 6% at 80 degrees. Simulations of total downward irradiance measurements, accounting for average angular responses of the investigated radiometers, were made with an accurate radiative transfer code. The estimated errors showed a significant dependence on wavelength, sun zenith, and aerosol optical thickness. For a clear sky maritime atmosphere, these errors displayed values spectrally varying and generally within , with extreme values of approximately 4–10% (absolute) at 40–80 degrees sun zenith for the channels at 412 and . Schemes for minimizing the cosine errors have also been proposed and discussed.
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