|The manuscript of Feng Yin proposes a method for atmospheric correction for radiometers with a typical spatial resolution of few tenths of meters relying on prior information determined at much lower spatial resolution (few hundredths of meters). The proposed method relies on a Bayesian approach. Though the title of the manuscript “BAYESIAN ATMOSPHERIC CORRECTION OVER LAND: SENTINEL-2/MSI AND LANDSAT 8/OLI” is very appealing, this research work misses delivering the expected Analysis Ready Data outcome, essentially due to several unjustified assumptions and approximations. As stated in the abstract, mitigating the impact of atmospheric effects on optical remote sensing data is critical for monitoring intrinsic land processes. In the context of delivering Analysis Ready Data, intrinsic land processes mean that no additional variables are needed to analyse the delivered surface reflectance. Despite the apparent complexity of the proposed SIAC Bayesian approach, the delivered geophysical variable, namely the bottom of atmosphere Bidirectional Reflectance Factor (factor and not function as stated in line 29) is erroneously presented as Analysis Ready Data. It still depends on the actual state of the atmosphere! |
In line 60, it is written “We wish to estimate the probability distribution function (PDF) of BOA spectral BRF, R with illumination and viewing vectors Ωs , Ωv respectively”. The Bidirectional Reflectance Factor is indeed an intrinsic property of the surface that depends on Ωs , Ωv. The BRF should be estimated at the surface to achieve this objective, removing all possible atmospheric effects as written in line 94. In line 120 Equation (3), it is however assumed that the surface reflectance rb represents a “Lambertian reflectance” which is equivalent to assuming that the sky radiation is perfectly diffuse which is true only when the sky is overcast. rb is however estimated with Equation (4) which provides the BRF for the Ωs , Ωv geometry. It is therefore not a Lambertian Equivalent Reflectance as assumed in Equation 3. The relationship between rb expressed in Equations (3) and (4) is thus inconsistent. Assuming that rb in Equation (3) can be estimated with Equation (4) is strictly correct for the direct contribution only. It is inconsistent with the definition of the terms representing the total (direct and diffuse) downwelling and upwelling atmospheric transmittance. Consequently, the retrieved surface reflectance still depends on the actual atmospheric state (diffuse contribution). Hence, as long as this issue is not properly solved, the proposed approach will not work for the delivery of Analysis Ready Data. It is a major flaw in the proposed approach that should be corrected, and they are elegant ways to do so! Confusing symbols have been used to hide this issue:
- In lines 19 and 20, R defines a correlation coefficient.
- In Table 1, the symbol r is not defined.
- In Table 2, R is defined as R~N(r,Cr) as the a posteriori PDF of the BRF but the symbols r and Cr are not defined
- In Table 2, Rb is defined as Rb~(rb,Cb) as the a priori PDF of the BRF but the symbols rb and Cb are not defined
- In line 200, r is defined as the mean surface reflectance.
Sections 3 and 5 have not been reviewed as the proposed approach is erroneous. Unfortunately, I would not recommend the publication of this manuscript as long as it relies on erroneous assumptions.