I am trying to manually correct the surface reflectance values of some Landsat 7 and 8 images. I'm using Minnaert modified method, which uses the cosine correction as one of its inputs (see below the formula, from Richter, 2009).
- ρMM stands for "Minnaert modified correction"
- ρL stands for the result of cosine correction (sun zenit angle cosine divided by the cosine of the solar illumination angle at each point)
- cosβ stands for the sun zenit angle for the center of the scene
- cosβT stands for the solar illumination angle at each point of the scene (plus 10, 15 or 20º depending on its value).
- b exponent depends on the coverage and the band (it can be 0.333, 0.5 or 0.75).
The problem is that the cosine of the solar illumination angle can be negative, and in that case cosβ/cosβT gives a negative value whose power cannot be calculated, if it's 0.5 or 0.75.
I have looked into many articles to try find out how to deal with this problem, but I have found nothing until the moment. Could anybody point me in the right direction?