The variable MODTRN selects the band model algorithm used for the radiative transport. This includes the normal MODTRAN band model (MODTRN=M) or the most accurate but slower MODTRAN correlated-k option in its medium speed (MODTRN=KM, 33 absorption coefficients per spectral bin) or slow speed (MODTRN=KS, default value, 17 coefficients) (Berk et al., 1998) .
Berk, A., Bernstein, L., Anderson, G., Acharya, P., Robertson, D., Chetwynd, J., & Adler-Golden, S. (1998), "MODTRAN cloud and multiple scattering upgrades with application to AVIRIS." Remote Sensing of Environment. Vol. 65, No. 3, pp. 367‒ 375.