Yes. There are no limitations in the physics or the model that stop the user from modeling down-looking, slant-range, up-looking, etc. sensor acquisitions. There are some tricks to get the DIRSIG make_adb tool and MODTRAN to work correctly together in slant-range and up-looking geometries. Read the section Section 24.2.6 in the The make_adb User Manual to learn more about such acquisitions.