It is important to evaluate sky radiance for a ground-to-air measurement system. Based on energy conservation law and the assumption that the underlying surface is a Lambert’s surface, a model was developed to calculate the additional sky radiance due to the ground reflectance. According to the ray trace principle, multiple reflectance between the earth’s albedo surface and the atmosphere was also taken into account. The hybrid modified delta-Eddington approximation was used to calculate the transmission, reflection and average reflection of the atmosphere from the ground to the top of the atmosphere. The additional sky radiance was calculated under surface albedos of 0.15, 0.25 and 0.35, and compared with the measured data. The results show that there is a limb-brightening effect for sk