Abstract: The GPS signal ray’s perigee from the top of neutral atmosphere to the ground drifts largely in horizontal at an occultation event. The occultation event is simulated using the ray-tracking technique on the situation of the specified atmospheric refractive index model and the GPS constellation. The effect of atmospheric refractive index profile, LEO satellite’s height and inclination on the horizontal drift of the perigee studied. The results indicate that the difference between the average horizontal drift on day and that at night is about 20 km, the minimum of the average horizontal drift appears at some inclination 130° and the average horizontal drift increases from inclination 130° to 0° or 180° for a LED satellite which is at an altitude of 735 km above ground, and the average horizontal drift decreases from 400 km to 1500 km above ground for a LEO satellite at inclination 70°.