Tropospheric Refraction Effects on Satellite Range Measurements

Introduction To DETERM INE THE ORBIT OF AN EARTH SATELLITE, or conversely, to use a known satellite orbit to find the location of a point on the earth, it is necessary to use some sort of electromagnetic signal (light or radio signal) passing between the satellite and the earth station. This signal traverses the earth's atmosphere, where its velocity is not the same as in free space (the index of refraction of the atmosphere is not unity). Thus the signal travel time is slightly altered by the presence of the atmosphere. This means that there is an error in the measured range; there is also an error in the range rate, since the amount of the range error varies. Even if the atmosphere is in a steady state, its effect is greater for an oblique than for a vertical signal path. In addition, the amount changes with weather, since it depends on the integrated effect of air pressure, temperature, and water vapor along the path. These effects are large enough to be significant when precise work is to be done with satellites, and an atmospheric correction is therefore needed. To make this correction, two parts of the atmosphere are so different that they must be con-