GPS technique has been widely applied in use for testing airborne radio navigation system as location and velocity references. In case of testing TACAN, the range information from TACAN is not equal to the reference range result from GPS position location due to tropospheric refraction. A scheme for correcting for the effect of atmospheric refraction is required. In this paper, a formula which can be used to compute the refraction error is derived on the basis of ray-tracing. In addition, an algorithm which obtains an approximate solution of the formula is presented. The performance of the method is illustrated with numerical examples.