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Previous studies of range and angular errors caused by variation of atmospheric refractivity with altitude generally have used functional expressions, averaged data, or data from a small number of radiosonde ascents. In this investigation a large data base was employed consisting of radiosonde measurements made at intervals of about 2 hr for a period of 1 week. In the analysis it was assumed that the refractivity varied only as a function of altitude and that there was no ducting. Curves showing the deterioration of range correction with time for selected terminal points were developed by tracing rays at a succession of elevation angles. Thus, for a ground range of 416.7 km and an aircraft at 15.24 km, the standard deviation of the ground-to-plane radio distance 4 hr after a radiosonde ascent is .9 m, increasing to 1.3 m after 8 hr, and 1.6 m after 12 hr.