Tropospheric Range Error at the Zenith

The atmospheric range error in an electromagnetic signal arriving vertically at the earth is measured by the height integral of the refractivity N through the atmosphere. (N = 10^ (n-1) where n is the index of refraction.) N in the non-ionized atmosphere is the sum of "dry" and "wet" components, which are considered separately. Only the dry part is important at optical wavelengths,and the present discussion deals principally with this dry part. Data from several thousand meteorological balloons were studied to develop a method for predicting the height integrals of Njjj.y. It is found theoretically and from observed data that,although N^ry depends on both temperature and pressure, its height integral is a linear function of surface pressure only. The slope is nearly the same, worldwide; there is, however, a very small but detectable latitude variation, corresponding to the latitude variation of g. It is shown that the vertical range error (dry component) at a given latitude can be predicted within 2 mm rms from surface pressure alone. Irregularities in the temperature profile do not affect the prediction in the zenith direction. In view of these results, the equivalent height for a quartic N^yy model must vary as surface temperature; it is approximately 40 km at 0C and becomes zero near 0K at all locations. Possible small differences between parameters for different stations are being examined