The establishment of a radio refractivity data base for Southern Africa

The authors discuss the use of radiosonde data to produce a radio refractivity database for Southern Africa. The data consist of values for nine stations situated over Southern Africa; data for six years were analyzed. Lag corrections have not been applied on the data because of differing radiosonde design; geopotential heights are used instead of height calculated from the ascent rate. It is noted that the calculation of ΔN, the gradient of the atmospheric refractive index from ground level to an altitude of 1 km, and the Earth radius factor k should be found at 200 m and not on 1 km, since radiosystems operate in the first 200 m above ground level. Following CCIR norms, values of refractivity were calculated at surface level and at 1 km above surface and the ΔN derived from these values. The extent to which this method is a misleading approximation of reality is noted