Some Applications of the Monthly Median Refractivity Gradient in Tropospheric Propagation

A consistent correlation has been found between the monthly median values of 100mc transmission loss and an atmospheric parameter ΔN which is determined from standard radiosonde observations. ΔN is defined as the difference between the refractivity at the earth's surface and at one kilometer above the earth's surface. ΔN is determined from the midpoint of the propagation path and is taken to represent an effective gradient of the refractive index. It is found to yield correlation coefficients with transmission loss of about 0.7 even in the far scattering region. This correlation is also used to derive estimates of the annual, geographic, and terrain variances of the transmission loss. Six-year average values of NΔ are presented for the United States and can be used as an aid in the prediction of the annual cycle of 100 mc transmission loss. The possibility of using surface observations of N for times of day other than the radiosonde observation hours is examined and found to be encouraging. One of the major conclusions is that the observed dependence of transmission loss upon ΔN is five times greater than that indicated by standard propagation theory.