Effects of the variability of atmospheric refractivity on propagation estimates

The temporal and spatial sampling requirements of atmospheric refractivity used to calculate electromagnetic (EM) propagation effects (propagation factor, equivalently path loss, etc.) is a subject of debate. Results are presented from a propagation experiment designed to address these questions. The objective in propagation estimation is assumed to be the minimization of the error of propagation estimates over the entire time interval for which individual estimates are used. It is found that the minimum error of propagation estimates over the interval of their use can be calculated from radio measurements. The error of propagation estimates calculated from horizontally homogeneous refractive structures is compared to the minimum error. It is found that for beyond-line-of-sight overwater EM propagation factor calculations at VHF and UHF in the southern California coastal region, the assumption of horizontal homogeneity leads to little more error than the described minimum error. It is also found that estimates based upon range-dependent refractive structures will provide substantially less error than estimates based upon homogeneous refractive structures only if they are sampled at intervals of two hours or less.