Statistical assessment of the variability of atmospheric propagation effects in the southern California coastal area

The effect from horizontal inhomogeneity in the atmospheric refractive structure on radio frequency (RF) propagation is a current subject of discussion. Meteorological measurements indicating range dependent refractive structures have been documented by several observers. A central concern arising from these observations is the effect the assumption of horizontal homogeneity has on the accuracy of propagation estimates, where accuracy is defined as the difference between estimated and actual propagation effects, typically pathloss. Whether the environment can be considered homogeneous is dependent upon the accuracy requirements of the application and the accuracy of pathloss estimates developed from environmental inputs, i.e. what is the accuracy obtained assuming horizontal homogeneity, how much does that accuracy improve with the inclusion of range dependent refractive structures, and what is the reduction in accuracy of either regime as a function of the time since the observation used for developing estimates. The Variability Of Coastal Atmospheric Refractivity (VOCAR) experiment was designed to assess the effects of spatial and temporal variability of the atmospheric refractive structure in the southern California coastal (SoCal) area on RF pathloss. Three months of RF pathloss measurements on two ~130 km over-water, over-the-horizon paths having a common endpoint but on radials separated by 120 degrees are used to assess the effects of spatial and temporal inhomogeneity on RF pathloss. Mid-path atmospheric soundings from a four day period are compared to measured pathloss values to examine the errors arising from the assumption of horizontal homogeneity. The increase in error when the path is moved away from the point of the sounding is also analyzed and compared