A climatology-based model for long-term prediction of radar beam refraction

Abstract

Hitherto, radar beam bending has been predicted using four-thirds Earth or standard atmosphere. A new and more accurate model has been developed using a mix of ray tracing and climatology. Usually a microwave beam traveling through the atmosphere bends towards the Earth with a radius of curvature greater than the Earth's surface. However, seasonal and climatic variations influence the amount and direction of bending, and at times create temperature or moisture inversions that tend to redirect the energy along the Earth's surface leaving gaping holes where there is no coverage. In this work, iterative ray tracing is used to determine the most direct path from the radar to the target through the climatologically predicted refractive atmosphere. Height measurement error is calculated by comparing the geographic path to the refracted path. Only vertical refractivity variation is taken into account, and the effects of ducting and exponential refractivity are both modeled. As a test, the model computed height errors at 17 locations world-wide for a hypothetical target at 10000 feet and 60 nautical miles. The predicted errors varied from 100 feet to 2260 feet as against the standard atmosphere predicted height error of 804 feet. The model traces to all targets when no ducting is modeled, to all targets outside the duct with surface ducting, and to some targets outside the duct with elevated ducting. In the remaining cases, adjacent rays sometimes cross, causing ambiguity in the estimation and, usually, tracing failure.