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Commercial windmill-driven power turbines ("wind turbines") are expanding in popularity and use in the commercial power industry, since they can generate significant electricity without using fuel or emitting carbon-dioxide ldquogreenhouse gasrdquo. In-country and near-off-shore wind turbines are becoming more common on the European continent. The United States has recently set long-term goals to generate 10% of national electric power using renewable sources. In order to make such turbines efficient, current 1.5 MW wind-turbine towers and rotors are very large, with blades exceeding 67 m in diameter, and tower heights exceeding 55 m. Newer 4.5 MW designs are expected to be even larger. The problem with such large, moving, metallic devices is the potential interference such structures present to an array of civilian air-traffic-control radars. A recent study by the Undersecretary of Defense for Space and Sensor Technology acknowledged the potential performance impact wind turbines introduce when located within line of site of air-traffic-control or air-route radars [Report to the Congressional Defense Committees on The Effect of Windmill Farms On Military Readiness, 2006]. In the spring of 2006, the Air Force Research Laboratory embarked on a rigorous measurement and prediction program to provide credible data to national decision makers on the magnitude of the signatures, so that the interference issues could be credibly studied. This paper, the first of two parts, will discuss the calibrated RCS measurement of the turbines, and compare this data (with its uncertainty) to modeled data.