Doppler vor ground equipment

VOR stands for "VHF Omnirange" a term used to describe a system of navigation using very-high-frequency omnidirectional radio beacons for determining the course of an aircraft. This system provides good bearing information over flat terrain but if the installation is sited, because of airway requirements, near reflecting objects, or the receiving aircraft flies over mountainous territory, errors occur due to propagation disturbances. These disturbances can be effectively reduced by increasing the antenna base line but this leads to bearing ambiguity. Doppler VOR enables the antenna base line to be extended whilst retaining unambiguous bearings. Trials using this technique show, in comparison with conventional VOR, a remarkable improvement in course information even in difficult sitting terrain or where the receiving aircraft is flying over a mountainous route.