VOR error tolerance, stability, and accuracy

The March, 1965, issue of IEEE Transactions on Aerospace and Navigational Electronics published some experimental results of monitoring VOR radial stability which the author had conducted at Ohio University. The data raised some questions concerning VOR error tolerance and stability. During the past 18 months we at Ohio University have continued to collect and analyze VOR data, and we are now in a position to provide the explanation for the phenomena observed. Since alluding to VOR difficulties caused some concern, I am using this mechanism to disseminate our further findings. The original Ohio University momtor system was operated at a diffraction site. This has been augmented to include three line-of-sight (LoS) stations ranging from 150 feet to 40 miles from the VOR transmitter. Receiving equipment was upgraded from that previously used to include Bendix RA21/MN21B VOR receiving systems calibrated by a Collins 479-S3 signal generator. The causes of the four types of disturbances discussed in the original article have been determined and are discussed.