ABSTRACT
The early use of aircraft radio navigation was based on low/medium frequency (LF/MF) transmissions. During the mid to late 1940s, it was evident that increased accuracy and reliability were needed. Since radio communication systems based on very high frequency (VHF) were being successfully deployed, a decision was made to develop a radio navigation system based on VHF. This system became the VHF omnidirectional range (VOR) system; a system that is in widespread use throughout the world today. VOR navigation was the basis for the network of navigation 'airways'. Locations of conventional VOR (CVOR) ground stations have to be carefully planned to take into account local terrain and obstacles. The Doppler effect is applied to the second-generation version of VOR ground transmitters. The DVOR ground station has an omnidirectional transmitter in the centre, amplitude modulated at 30 Hz; this is the reference phase.
