The airway system is based primarily on the very high frequency omnidirectional range (VOR). This extensive system consists of several hundred ground stations that transmit navigation track guidance signals used by aircraft in flight.
The VOR navigational system has many advantages for the IFR pilot. The VOR transmits in the very high frequency range of 108.1 through 117.95 MegaHertz (MHz); therefore, it is relatively free from precipitation static and annoying interference caused by storms or other weather phenomena. Accuracy is another advantage: a track accuracy of plus or minus 1° is possible when flying a VOR radial. Wind drift is compensated for by flying to center the track bar indicator.
VOR signals are transmitted on line-of-sight. Any obstacles (buildings, mountains or other terrain features, including the curvature of the earth) block VOR signals and restrict the distance over which they are received at a given altitude. This can result in a sudden scalloping fluctuation of the cockpit indicators -normally for short time intervals. Certain terrain features may produce areas where VOR navigation signals are unusable, so every instrument pilot making an "off airways" flight should be aware of the restrictions along the route.