Site error is caused by uneven terrain such as hills and man-made structures, trees and even long grass, in the vicinity of the transmitter. The error to radiated bearings is termed ‘VOR course-displacement error’. Ground VOR beacon site error is monitored to ± 1° accuracy. Propagation error is caused by the fact that, having left the VOR site with ±1° accuracy, the transmissions are further affected by terrain and distance. At considerable range from the VOR, ‘bends’ or ‘scalloping’ can occur. VOR scalloping is defined as an imperfection or deviation in the received VOR signal. It causes the signal to ‘bend’ as a result of reflections from buildings or terrain; it causes the Course Deviation Indicator to slowly or rapidly shift from side to side. Airborne equipment errors are caused by aircraft equipment assessing and converting the phase differences to 1° of bearing; maximum aircraft equipment error should be ± 3°.
The above errors are aggregated to give a total error of ± 5°. In addition there is pilotage error due to the fact that as an aircraft approaches the VOR the 1° radials get closer together.
The Cone of Ambiguity
As the VOR is approached, the radials converge and the VOR needle becomes more sensitive. Near the VOR overhead the needle oscillates rapidly and the ‘OFF’ flag may appear momentarily; also the ‘TO/FROM’ display alternates. This is all caused by the cone where there is no planned radiation. This is known as the cone of ambiguity or confusion. Once the aircraft has flown through this cone the readings at the aircraft will stabilize.
The VOR shall provide signals to permit satisfactory operation of a typical aircraft installation at the levels and distances required for operational reasons, and up to a minimum elevation angle of 40°. In practice, modern VOR beacons are capable of providing usable signals within 60° to 80° above the horizon.