INSTRUMENT FLIGHT RULES WORKBOOK
VOR navigation facilities transmit either a three-letter Morse code identifier or a voice call sign
alternating with the usual Morse code identification.
VOR station passage is indicated by:
TO/FROM indicator switches from "TO" to "FROM" (primary indication),
VOR needle rotates 180º, and
Course (CDI) oscillates from side to side.
VOR operation is based on the phase difference between two signals transmitted from a VOR
facility (Figure 1-4). The reference signals, or the omnidirectional signal, radiates from the
station in a circular pattern. The phase of the signal is constant. The other signal rotates
uniformly and its phase varies at a constant rate. Magnetic North is used as a baseline for
electronically measuring the phase relationship between the two signals. At Magnetic North, the
signals are in phase; however, a phase difference exists at any other point around the station and
is electronically measured by the aircraft's VOR receiver. The VOR provides 360 magnetic
courses called radials, which radiate from the station like spokes from the hub of a wheel.
Unlike the NDB system, the VOR system is not affected by lightning or severe weather. The
VOR system is also more accurate, but is limited to line-of-sight reception which varies with the
altitude of the aircraft and elevation of the transmitter. Normal range is 40 NM at 1000 feet
AGL; this range increases with altitude.
The line-of-sight transmission pattern from VOR and TACAN facilities creates an area directly
over the facility where signal reception is very weak or impossible. This area where navigation
information is unreliable is called the cone of confusion. The cone of confusion affects course
information received from either a VOR or TACAN station.
1-6 INTRODUCTION TO AIRBORNE NAVIGATION AND COMMUNICATIONS
EQUIPMENT AND PRINCIPLES OF OPERATION