VOR Principles of Operation -Cont.

CHAPTER ONE

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 180, 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