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Aircraft Navigation
Navigation Introduction
VHF Omni Range
Automatic Direction Finder
Long Range Navigation
Global Positioning System
VOR (VHF Omni-Range) is the basic Electronic navigation that in use today . This VHF Omni-Range navigation method relies on the ground based transmitters which emitted signals to VOR receiver. The VOR system operates in the VHF frequency band , from 108.0 to 117.95 MHz. The reception of VHF signals is a line of sight situation . You must be on the minimum altitude of 1000 feet (AGL) above ground level in order to pick up an Omni signals service range.
VOR Range
VOR Class= Low Altitude       1,000-18,000 feet     Range 40 nautical miles
VOR Class=High Altitude       1,000-14,500 feet     Range 40 nautical miles
VOR Class=High Altitude      14,500-60,000 feet     Range 100 nautical miles
VOR Class=High Altitude      18,000-45,000 feet     Range 130 nautical miles
The VOR facility at ground base transmits two signals at the same time. One signal is constant in all directions as a reference phase. Another signal, it is variable-phase signal and it rotates through 360 degrees, like the beam from the lighthouse. Both signals are in phase when the variable signal passes 360 degrees (reference to magnetic north) and they are 180 degrees out of phase when the rotating signal passes 180 degrees The aircraft equipment receives both signals. The receiver will calculate the difference between the two signals, and interprets the result as a radial from the station to pilots on the aircraft.

RADIALS: The two signals from VOR transmitter generate 360 lines like spokes in a wheel . Each line is called a Radial . VOR navigation equipment on the airplane will determine which of those 360 radials the airplane is on.
VOR Indicator
A : Rotating Course Card is calibrated from 0 to 360 degrees, which indicates the VOR bearing chosen as the reference to fly by pilot.
B : Omni Bearing Selector or OBS knob , used to manually rotate the course card to where the point to fly to.
C : TO-FROM indicator . The triangle arrow will point UP when flying to the VOR station. The arrow will point DOWN when flying away from the VOR station. A red flag replaces these TO-FROM arrows when the VOR is beyond reception range or the station is out.
D : Course Deviation Indicator (CDI). This needle moves left or right indicating the direction to turn the aircraft to return to course.
DOT : The horizontal dots at center are represent the aircraft away from the course . Each dot represent 2 degrees deviate from desired course.
How It Works
The followings are just the typical, some aircraft may be vary in details . The pilot can set VOR receiver to selected ground station or another word is to select a radial to define a magnetic course toward or away from VOR station on receiver. The Radial of the VOR receiver is divided into 360 degrees, at the point 360 is representing Magnetic North . When we called out , we called in three digits such as 090 that means on the East and 270 means on the West .

The proper time to tune navigation receivers is while the aircraft is on the ground because the pilot has to do the flight planned and known where to go. After takeoff, usually start from altutude of 1000 feet minimum above ground level, the VOR receiver will get signals from transmitter and the flag will show arrow FROM (left picture).
When the aircraft has gone half way or close to next VOR station and VOR receiver got that signals from next station . The arrow flag will change from FROM to TO arrow (from right picture) . At this time, pilot should select OBS to Radial of next VOR station.

CDI on the indicator shown off center by four dots and that means eight degrees off the course, the pilot must correct the heading of aircraft.
If the aircraft out of transmitter range or VOR station not operates, the VOR receiver will show red flag or indication to tell pilot that don't misunderstand because CDI needle will stay at center all the time.

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