VOR Navigation

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VOR’s derived from the old 4-course radio range from the late 1920’s and 1930’s
Gained widespread use for navigation in the 1950’s
Made instrument navigation commonplace
Remain the basis for most of the world’s air navigation systems- and will be for 5-10 yrs.
There are about 3,000 VOR stations worldwide
Just under 1,000 of those are in the US

Frequency range from 108.0 to 117.95 MHz
Uses even tenths
ILS uses odd tenths
Different power outputs depending on service volumes
Restricted to line of sight
Stations are orientated to mag north
Transmit 360 courses To or From

VOR Types
Azimuth only
No distance information
Gives both azimuth and distance
TACAN is the military version
Provides both azimuth and distance
Specs are a little different
Can be portable, set up in an hour
3 times more accurate than VOR
Combination VOR and TACAN
Gives both azimuth and distance to both military and civilian users


VOR Basics
Courses orientated FROM the station are radials
Not influenced by aircraft heading
Morse code is used to identify the station
Voice ID may be used by some
If no voice, the class identifier is VORW (without voice)
If under maintenance the T-E-S-T (-. …-) code may be radiated
No code at all may indicate maintenance as well

VOR Basics
Power output
Standard output is 200 watts
From the unit generally + or – 1°
Shows up as an oscillation on the CDI
Usually denoted in the AFD
Some receivers are more susceptible than others
Propellers and rotors may cause an oscillation to develop as well
Change RPM to smooth out the needle
Especially when doing the VOR accuracy checks

VOR Basics
Influenced by lots of factors
Aircraft altitude
Class of the facility
Useable area (line of sight)
If used for off airway nav VORs range is 40 NM
So VOR stations must be no more than 80 NM apart
Automatic monitoring
Automatically turns off the VOR if defective
Turns on the standby transmitter
156 Miles!

VOR Basics
Standard Service Volume (SSV)
These are standard values
They do not apply to published IFR routes and procedures
The signal can be certified to a greater range and or coverage area when needed
SSV restrictions are published in the NOTAMS first then in the A/FD
There are 3 service volumes:
The designation may be found in the A/FD
The terminal VOR has a (T) in the frequency box


VOR Components
Antenna, Receiver, VOR head
Omnibearing Selector (OBS)
Course Deviation Indicator (CDI)
To/From Indicator or Ambiguity Indicator
HSI – combo HI and CDI

Operational Errors
Careless tuning and identification
Failure to check receiver for accuracy
Turning the wrong direction
Failure to check the correct TO/FROM
Reverse sensing
Failure to parallel the course
Overshoot and undershoot
Over controlling during tracking
Chasing the needle
Misreading station passage
Failure to bracket the course correctly for wind correction

VOR Checks
VOR check has to be done within 30 days
When checking, center the needle and read the bearing error on the OBS
In addition swing the CDI 10° right and left and see if the bearing matches the movement
Turn the OBS 180° verify the TO/From flag flips and centers on the reciprocal
Record the date, place, bearing error and signature
Radiates one radial, the 360
May be done airborne or on the ground
A/FD will indicate if ground (G) or airborne (A)
Locations listed in the A/FD
Usually uses 108.0 for the frequency
Certified ground checkpoints
Certified airborne checkpoints
Homemade airborne checkpoints
More than 20 miles out
Over a prominent landmark
One system against the other

Reading the VOR
Each dot is 2 degrees
Station passage is considered to be the first full reversal of the ambiguity indicator
When setting a VOR course use the add/subtract method to eliminate errors
For example to set a bearing of 216, add 6 to 210 and subtract 4 from 220
Full scale deflection indicates 12 degrees or more off course
If there is no TO/FROM flag:
You are over the station (cone of confusion)
You are 90 degrees to the set course
You may not be receiving a signal

Reading the HSI
Pretty much the same as the VOR except for the ambiguity indicator
The triangle always points TO
The rules for absence of the ambiguity indicator are the same:
You are over the station
You are 90 degrees to the set course
You may not be receiving a signal

Radio Wave Primer
Radio Waves – radiated energy that in free space travels in straight lines, and at the speed of light (186,000 miles per second).  Radio waves have the same properties as light and heat waves, but are in lower frequency.
Radio Wave Cycle – the interval between any two points that measures the completion of a single wave movement.
Wave Length – The actual linear measurement, in meters, of one wave.
Amplitude – The strength or width of one wave—the greater the distance from the transmitting site, the smaller this distance.
Frequency-The number of cycles per second, expressed in three units—Kilohertz (KHz), which measures the number of cycles per second, Megahertz (MHz), which measures millions of cycles per second, and Gigahertz (GHz), which measure billions of cycles per second.

Basic Radio Wave Stuff
The transmitting antenna converts electric current to electromagnetic radiation which travels to the airplane’s antenna which converts it back to electrical current
The earth, trees, buildings, air, water, dust and mineral deposits decrease the strength of a radio wave

Wave Types
There are 3 types of waves:
Ground wave
These travel along the surface between the earth and the ionosphere
The lower the frequency the farther it will travel
100 Hz to 1000 kHz (1MHz)
Great for navigation, very predictable
NDB’s live here
Sky wave
Refracted by the ionosphere back to earth
1 MHz to 30 MHz
High frequency radio lives here
Not for navigation
The height of the ionosphere changes with the sun’s radiation and is therefore variable
80 to 90% reliable
Replaced by sat radios
Space wave
15 MHz and above
Most of our nav stuff lives here
Is reflected by hard surfaces
ILS critical areas are the result, as well as propeller interference on the VOR
Line of sight

How does it work?
VOR’s broadcast 2 signals
One antenna rotates 30 times a second and is directional
This means the phase varies as it rotates through 360°
The other signal is omnidirectional meaning the same in all directions
The 2 signal’s phase difference is compared to each other and the difference is measured
The receiver in the airplane determines bearing by phase comparison

Rule of 60
At 60 miles 1° = 1 mile
So if you’re 10° off your 10 miles off course
At 30 miles 1 dot = 1 mile
At 30 miles 10° = 5 miles
At 15 miles 10° = 2.5 miles
Take a circle with a circumference with 360 miles
To get the radius you actually get 57.3

So 1 degree around the circle gives you 1 mile
This comes up over and over again especially with DME arcs

Rule of 60
Another way not involving mental math is to use the equation
200 feet per dot per NM
So lets say your 2.5 dots off 10 NM out
200 ft x 2.5 = 500
500 x 10 NM = 5000 feet
Same deflection at 20 Nm
200 x 2.5 = 500
500 x 20 = 10,000
Obviously this would be hard to work out in the plane so the 60, 30, 15 rules of thumb work better there

The Radio Magnetic Indicator is invaluable for DME arcs
The tail of the needle tells you what radial you are on
The rotating card tells heading at the top
The head of the needle points to the station
In this case turn to a heading of 270 to go TO the VOR
So we are on the 090 radial east of the station

AKT Problems
First step: find out what radial from the station you are indicating
This tells you what quadrant you are in
Second step: determine how far off that radial you are if the CDI isn’t centered
Third step: examine the answer choices, 1 should make sense, if 2 answers are possible, repeat the process looking for your error

These are just plain mean
No triangle
Look at the heading first
Use process of elimination

Check the heading
Find the radial
Which side of the plane would the radial be if you paralleled the course?

Time/Distance Checks
Wing tip bearing change method
This formula uses elapsed time over a predetermined bearing change
Tune and identify
Set HI
1. determine the radial you are on
2. turn inbound and re-center the needle if necessary
3. turn 80 degrees right or left and rotate the OBS 10 degrees opposite the direction of turn
4. maintain heading when CDI centers, note the time
5. maintain the same heading rotate the CDI 10 degrees ahead
6. note the elapsed time when CDI again centers
7. use the formulas:

Time/Distance Checks
Isosceles Triangle Method
If the 2 angles of a triangle are equal the length of the sides are also equal
1. with the aircraft established inbound on a radial rotate the OBS 10 degrees left
2. turn 10 degrees right and note the time
3. maintain a constant heading until the CDI centers, note the elapsed time
4. time to the station is the same as the time taken to complete the 10 degree change of bearing

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