VOR (VHF Omnidirectional Range) Navigation System Overview

Political FundaSeptember 24, 2021

VOR Navigation System

One of the oldest and most useful navigational aids for aircraft is the VOR system. The
system was constructed after WWII and is still in use today. It consists of thousands of land-
based transmitter stations that are called VORs. The ground VOR stations communicate with
radio receiving equipment on board aircraft.
How VORs Works?
The position of all VOR stations is marked on aeronautical charts along with the name of the
station, the frequency of the station which an airport can tune to use, and also a Morse code
designation for the station.
VOR uses VHF (Very High Frequency) radio (frequency range 108–117.95 MHz) with a 50 kHz
separation between each channel. This keeps atmospheric interference to a minimum but limits
the VOR to line of sight usage. To receive VOR VHF radio waves, generally a V-shaped,
horizontally polarized, bi-pole antenna is used by aircraft. Other types of antennas are also
certified. The manufacturer’s instructions for installation location must be followed.
The signals emitted by a VOR transmitter travel 360 degrees around the unit and are used by
aircraft to navigate to and from the station using an onboard VOR receiver and display
equipment. Because the signal from a VOR station propagates in all directions, a pilot does not
need to fly a pattern to intersect it. The radio waves are received regardless of the aircraft's
direction of travel as long as it is within range of the ground unit.
VOR/DME Ground Station

A VOR transmitter sends out two signals that a plane's receiver uses to figure out where it is in
relation to the ground station. One of the signals is a reference signal. The second is made by
electronically spinning a variable signal. The variable signal is in phase with the reference signal
when it is at magnetic north, but as it rotates to 180°, it becomes increasingly out of phase. As it
spins to 360° (0°), the signals become increasingly in phase, until they are once again in phase at
magnetic north.The aircraft's receiver decodes the phase difference and calculates the aircraft's
position in degrees from the VOR ground station. Most planes have two VOR receivers.
Types of VORs Receivers
VOR Receiver

VOR receivers are sometimes found in the same avionics unit as the VHF communication
transceiver (s). NAV/COM radios are what they're called. Internal components are shared since
their frequency bands are close together. Larger aircraft may have two dual receivers, as well as
two antennas. Normally, one receiver is chosen for usage, while the other is tuned to the
conditions encountered along the way. There is a button for selecting the active or standby
frequency, as well as a way to switch between NAV 1 and NAV 2.
Instrument landing systems (ILS) and glideslope receivers are also used in conjunction with
VOR receivers. The bearing in degrees to (or from) the VOR station where the signals are
generated is interpreted by a VOR receiver. It also generates Dc power, which is used to power
the display indicating the deviation from the planned course centerline to (or from) the chosen
station. In addition, the receiver determines whether the aircraft is flying toward or away from
the VOR.
VOR Display Function
VOR Signal Propagate in all Direction

These things can be exhibited in a variety of ways on a variety of instruments. An older aircraft
may be fitted with a VOR gauge that displays just VOR information.
An omni-bearing selector (OBS) or a course deviation indication is another name for this device
(CDI). The CDI linear indicator is essentially vertical, but it moves left and right across the
graduations on the instrument face to reflect deviation from the target. Each degree is
represented by a graduation. The azimuth ring is rotated by the OBS knob. The pilot spins the
OBS until the course deviation indication centres when in range of a VOR. The OBS can be
rotated to two settings where the CDI will be centred on each aircraft location.
One causes an arrow to appear in the TO window of the gauge, indicating that the plane is
approaching the VOR station. The other possible bearing is at an angle of 180 degrees to this
one. When this option is selected, an arrow appears in the FROM window, showing that the
aircraft is travelling away from the VOR on the set course. To fly directly to or from the VOR,
the pilot must guide the aircraft to the heading with the CDI centred. The phase connection
between the two simultaneously broadcast signals from the VOR ground station is used to create
the displayed VOR information. A NAV warning flag appears when power is lost or the VOR
signal is poor or interrupted.
VOR Cockpit Display
VOR Display on HSI/EHSI

VOR Dispaly

A separate gauge for the VOR information is not always used. As flight instruments and
displays have evolved, VOR navigation information has been integrated into other instrument
displays, such as the radio magnetic indicator (RMI), the HSI, an EFIS display or an electronic
attitude director indicator (EADI). Flight management systems and automatic flight control
systems are also made to integrate VOR information to automatically control the aircraft on
its planned flight segments. Flat panel MFDs integrate VOR information into moving map
presentations and other selected displays.
The basic information of the radial bearing in degrees, course deviation indication, and to/from
information remains unchanged, however. At large airports, an instrument landing system
(ILS) guides the aircraft to the runway while on an instrument landing approach.
The radio signals are decoded using the aircraft's VOR receiver. On the same instrument display
as the VOR CDI display, it produces a more sensitive course deviation signal. The localizer is a
component of the ILS that is detailed in the previous article. The VOR circuitry of the VOR/ILS
receiver remains dormant while tuned to the ILS localizer frequency. At VOR stations, the VOR
transmitter is frequently combined with distance measuring equipment (DME) or a
nondirectional beacon (NDB), such as an ADF transmitter and antenna. Pilots can use the VOR
and DME together to achieve a precise fix on their location when combined with a DME.
The pilot is relieved of needing to fly over the station to know with certainty his or her location
because the VOR shows the aircraft's bearing to the VOR transmitter and a co-located DME
indicates how far away the station is. The next sections go through each of these navigational
aids independently. The operational precision of VOR technology is crucial to flight safety. VOR
receivers are put through their paces utilising VOR test facilities (VOT). These can be found at a
number of airports, which can be found in the Airport Facilities Directory for the area in
question.
VOR Testing
To conduct the test, specific points on the airport's surface are designated. Most VOTs
necessitate adjusting the VOR radio to 108.0 MHz and focusing the CDI. On the indicator, the
OBS should show 0° when showing FROM and 180° when showing TO. The test heading
should always indicate 180° if an RMI is utilised as the indicator. although not on 108.0 MHz.
Distance Measuring Equipment (DME) overview & functions
Political FundaSeptember 22, 2021

Distance Measuring Equipment (DME)

 The distance between the aircraft and the DME unit at the ground station is calculated by a
DME system and displayed on the flight deck. When the aircraft is flying to the station, it can
also show the calculated aircraft speed and elapsed time for arrival.

DME Components
DME ground stations have subsequently been installed at VOR stations as well as in conjunction
with ILS localizers. These systems are known as VOR/DME and ILS/DME or LOC/DME.
The ILS/DME system used during approach to the runway during landings. The DME system
consists of an airborne DME transceiver, display unit, and antenna (in aircraft), as well as the
ground based DME unit and its antenna.
How DME works?
The DME is very helpful because a pilot can positively identify the location of the aircraft using
the bearing from the VOR and the distance to a known point ( from DME antenna at the VOR to
the distance from aircraft antenna). DME (Distance Measuring Equipment) operates in the UHF
frequency range from 962 MHz to 1213 MHz. A carrier signal wave transmitted from the aircraft
unit is modulated with a string of integration pulses. The ground DME unit receives the pulses
and returns a signal to the aircraft. The time that transpires for the signal to be sent and returned
between aircraft & ground unit is calculated and converted into nautical miles for display. The
time to reach and the aircraft's speed are both computed and displayed on a dedicated DME
display, as well as an EHSI, EADI, EFIS, or the primary flight display in a glass cockpit.

DME Slant & Actual Distance

The co-located VOR or VORTAC frequency is coupled with the DME frequency. The DME is
automatically tuned when the VOR signal is tuned to the correct frequency. The VOR station
identification tones are broadcast first, followed by the DME tones. While the VOR selector is
tuned to a different VOR, the hold selector on a DME panel keeps the DME tuned in. In most
situations, a small blade-type antenna mounted to the underside of the fuselage centerline
transmits and receives the DME's UHF.

VOR/ DME Ground Location

The distance between the DME transmitter antenna and the aeroplane is displayed on a typical
DME, The slant distance is what it's called. It's quite accurate. The distance to the DME ground
antenna from a position directly beneath the aeroplane is shorter since the aircraft is at altitude.
Some newer DMEs can calculate and show this ground Distance.