VOR Navigation in IFR 

VOR (Very high frequency omnidirectional range) was developed by the United States in 1937 and since then, has been one of the most important radio aids used in IFR flights. 

From standard instrument departures, to cruise navigation and non-precision approach procedures, VOR operations can cover the navigation of all the phases on our flight.

 

What is VOR?

A VOR is a ground based radio navigation aid.  The antennas in the aircraft receive the radio frequency emitted from the VOR and translate it to give us, via the navigation instruments, the course from the station to the aircraft in reference to the magnetic north in the position of the VOR.

A VOR receiver is an essential navigation equipment for IFR certification of our aircraft and it is mandatory for flight under Instrument Flight Rules (IFR). 

VOR instruments provide us with a precision of not more than 2º. 

While using the magnetic north as a reference is very useful for our navigation, it also makes the VOR signals change with changing magnetic north variation.

 

How does a VOR work? 

VOR emits 2 waves in the VHF range, both modulated at 30 Hz. A first reference phase signal will be emitted in all directions with a constant phase and frequency modulated (FM) at 30 Hz. A second signal will rotate clockwise at around 1.800 RPM, at such speed that it will emit 1 complete wave from 0º to 360º in one revolution and will be amplitude modulated (AM). This is called the variable phase signal. Thus, if we are North of our station, the second wave our aircraft will detect will be 0º, while if we are South, it will be 180º. Then, our VOR receiver will compare the phase of both waves and it will deduce the radial we are currently navigating.It will also transmit a code identification signal or COM/ID, modulated at 1.020 Khz which will carry the morse code associated with the 3/4 letters identifier code assigned to the station. 

VOR will operate in the range of 111.975 MHZ to 117.975 MHZ with a horizontally polarised wave and increments of 50 HZs between signals. 

 

How to use a VOR?

To sintonize a VOR to our instruments, we will set the frequency from the charts in our NAV equipment.

This is a representation of how we will see the VOR of Salamanca, called BBI, in the charts.

In the NAV equipment, the frequency selection under USE coincides with the frequency of BBI (122.20).

Before using a VOR and trusting its indication for our navigation, we should listen to the morse code while comparing it to the morse code in our charts.

To listen to the morse code from the VOR, we will press the NAV button in the communication box.

VOR are also useful for establishing fixed points approach charts and Area Navigation (RNAV). A fixed point can consist either on a radial and a distance from a VOR/DME or the interception of the radials of two different VOR or VOR/DME stations.

 

VOR DME

Sometimes VOR will be equipped with DME, allowing for increased precision, advanced manoeuvres like DME arcs and a decreased MDA/H in approach procedures. 

Doppler VOR

There is a variation of VOR called Doppler VOR (DVOR). While DVOR has decreased range, it is less sensitive to multipath errors (Radio signals reaching the receiver at multiple paths due to reflection and refraction of the terrain) and has an increased precision of 0.5º, making it a better option in airports where obstacles may be a problem for radio signal propagation, with the advantage of the increased precision for approach procedures. DVOR consists of 53 typically 48-50 antenas, one in the centre and the rest of them making a circle with a diameter of 13.5-14 m, making an Alford Loop.

While for a pilot, there is no difference in using a CVOR (Conventional VOR) or a DVOR, it functions in a completely different way. The reference signal will be provided by the antenna in the center. Instead of being a fixed FM signal,  this signal will be AM. The variable signal will be provided by the rest of the antennas on the surroundings and will be FM. Of the +/- 48 antennas, only 2 will be active at the same time, and they will be just at opposite directions. The antennas will switch on and off in a circular pattern moving at a speed of 1,800 RPM.  As it rotates, Doppler effect will cause the frequency to either increase or decrease. If the signal moves away from our aircraft, it will decrease its frequency, while if it moves towards us, it will increase the frequency. Depending on our relative position to the station, the pattern of frequency variations caused by the Doppler effect will change. By comparing this to the reference signal provided, the receiver will deduce our radial. 

 

	Reference Signal	Variable Signal
CVOR	FM	AM
DVOR	AM	FM

 

Other types of VOR

En-Route VOR: Uses high-powered transmitters of up to 200W to achieve ranges of up to 200 Nm. It’s used to define airways.

Terminal VOR (TVOR): Uses low-powered transmitters to achieve ranges of about 25 NM. Typically used for approach and departure procedures.

Broadcast VOR (BVOR): Additionally transmits audio information, normally in the form of an ATIS (Automatic Terminal Information Service).

TEST VOR (VOT): Emmits a test signal for pre-flight check of VOR airborne equipment.

 

How to Calculate the Range of a VOR.

As we increase the distance to our VOR, our precision will degrade, as we need to cover more distance to change from one radial to another. To know our precision, we will use the 1/60 rule. For each 60 NM of distance, we will need to travel 1 NM to change 1 radial. If for example we are at 30 NM, we will need 0.5 NM. This rule is also very useful for calculating the length of DME arcs. 

We can also use this formula to calculate the effective range of a VOR station.

 

What Instruments To Use For VOR

The main instruments we will use for navigating with VOR will be the Horizontal Situation Indicator (HSI), Omni-bearing Indicator (OBI) and Radio Magnetic Indicator (RMI), as well as the Navigation Display (ND) of the Electronic Flight Instrument System (EFIS). 

HSI and OBI will be equipped with a CDI (Course deviation indicator), allowing us to select a specific radial and navigate it with precision. 

CDI will feature dots that will provide information on our accuracy. Each dot represents a deflection of 2º. 

VOR Procedures

VOR can be used to perform type A non-precision approaches. When planning to land with a VOR approach procedure, our MDA/H will be 300 ft and 250 ft for VOR/DME. When planning to use it as an alternate aerodrome landing procedure, as it is a type A approach, our minima will be MDA/H + 400 ft and RVR/VIS requirement +1500 m.

 

Cone of silence of a VOR

One of the inconvenients of VOR is the cone of silence. Just above the station, there will be a cone of 40º where the signal will not be reliable.

 

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