VOR stands for very high-frequency omnidirectional range systems. They are used for air navigation and have existed for longer than GPS. VOR systems are commonly used as a backup system in countries where the primary navigation method is GNSS navigation (global navigation satellite system). This is due to fears that the GNSS could be interfered with and sabotaged.
VOR systems are highly accurate and reliable, particularly when compared to NDBs. The estimated accuracy rate for VOR systems is ±1.4°, but 99.94% of the time this margin of error is less than ±0.35°. This means that the system is not heading sensitive.
VOR systems are used by commercial and general aircraft alike across the globe. The method of navigation used is very concise, meaning that it is easy to interpret an exact position. The bearing from the VOR station to the airborne vehicle remains the same regardless of the orientation of the aircraft and the wind speed.
The radio used, VHF, bends less around features of the landscape and varying terrains. This is a process known as course bending or diffraction. There is also far less interference on the radio signal in thunderstorms.
How many VOR stations are in the US?
VOR stations began development in the United States in the year of 1937. By 1946, they were ready to be deployed across the nation.
In 2000 there were approximately 1,033 VOR stations situated in the United States. They are gradually being phased out and replaced by GPS. By 2013, there were only 967 stations remaining in the US.
The FAA has a program that is working to decommission around 34% of the VOR stations across the United States. This is equivalent to 307 VOR stations, of which 82 have already been discontinued. By 2030 the program is set to be completed and there will be 589 VOR stations remaining across the country.
This is following a program called the MON (minimum operational network). This program aims to enhance the service volumes of the remaining ground stations to ensure the country is still covered. This system is designed to be a backup if the GPS fails or is disrupted for some reason.
How many radials does a VOR have?
VOR systems work by sending a highly directional signal using a phased antenna array from a station located on the ground. This signal rotates clockwise horizontally at a speed of 30 spins per second.
Simultaneously, a 30 Hz reference signal is emitted on a subcarrier. This is timed so that the signals are in phase as the subcarrier crosses magnetic north. This line of position is the radial of the VOR.
A VOR station has 360 different radials, each of which can be flown in 2 different directions. They all run through the center of the station before exiting. Each radial has an alignment with a certain compass degree - 0 degrees points North, 90 degrees points East, and 270 degrees points East.
The point at which the radials of 2 separate VOR ground stations intersect is used to fix the position of the aircraft in the sky. This is similar to how RDF (radio direction finding) systems operate.
To figure out which radial you are on, you will first need to tune into the signal and identify the station it is coming from. Turn the OBS until the needle has been centered and you see a FROM indicator. Look at the top of the display for a number - this is the radial that you are on.
VOR navigation is not yet obsolete. GPS has taken over and is the primary source of navigation, but many airlines and commercial planes will use VOR navigation. This is because this system makes it easier for the aircrafts to follow airways (like highways but in the sky).
VOR navigation is much more simple and intuitive to operate than GPS. It is found in almost all aircraft originating from Europe or the United States, meaning there is a way to go before the system becomes obsolete.
As mentioned above, VOR ground stations are being decommissioned by the FAA. This means that eventually the VOR navigation system will become obsolete. This is partly due to the expense required to maintain the VOR ground systems, but also because the GPS navigation system is becoming more accurate and easy to operate as technology continues to develop.
The FAA wishes to keep the ability to have full VOR coverage above 5,000 feet AGL. They wish there to still be the capacity to transition the United States using airways. They also wish to retain enough VOR ground stations to enable landings under IFR at airports within 100 nm of any location across the country, in the event of GPS and other satellite outages.