How Do Pilots Navigate?

Have you ever wondered how jet transport aircraft know how to get where they are going? There are no road signs in the sky, and even if there were, they would be hidden behind clouds. So how do pilots know how to guide aircraft around the world?

The most common type of electronic navigation system uses a Very High Frequency Omnidirectional Radio Range, or VOR, which can be explained by using a lighthouse metaphor. Imagine a light beam timed to pass through due north every minute exactly on the minute. At this precise instant, an omnidirectional strobe light atop the lighthouse is sequenced to flash. If one sees the flash and beacon simultaneously, they must be exactly to the north of the station. If the delay between the light flashes is 15 seconds, they are due east, 30 seconds, south, and so on. In actuality, the VOR uses two Very High Frequency (VHF) radio signals of differing phase shifts to accomplish the task of the lights. VOR is the backbone of the National Airspace System's network of airways, the highways in the sky used by airliners as well as by general aviation airplanes. Military aircraft use a similar system on the Ultra High Frequency (UHF).

When close to its destination airport, an aircraft uses an instrument landing system (ILS) similar to the VOR, which can provide guidance right down to the runway. The pilot is provided headings, called vectors, from air traffic control radar facilities when transitioning from en route airways to the ILS final approach course. This precision guidance tells the pilot not only whether the aircraft is to the left or right of centerline but also whether it is too high or too low in the glide path (glide slope). The radar controllers also keep aircraft flying in the clouds a safe distance apart.

The most recent revelation in navigation is the Global Positioning System (GPS), which uses a network of orbiting satellites. The receiver measures the time it takes for the signal to reach the aircraft from the satellite. Because the signal travels at the speed of light, the distance from the satellite can be computed. By analyzing signals received from several satellites, an exact three-dimensional position can be determined anywhere in the world, to within a matter of feet.

Airliners also use a gyroscopic system of navigation known as the inertial navigation system (INS). The pilot initializes this system by programming the latitude and longitude of the starting airport. The INS, which can detect the aircraft's motion and provide guidance to the destination, requires no external signal to operate. The self-contained nature of the INS is essential to transcontinental aircraft as there are no VOR stations in the middle of the ocean.

In time, GPS will most likely become the primary means of electronic navigation. As the cost of GPS receivers continues to decrease, their applications will continue to increase. Imagine a car telling you when to turn, or even a grocery cart guiding you to the aisle where the pickles are. Ancient mariners steered their ships by celestial navigation. Five hundred years later we again find ourselves looking to the stars for guidance.

Return to Feature Articles