Prior to this post, we’ve highlighted airline pilots’ duties from pre-takeoff to leveling at cruise altitude. In this installment, we’ll examine a major component of the cruise leg: navigating from Point A to Point B. Unlike with ground-bound modes of transport, flight crews can’t rely on a solid network of roads or rails. However, vast as the wild blue yonder might be; the national airspace system offers several types of navigational assistance.
Methods of Navigation
Air Traffic Control (ATC)
The best-known method of navigational aid, ATC provides radar & communication services to all instrument flight rules (IFR) aircraft, which includes all airline traffic. Through transponder and/or ADS-B signals (see Avoiding Other Aircraft Part II), ATC can easily identify aircraft and provide navigational assistance. Many times, controllers instruct aircraft to fly to a known point using onboard navigation. At other times, they’ll provide pilots with directional headings to fly, a method known as vectoring traffic.
VOR: The VHF omnidirectional range (VOR) network has been the backbone of America’s air navigation system since the 1960s. This network consists of hundreds of VHF stations scattered across the country (and abroad). Each VOR emits a signal that can be used to navigate to/from the station on any of the 360 (1̊ each) radials surrounding the unit. Many VORs are also equipped with distance measuring equipment (DME), and thus provide mileage as well as bearing to/from the station.
While VORs are multifunctional and easy to use, this antiquated network is not without its drawbacks. Chief among its limitations is the finite range of VOR signals. Of the three classes of VORs, the maximum guaranteed range (with certain exceptions) is 130 nautical miles. While that might sound like a significant range, many airliners can cover that distance in about 15 minutes. The second major drawback is the need to operate directly to/from VOR stations (in most cases). While such navigation is simple, navigating by VORs rarely results in a direct course from departure to destination. Despite these shortcomings, VORs have reliably upheld the national airspace system for more than half a century.
GPS: Since the mid 1990s, the global positioning system (GPS) has significantly modernized aerial navigation practices. GPS lacks the range limitations associated with VORs while also allowing point-to-point (i.e. direct) navigation to virtually anywhere on earth. Additionally, after continual improvements over the years, the GPS network now permits pilots to fly instrument approaches without the aid of
any secondary navigation systems. In fact, aircraft can now navigate from takeoff to touchdown entirely by GPS. As future upgrades continue to enhance the system, additional GPS benefits will undoubtedly refine air navigation practices.
Although GPS permits “direct-to” navigation with the push of a button, ATC procedures and air traffic congestion usually prevent aircraft from flying directly from their departure airports to their destinations. Instead, airplanes are often instructed to fly to waypoints, points in space that can be determined through navigation systems. Often these waypoints mark the beginning of a standard terminal arrival route (STAR, see Avoiding Other Aircraft) into the destination airport. Though not as efficient at “direct-to” flight, waypoints often shave off many air miles that would otherwise be flown with VOR navigation.
INS: Limited mainly to airliners, the inertial navigation system (INS) is unique in that it is a completely self-sufficient system. Through the use of a computer and motion-sensing components (chiefly accelerometers and gyroscopes), the INS is capable of calculating its own speed, location, and orientation without external reference. As such, INS is great for supplementing other systems or for backup navigation. The system does, however, require an external source of position & velocity data (pilot, GPS, etc.) during initialization. In addition, small calculation errors will, with time, lead to increasingly greater speed/position errors (known as integration drift). However, INS’s shortcomings are generally minor and unlikely to affect flight safety.
Although pilots still carry maps (aeronautical charts), most modern navigation is done through a combination of electronic sources and ATC. As technology continues to advance, future navigation procedures will likely become more efficient and reliable, further increasing the safety of the national airspace system.