Commercial airliners navigate with pinpoint accuracy since modern cockpit computers give precise route coordination. The GPS (Global Positioning System) is typically the most accurate navigation system on most of today’s aircraft, in some circumstances even allowing the airplane to perform maneuvers down to an accuracy of 0.1 of a nautical mile. Aviators throughout the world now use GPS to increase the safety and efficiency of flight. With its accurate, continuous and global capabilities, the system offers seamless satellite navigation services that satisfy many aviation user requirements. Space-based GPS location and navigation enables three-dimensional position determination for all phases of flight from departure, en route, and arrival, to airport surface navigation. But, did you every wonder how aircraft navigated before GPS became a standard navigational tool? Airplanes used different methods to find their way from place to place in days gone by, including dead reckoning, celestial navigation and more.
When aircraft first took to the skies, pilots used visual aids for all navigational purposes, with very little in the way of hardware. However, accurate navigation became essential during the First World War when military aircraft were flying at higher altitudes and longer distances, Of course, planes used onboard radios to communicate, receiving instructions from their ground crews. Even though this was workable during takeoff and landing phases, radios had a limited range so communication became impossible once aircraft were a few hundred miles away. Instead, crews calculated their position manually. Celestial Navigation was one standard method for finding a plane's location and navigators later employed a bubble sextant to calculate the aircraft's position relative to the sun, moon, or stars. This method was even used in early Boeing 747s, which featured a sextant port on the cockpit roof.
Dead Reckoning was another common navigation method used on long flights. Navigators used previously known positions to estimate the plane's current location using speed and flight time. While the weather could hamper these estimates, it was a relatively accurate way to calculate an aircraft’s location To better provide information while in flight, ground bases started using a system known as Long Range Navigation (LORAN). Two land-based radio transmitters traded signals at set intervals, allowing navigators to use the time difference to find their exact location. While this was an ideal solution, weather and frequency disruptions often distorted the transmission, leaving the crew with unreadable data.
Over time, aircraft used a radio-based system known as Very High Frequency Omni-Directional Range (VOR). Using this system, aircraft received communications from fixed ground beacons, allowing pilots to continue on their flight path and determine their position. This navigation method was quite reliable in areas with radio coverage and continued to be widely used until GPS became the norm. (I vividly recall learning to fly “to” and “from” VORs during my flight training. However, the FAA has been decommissioning many VOR stations as they are not as necessary as they one were.) The beginning of the jet age also marked the introduction of another navigation method known as the Inertial Navigation System (INS). The INS phased out older celestial systems, relying on highly sensitive motion and rotation sensors instead. This marked the first use of partially-computerized navigation sensors. Military and commercial aircraft still use the system in conjunction with GPS.
Originally intended for US military applications, the first experimental GPS satellite was launched in 1978. Later, in 1983, President Reagan signed an Executive Order allowing commercial passenger aircraft to use the system once it was fully operational. His decision resulted from the Korean Air Lines disaster, which occurred in 1983. The airliner was shot down by Soviet fighter aircraft as the plane mistakenly entered Soviet airspace on its way to Seoul. In response to the tragedy, the US authorized the use of GPS for commercial flights to enable more accurate navigation. The system was enhanced with additional satellites and by 1994, some 24 satellites were in orbit above the planet.
First called Navstar GPS, this space-based technology is owned by the US Government and operated by the US Space Force. It now relies on a bevy of 31 satellites to provide users with an accuracy within 23 feet up to 95% of the time, anywhere on Earth, according to the Federal Aviation Administration (FAA). This vital satellite constellation is in geosynchronous orbit some 12,500 miles above Earth, circling our planet every 12 hours and emitting continuous navigation signals. Receivers on the ground capture these signals, using them to calculate time, location and velocity with a high degree of accuracy.
GPS technology requires three elements, the user, satellites, and ground stations. A transmitter onboard the aircraft connects to multiple satellites. These satellites measure the signal to determine latitude, longitude and altitude. Additionally, atomic clocks built into the satellites measure the time it takes for the signal to travel from the aircraft to the satellite, which provides a more precise determination of the aircraft’s location. The satellites then send this information to a ground station which further corrects for position error before the information is re-uploaded to a geostationary satellite (one that does not move relative to the Earth's rotation). This corrected information is then sent back to the receiver onboard the aircraft, and the information is displayed on the pilot’s display. And all of it pretty much happens in real time.
Aircraft navigation has come a long way since the days when pilots flew with little information to now having everything displayed on multiple screens right before their eyes. While many older methods of navigation are gone from the cockpit, the FAA does require pilots to demonstrate “the ability to use an airborne electronic navigation system.” So, as a practical matter, most pilots are more comfortable having a backup to the electronic GPS since technology can fail, forcing those at the controls to manager the aircraft manually.
Until next time…safe travels.