Forum: aviation.stackexchange.com
Tags: <navigation><gnss><oceanic><navaids>
Title: How do airliners cross the ocean without GPS?
Question: How do airliners cross the ocean without GPS?  Do they use dead-reckoning or are there navigation aids floating out there?

Answers:
sents: text: Well, most airlines do cross the ocean with GPS in today's world.
text: That being said, most (if not all) transcontinental airliners, and many flying domestic routes as well, have what's called an inertial navigation system (a form of dead-reckoning where gyros and accelerometers are used to compute changes in position).
text: The INS feeds into the flight management computer which is used to cross-check the GPS position, and can be used as a primary navigation source should the GPS signal be lost or corrupted.
text: Prior to GPS, inertial navigation was the primary means of navigating across oceans.
text: However, a great many airplanes did cross oceans before INS... with varying success.
text: The general idea was to use a combination of dead-reckoning, radio beacons where you can find them (small islands and coastlines), the sun (with the help of a sextant ) during the day, and the stars at night.
text: Hopefully, this puts you in position after some period of time to pickup ground-based navigational aids, or to identify a coastline.
text: This actually works pretty well if your wind estimates are good, and the margin for error is high enough.
text: As DeltaLima points out, long-range radio navigation systems also started to emerge around World War II (LORAN and Decca) which had a much greater range than previous radio beacons, and therefore decreased the amount of time required to fly by dead-reckoning alone.
text: By the time inertial navigation became prevalent in the 1970's, airlines were already providing reliable transcontinental passenger flights every day.
text: Another possible outcome is that your plane is lost in the ocean, never found, and the Discovery Channel funds an expedition to look for you 75 years later.
answer_details: author: aviation.stackexchange.com/users/6/Bret Copeland
score: 26
sents: text: We were crossing the seas centuries before GPS, INS or really any other form of modern technology.
text: All you need is a clock, a compass and a sextant.
text: And some largely-forgotten skills, like how to do math without a smartphone.
answer_details: author: aviation.stackexchange.com/users/1632/paul
score: 20
sents: text: In the days before GPS, we routinely crossed the oceans using inertial navigation systems.
text: The system I was familiar with used 3 separate inertial systems (Carousel was the brand name).
text: You could choose to navigate by any single one, but the most common way of using them was to have the autopilot "average" the positions.
text: You could also choose to exclude any given INS, in which case if you were averaging, you were averaging only two.
text: For the North Atlantic MNPS airspace system, You manually updated each INS passing over your last known ground position fix.
text: When you passed over your first ground-based nav fix on the other side, you again manually updated the INSes with that information and then recorded in the maintenance log how far off left or right
text: each INS was in the maintenance log.
text: There were standards.
text: Offhand
text: I'm not sure I am remembering them correctly, but what I seem to recall was a max error of 2 nautical miles per hour flown without update.
text: The MNPS had at the time a max error allowed without penalty of 10 nautical miles left or right of course.
text: If that was exceeded, points were deducted from your company's score.
text: Lose enough points and the company would not be allowed in fly in MNPS.
text: Off course errors of two to six nautical miles for a crossing were typical.
text: There is the story, true or not I do not know, of the captain who, when hand held GPSs first became available, used one to cross the North Atlantic, and had a near zero off course error.
text: When ATC complimented him on that, he blurted out that he had used his hand-held.
text: He was subsequently violated for using a non-approved navigational system.
text: Whether the incident really happened, I don't know, but an airline's FAA approved op specs specifies not only which navigation methods are to be used but also which systems by brand and model name (at least back when I was flying).
answer_details: author: aviation.stackexchange.com/users/394/Terry
score: 15
sents: text: Most aircraft cross the atlantic by GPS, usually with INS as backup.
text: INS as the primary form of navigation is still used as well.
text: Historically, a combination of dead reckoning and long range radio navigation systems were used on transatlantic routes.
text: These include Decca , LORAN C and Omega .
text: Omega was shut down in 1997,
text: Decca in 2000 and LORAN C ceased in 2010.
answer_details: author: aviation.stackexchange.com/users/19/DeltaLima
score: 7
sents: text: They do use GPS, with backups, as the other answers describe.
text: If your question is inspired by MH370, remember that GPS tells you where you are: it doesn't tell anyone else where you are.
text: GPS works by having a bunch of satellites sending out signals and, based on the signals you hear, you can calculate your own position.
text: A GPS user doesn't transmit anything back to the satellites.
text: What MH370 lacked was a system that was reporting its location back to base.
answer_details: author: aviation.stackexchange.com/users/946/David Richerby
score: 1
sents: text: Mode of air navigation I've witnessed: Celestial - sextant and charts.
text: Like, no kidding, right out of days of sailing ships.
text: Atmospheric pressure - driving along specific barometric pressure lines.
text: By no means a primary form of navigation.
text: Omega - now obsolete.
text: Triangulates on a small number of very powerful radio transmitters spread around the globe.
text: At least good enough to hit the ADIZ w/in limits.
text: I'm told when particularly close to a transmitter it can be as accurate as A Cat 2 ILS beam.
text: Inertial Nav - GPS.
text: what's that?
answer_details: author: aviation.stackexchange.com/users/1798/radarbob
score: 1

Summary of the answers:
To cross the ocean without GPS only a clock, compass and sextant are needed. In the past, a combination of dead reckoning and long range radio navigation were used. The INS is now used and it feeds into the flight management computer, it can be used as a primary navigation form without GPS.
Methods of crossing the ocean without GPS include INS; a clock, compass and sextant; celestial navigation; atmospheric pressure. In the past, dead reckoning and long-range radio navigation were combined and used for transatlantic routes.