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How do airliners cross the ocean without GPS?

Do they use dead-reckoning or are there navigation aids floating out there?

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It's worth noting that ICBMs cross the oceans on combination of INS (as mentioned in some answers) and automated star tracking devices data (as it will be unwise to rely on any sort of ground/satellite assisted navigation during the global nuclear meltdown). Same applies to many military planes, such as a venerable SR71 (which was equipped with a daylight capable star tracker). – oakad Mar 24 '14 at 1:46
@oakad The U-2 also used celestial navigation, especially when flying close to the north pole, which they often did to see what the USSR was doing at it's far-northern nuclear test sites. During the early stages of the Cuban Missile Crisis, one of these flights accidentally crossed the Russian far-east after the pilot miscalculated his return to Alaska. – David Richerby Mar 24 '14 at 8:43
up vote 19 down vote accepted

Well, most airlines do cross the ocean with GPS in today's world.

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). 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.

Prior to GPS, inertial navigation was the primary means of navigating across oceans. However, a great many airplanes did cross oceans before INS... with varying success. 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. Hopefully, this puts you in position after some period of time to pickup ground-based navigational aids, or to identify a coastline. This actually works pretty well if your wind estimates are good, and the margin for error is high enough. 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. By the time inertial navigation became prevalent in the 1970's, airlines were already providing reliable transcontinental passenger flights every day.

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.

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Many aircraft, the Vulcan included (first hand experience) have a sextant protruding from the fuselage. The navigator would take star fixes - just as the Vikings did when they crossed the Atlantic. – Simon Mar 23 '14 at 11:26
Do i read correctly that planes don't have compasses? – atk Mar 23 '14 at 13:26
@atk No, a compass is required equipment on all certified airplanes. – Lnafziger Mar 23 '14 at 13:31
@atk Note that the usefulness of copmasses decreases as you get close to the poles, which some intercontinental flights do. – David Richerby Mar 23 '14 at 13:36
@Simon it would help you maintain a particular heading for a given amount of time. – Bret Copeland Mar 23 '14 at 22:09

We were crossing the seas centuries before GPS, INS or really any other form of modern technology.

All you need is a clock, a compass and a sextant. And some largely-forgotten skills, like how to do math without a smartphone.

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Surely you can use a smartphone if you put it in flight mode. – tobyink Mar 23 '14 at 14:54
+1 for "math without a smartphone" :) – dvnrrs Mar 24 '14 at 14:23
Did the Vikings not have Calculator apps on their smartphones, or something? :p – Jon Story Feb 19 '15 at 13:41

In the days before GPS, we routinely crossed the oceans using inertial navigation systems.

The system I was familiar with used 3 separate inertial systems (Carousel was the brand name). 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. You could also choose to exclude any given INS, in which case if you were averaging, you were averaging only two.

For the North Atlantic MNPS airspace system, You manually updated each INS passing over your last known ground position fix. 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 each INS was in the maintenance log. There were standards. Offhand 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. The MNPS had at the time a max error allowed without penalty of 10 nautical miles left or right of course. If that was exceeded, points were deducted from your company's score. Lose enough points and the company would not be allowed in fly in MNPS.

Off course errors of two to six nautical miles for a crossing were typical.

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. When ATC complimented him on that, he blurted out that he had used his hand-held. He was subsequently violated for using a non-approved navigational system. 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).

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At least until the early 2000s hand held GPS sold in the US would lock out at 99/199knots to prevent you using them in aircraft. The makers were scared they would be sued. To use them for an airborne science experiment we had to buy special FAA grade GPS – NobodySpecial Mar 25 '14 at 22:39

Most aircraft cross the atlantic by GPS, usually with INS as backup. INS as the primary form of navigation is still used as well.

Historically, a combination of dead reckoning and long range radio navigation systems were used on transatlantic routes. These include Decca, LORAN C and Omega.

Omega was shut down in 1997, Decca in 2000 and LORAN C ceased in 2010.

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LORAN C/eLORAN is still in operation in some parts of the world... – UnrecognizedFallingObject Mar 10 '15 at 0:31

They do use GPS, with backups, as the other answers describe.

If your question is inspired by MH370, remember that GPS tells you where you are: it doesn't tell anyone else where you are. GPS works by having a bunch of satellites sending out signals and, based on the signals you hear, you can calculate your own position. A GPS user doesn't transmit anything back to the satellites. What MH370 lacked was a system that was reporting its location back to base.

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Thanks, I did know that about GPS, this wasn't inspired by MH370. I was just wondering how it would be done in aircraft without GPS ;) – flyingfisch Mar 23 '14 at 16:58

Mode of air navigation I've witnessed:

  • Celestial - sextant and charts. Like, no kidding, right out of days of sailing ships.
  • Atmospheric pressure - driving along specific barometric pressure lines. By no means a primary form of navigation.
  • Omega - now obsolete. Triangulates on a small number of very powerful radio transmitters spread around the globe. At least good enough to hit the ADIZ w/in limits. I'm told when particularly close to a transmitter it can be as accurate as A Cat 2 ILS beam.
  • Inertial Nav -
  • GPS. what's that?
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Omega were effectively high power NBDs. Pretty accurate if you knew how to use them properly. Of course pilots could also tune their NBDs to commercial stations with known locations (as the ill fated B-17 group did that flew into Hawaii on 7 December 1941) and get the same effect. – jwenting Mar 26 '14 at 10:57

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