In case of loss of GPS: how can an aircraft not only be navigated but navigated so precisely that it can pinpoint its destination airport ?
(case in point: Finland and Norway are telling airline pilots to be ready to fly without GPS )
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4$\begingroup$ Before 1974, aircraft navigated without GPS and were able to cross oceans. $\endgroup$ – Ron Beyer Nov 13 '18 at 15:18
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3$\begingroup$ @RonBeyer - Many aircraft (air carriers, business Jets, etc.) crossing the oceans were using INS/IRS (Inertial nav systems) as the primary means of navigation well into the 1990s. I'm pretty sure there are few aircraft still not using GPS to cross the oceans (probably not many). $\endgroup$ – 757toga Nov 13 '18 at 15:27
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1$\begingroup$ @757toga The "1974" figure was just an example of when the Air Force first launched Block 1. GPS wasn't even certified until 1984 for use in IFR. There are quite a few GA aircraft that don't have it, and 10's of thousands of smaller ultra-light or even LSA's that don't. I'm not sure about commercial aircraft though, I would think by now all of them would be equipped with some kind of GNSS. $\endgroup$ – Ron Beyer Nov 13 '18 at 15:30
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$\begingroup$ @RonBeyer - my comment was not a criticism just adding info. I edited my comment to clarify that there are still (probably only a few) aircraft crossing oceans without GPS. I'm fully aware there are thousands of other aircraft that do not have GPS, and may never will. Also, likely there are some older B757/B767 cargo haulers (or similar) still relying on their INS/DME/DME (non-GPS) systems. $\endgroup$ – 757toga Nov 13 '18 at 15:35
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$\begingroup$ @757toga Same here, just adding info, no worries. I'd still be surprised though if they didn't have GNSS, mostly because the ADS-B mandate will affect all larger aircraft types as they typically don't hop around between Class-D/E and use A/B/C. $\endgroup$ – Ron Beyer Nov 13 '18 at 15:36
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The question you should be asking is rather, how did airliners navigate before the widespread adoption of satellite-based navigation systems such as civilian GPS?
One can navigate by way of landmarks (as is typical in low-altitude VFR general aviation), but more typical in commercial aviation in the absence of GPS would probably be to navigate by either inertial navigation, or by radio beacons. There are or have recently been several systems that can be lumped together as "radio beacons", such as for example NDB, VOR/DME and ILS, which are used for different purposes, and have different strengths and weaknesses. Until recently, another such system was LORAN.
Major airports typically offer a number of radio beacons that can be used for locating the airport, and there are normally waypoints, sometimes associated with radio beacons of their own, along which a route can be flown even without external references (such as in instrument meteorological conditions, IMC). 757toga's answer to this question shows what such a navigation system for long-distance flight can look like.
GPS does make it easier to fly the most efficient route possible under current conditions, but it is hardly required in order to fly some route from one airport to another. Being prepared to fly without GPS might, in the simplest of cases, simply mean that you plan for a slightly less direct route and treat your non-GPS navigational aid equipment as required, possibly even primary, rather than secondary or nice to have, since you know there's a clear risk of losing GPS through no fault of your own or of the onboard equipment.
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$\begingroup$ In addition to the rather short range radio beacons you mentioned there was the LORAN long range hyperbolic radio navigation which might also be worth mentinoning. $\endgroup$ – altocumulus Nov 13 '18 at 15:08
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$\begingroup$ Good point; edited. The list was never meant to be exhaustive, but I agree that LORAN makes a good addition. $\endgroup$ – a CVn Nov 13 '18 at 15:20
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1$\begingroup$ might be a shortage of LORAN receivers on aircraft these days. $\endgroup$ – 757toga Nov 13 '18 at 15:45
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1$\begingroup$ It’s rare but there are a few small aircraft left with LORAN receivers. It was the rage in the late 1970s and early 1980s but never achieved a large place in general or commercial aviation. By the mid 1990s GPS was the default new navigation system for military use and quickly acquired the civilian market when access was allowed. $\endgroup$ – Carlo Felicione Nov 13 '18 at 16:16
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Below is a map of various airways in Finland that extend from VORs (including VOR/DMEs), which are ground based (non-GPS) navigation facilities. It's likely that all aircraft currently navigating primarily using GPS also have 1 or 2 onboard VOR receivers. Long before the development and widespread use of GPS, these route structures were the primary source of (non-visual) navigation for aircraft operating throughout the country.
Additionally, aircraft are not constrained to using these straight-line airways between the VORs. Each VOR generates 360 courses (known as 'radials') that permit different routes not depicted on the chart below.
Importantly, many aircraft have multi-sensor navigation components that allow "Area Navigation" (RNAV) without using GPS. With RNAV (non-GPS), aircraft can fly direct point to point, similar to using GPS.
Lastly, the lack of GPS for navigation would be disruptive to varying degrees, especially for operations that are beyond the service volume range of VORs. But, not that many years ago before the widespread use of GPS, day to day navigation operations were conducted efficiently using the VOR system as well as other non-GPS electronic navigation aids.
Other countries have a similar (Non-GPS) route structure as well.
Source: Finland AIP FIN AIP
