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It's not uncommon for polar flights to lose GPS signal (ionosphere issues due to the high latitudes). But it is not a concern because the typical IRS drift is 650 meters for every hour of flight, and the skies are empty (compared to the Atlantic tracks) in those regions.

Sooner or later GPS will be back, navaids will be within reach, and radar coverage will resume.

Q: Now, what happens when the GPS signal is lost? Does the FMS navigation start to drift from this point on, or does it switch to all the accumulated IRS drifting that's been going on for hours prior?

One way to picture the latter (I guess), is the magenta line on the navigation display to abruptly shift away from the plane-symbol, even if it is just by a mere few nautical miles.

My question applies to the older IRS systems that require stationary alignment, unlike the new GPS-coupled align-in-motion systems found on the 787, etc.

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  • $\begingroup$ "sooner or later we'll pick up ground navaids or we'll be picked up by radar". I wouldn't be so sure, especially operating under IFR conditions where spatial disorientation can get you, if you don't have trustworthy instruments thats a dire situation. Can you tell us what avionics package you are talking about? We aren't biased and it would help understand what you are talking about. $\endgroup$
    – Ron Beyer
    Commented Jun 12, 2016 at 2:30
  • $\begingroup$ KAL007 is probably the best example of disaster due to drifting off course a couple of degrees, although that wasn't due to INS drift $\endgroup$
    – TomMcW
    Commented Jun 12, 2016 at 4:15
  • $\begingroup$ Why would polar flights lose GPS signal? The satellite orbits are inclined 55° from the equator so they cover the poles just fine $\endgroup$
    – TomMcW
    Commented Jun 12, 2016 at 4:24
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    $\begingroup$ @TomMcW: This degradation is due to auroras which can block the reception. $\endgroup$
    – mins
    Commented Jun 12, 2016 at 8:53
  • $\begingroup$ @RonBeyer -- he's referring to a loss of navigation aids, not a loss of inertial reference. Your IRUs will still keep you dirty-side-down without any external help, they just might get you lost unless you can find a navaid somewhere to tune ;) $\endgroup$
    – UnrecognizedFallingObject
    Commented Feb 15, 2017 at 1:13

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Turns out indeed it's an issue affecting polar operations. It differs from type to type, but for the 737-600/-700/-800/-900 as an example, the message UNABLE REQD NAV PERF-RNP will be displayed.

For oceanic areas this will only happen if ANP shifts by over 10 NM, i.e., after losing the RNP-10 capability.

At an IRS drift rate of 650 metres per hour, it would take 28 hours of being airborne or a rather shifty IRS unit—no pun intended—for the message to be displayed.

So the answer is that the magenta line won't shift, unless the pilots choose an IRU over the rest by using the position shift function of the FMS.

For the 747-400:

When no global positioning system (GPS) updating occurs, all position and velocity corrections gradually are phased out until the FMC navigation parameters equal the selected IRU position and velocity.

This logic is part of the RNP standards and regulations, to stop an auto flight system from chasing the course when navigational accuracy becomes questionable. Any loss of GPS (away from ground navaids) inhibits the FMS from applying any corrections without crew intervention.

Source: Polar Route Navigation

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    $\begingroup$ Rules for answering your own question is that it is welcome. It is even welcome to write a question with intention to answer it yourself for purpose of documenting research you've done for the benefit of greater public. $\endgroup$
    – Jan Hudec
    Commented Jun 13, 2016 at 12:42
  • $\begingroup$ Great question, but your answer is a bit simplistic. The Boeing document you reference (very good article) indicates there's a lot more going on between the time GPS is lost and you get the RNP message. And the behavior is different based on the aircraft's IRS and FMS system designs and architectures. $\endgroup$
    – Gerry
    Commented Feb 16, 2017 at 13:34
  • $\begingroup$ @Gerry 100% agreed, is the answer better now? $\endgroup$
    – user14897
    Commented Feb 17, 2017 at 20:18
  • $\begingroup$ @ymb1 Much better. The FMS uses all available inputs to compute an aircraft position. How it does that would get into a discussion of proprietary algorithms and Kalmann filters. RNP exists to allow for reduced separation of aircraft. The increased performance of GPS is the primary enabler of RNP. It's useful to remember that before GPS and RNP, B747s were flying similar routes using only triple IRS systems. $\endgroup$
    – Gerry
    Commented Feb 19, 2017 at 3:07

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