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A comment here Why does "no aircraft cross directly over the pole"? indicates that modern aircraft use GPS for navigational purposes, which makes perfect sense. However, do these aircraft also have backup mechanical sensors (altimiter, magnetic compass, etc.) in case GPS signals go haywire or are not available?

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    $\begingroup$ Clarification might be helpful. GPS is a system for precision location determination. Are you asking if there are backup precision navigation instruments (e.g the use of terrestrial navigation radials or the presence of onboard inertial measurement systems) ? An altimeter, magnetic compass and airspeed indicator are not precision navigation instruments, they are primary flight instruments. $\endgroup$ – tillmas Dec 17 '16 at 0:03
  • $\begingroup$ Note that GPS is not used for measuring altitude except for the terrain proximity warning. This is because barometric altitude and altitude are different quantities, but all aircraft must agree on one, which is the barometric one. $\endgroup$ – Jan Hudec Dec 19 '16 at 18:55
  • $\begingroup$ @tillmas, I was primarily wondering about primary flight instrument backup capabilities in case the primary precision electronics-based instruments became inoperative. Essentially redundancy in general navigation function by two completely differently-designed systems. $\endgroup$ – Milwrdfan Dec 19 '16 at 19:44
  • $\begingroup$ @ScottEvans, then I think your question should be edited to say that. You should also understand that primary flight instruments are not backups to GPS. GPS is not a primary flight instrument, it is a navigational instrument. $\endgroup$ – tillmas Dec 19 '16 at 19:46
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In light aircraft they typically do: Glass-Panel aircraft will also have traditional "steam gauge" backup instruments of some kind.

Piper Glass Panel - Backup instruments on the left
Note the airspeed indicator, attitude indicator, and altimeter to the left of the pilot's PFD.

This is however no longer guaranteed: A completely independent electronic backup system with its own power source may be used in place of the traditional "steam gauges".
This can be seen on new Cirrus aircraft, which have digital standby instruments (in the green box):
Cirrus standby instruments

Similarly transport category jets may have traditional backup instruments:
Boeing 737-800 with traditional steam gauge backups

Or they may have an electronic backup system:
Boeing 787 with electronic backup instrumentation

Note that the electronic instruments in these cases aren't GPS-referenced: They are gyroscopic instruments with pitot and static input for airspeed and altitude/vertical speed calculation.

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    $\begingroup$ The question appears to be asking about backup for navigation systems, not flight instruments. $\endgroup$ – DJClayworth Dec 17 '16 at 4:57
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Basic VOR navigation equipment (and sometimes NDB or DME aswell), altimeter and magentic compas are mandatory equipment for any IFR aircraft

It's a pretty safe bet any aircraft will have them onboard.

Also, a good side story to tell is that mechanical sensors do not act as backup to GPS: in case of altitude the 'mechanical' baro-altimeter is still the primary instrument, as for navigation the radios & IRS are still used to enhance the estimated GPS position

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  • $\begingroup$ There's no requirement in the US to have a VOR receiver; the regulation only says "navigation equipment suitable for the route to be flown". $\endgroup$ – Pondlife Dec 17 '16 at 19:31
  • $\begingroup$ Yeah, the FAR is surprisingly vague about this, whereas the ICAO (and EASA) make this a little bit easy: check EU OPS 1.865 d) (1) (at least 1 VOR , or 2 if it is the sole means of navigation) ... FAR maybe has an equivalent I'm just not so familiar with FAR $\endgroup$ – Radu094 Dec 18 '16 at 12:46

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