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One type of instrument commonly referred to as a "gyrocompass" is commonly used in ships. It detects true north based on gyroscopic precession, and is not slaved to any magnetic compass or any magnetic sensor of any kind, nor is it necessary that the instrument be frequently manually corrected by reference to a magnetic compass. Has this instrument ever been used in aircraft, and if so, what are some examples?

The intent of the question is to ask about specifically about instruments that benefit from an intrinsic north-finding property related to the earth's rotation, including cases where this property is used to correct for various errors that would otherwise eventually cause the system to drift out of its initial orientation.

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  • $\begingroup$ Since aircraft will regularly go electrically dark for extended periods of time I don't see how this could be a viable option for navigation. All the gyro based compass systems I have seen are either slaveable to current heading or get magnetic info. $\endgroup$ – Jeff A Mar 26 at 0:30
  • $\begingroup$ Aircraft use Inertial Navigation Systems (INS) and Gyroscopes are part of the INS. en.wikipedia.org/wiki/Inertial_navigation_system $\endgroup$ – Mike Sowsun Mar 26 at 1:35
  • $\begingroup$ In hindsight the "steaming error" thing should have been obvious, but the question gathered an interesting answer nonetheless. $\endgroup$ – quiet flyer Mar 26 at 13:34
  • $\begingroup$ @Mike Sowsun: SOME aircraft may use inertial navigation systems, but a great many don't. $\endgroup$ – jamesqf Mar 26 at 18:26
  • $\begingroup$ Yes, I should have said some aircraft use INS and some INS use Gyroscopes. $\endgroup$ – Mike Sowsun Mar 26 at 22:04
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I think the answer is "no and yes". Bear with me... That sort of device as a stand alone heading sensor has never been used in an aircraft and if you parse the Wikipedia article carefully it becomes apparent why, under the paragraph titled "Errors".

A gyrocompass is subject to certain errors. These include steaming error, where rapid changes in course, speed and latitude cause deviation before the gyro can adjust itself.[12] On most modern ships the GPS or other navigational aids feed data to the gyrocompass allowing a small computer to apply a correction. Alternatively a design based on a strapdown architecture (including a triad of fibre optic gyroscopes, ring laser gyroscopes or hemispherical resonator gyroscopes and a triad of accelerometers) will eliminate these errors, as they do not depend upon mechanical parts to determinate rate of rotation.[13]

To work it requires a very slow moving and stable platform (like a ship). Anything approaching rapid motion, even by ship standards, let alone by aircraft standards, requires external help of some kind.

This rules out aircraft, and small boats for that matter I expect.

"Gyrocompass" in aviation has always referred to a directional gyro that is continuously slaved to an external magnetic north sensor. The traditional system has some form of directional gyro, either right in the instrument panel or built into an Attitude/Heading Reference System (AHRS) unit that drives the cockpit heading indication, with the actual magnetic field sensor in a remote location, usually out at the wing tip, called a Flux Valve, that provides a continuous magnetic north reference, to which the gyro is aligned.

When not slaved to the Flux Valve, an aircraft gyrocompass becomes just a directional gyro, which must be set manually and corrected for precession manually (on an AHRS system you will have a MAG/DG switch that allows you to select slaved or unslaved indications - in the high arctic where compass indications are useless, you have to operate in DG mode).

Now, I said "stand alone" for a reason. Inertial Reference Systems (or Inertial Navigation) used for aircraft navigation do use this phenomenon for their initialization alignment process. IRS initialization/alignment is done normally on the ground with the aircraft stationary, but can also be done in flight if heading is held steady during the alignment. The system is told where the aircraft is geographically, by entering lat/long or by GPS/FMS input if equipped, and the IRS senses precession from the earth's rotation to figure out what direction the airplane is pointed relative to true north (because it can sense the west-east axis of the earth's movement and knows the airplane longitudinal axis relative to that). This is how IRS systems get their directional starting point, from which future indications are derived from inertial changes from the starting point. If inertial errors build up in the system (you get left right IRS position comparator errors), the system has to be aligned again (which is why there is usually a procedure to do an "IRS Align" in flight where you fly a constant heading, speed and altitude for several minutes while the IRS does its thing).

So the answer is, yes, but limited to IRS systems when starting up, and no as a stand-alone heading device.

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    $\begingroup$ Please tell me the Flux Valve feeds into a Flux Capacitor on its way to the AHRS! That would just make my day complete! $\endgroup$ – FreeMan Mar 26 at 12:48
  • $\begingroup$ Phhhhhht.. that gag is, like, so 80s lol. $\endgroup$ – John K Mar 26 at 16:15
  • $\begingroup$ @JohnK Don't you mean "Will be so 80s"? $\endgroup$ – HiddenWindshield Mar 27 at 0:42
  • $\begingroup$ @HiddenWindshield Just clicked that that's another Back To The Future gag. Clever! $\endgroup$ – John K Mar 27 at 2:59
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The modern equivalent is the ring laser gyro, a gyrocompass made out of light. A laser beam is split, with each half being bounced round the same triangular cavity but in opposite directions. The interference patterns between the beams are very sensitive to rotation.

Laser gyros are cheaper and all-round better than mechanical gyros. Most modern inertial navigation systems (INS), be they land sea or air, are nowadays based on them. One unit is typically adequate for land and sea, but three are needed for an aircraft; one for each axis of rotation in pitch, yaw and roll.

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