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Finding True North without a compass

10 Posts

This is obviously impossible (under cloud) unless you can somehow detect and characterise the very slow rotation of the earth, causing your position to move through a curved trajectory, whose curve will tell you where the rotation axis is, and thus where True North is.

I don’t know if you need GPS position for it – I suspect not. You would need GPS if you want the magnetic north – use it to get a lat/long so you can use a lookup table (etc) to lookup the magnetic deviation and do the correction.

Airliner inertial reference systems have for many years had the True North capability but I have just come across this gadget

here

They have no data on it and the image is clearly 3D CAD generated, but it shows what can be done using a solid state gyro, not the far more expensive fibre optic (FOG) gyro.

From a 2010 press release

Background (serious maths in there)

Last Edited by Peter at 09 Jan 19:10
Administrator
Shoreham EGKA, United Kingdom

This a more fun way of finding it. Great book.

http://www.amazon.com/True-North-Exploring-Great-Wilderness/dp/1592281567

True North by George Erickson, goes trekking around northern Canada in his Piper Cub on floats…

Last Edited by podair at 09 Jan 19:57
ORTAC

An hour ago I finished installing a new magnetic compass in my plane, and did a preliminary setup to get it close before my A&P and I run it over to the compass rose and make the correction table. The way I find north is fairly simple: the taxiway in front of my hangar is perfectly aligned true north/south! I use the published variation to set up the compass on the taxiway.

For most practical navigation purposes, you need to know your track rather than heading, which you can directly read off your GPS. You have a choice of magnetic or true track: magnetic with a valid data card in place, true if you pull the card out:


(Peter’s picture hijacked)

LKBU (near Prague), Czech Republic

For most practical navigation purposes, you need to know your track rather than heading,

As I was fiddling with my new compass today, it was admittedly hard not to ask myself “why am I doing this?” to which the obvious answer is “so I can keep the DG accurate”…. Which then begs the question why I have a DG. What I actually do when flying is take off and fly in the general direction of my destination, eventually arriving. A moving map or two in the cockpit, with track data on display makes most of that old stuff redundant for my use, most of the time.

The best answer to the original question is actually “because as a point of pride I want everything on my timeless aeronautical conveyance to work properly”

Last Edited by Silvaire at 10 Jan 02:35

you need to know your track rather than heading, which you can directly read off your GPS

True, though only if you are moving, with some reasonable speed and through a distance sufficient to make the GPS “position noise” irrelevant.

Administrator
Shoreham EGKA, United Kingdom

Sorry can’t resist – I have to post this here as well.

So, in case you have a sufficient number of dogs (or a sufficiently productive dog) along for your flight:
http://motherboard.vice.com/blog/dogs-poop-in-alignment-with-earths-magnetic-field

EDDS, Germany

Hello!

Am I overlooking something? I can’t see anything new in that. This is the classical gyro compass that has been in use on ships since 110 years. Only with an electonic sensor.

If you google “north seeking gyro” or “solid state rate gyro” you will get thousands of results, some even available directly from Amazon: http://www.amazon.com/Maretron-SSC200-Solid-State-Compass/dp/B000Z4PXYY
Or straight from the Chinese manufacturer in bulk loads: http://en.ofweek.com/Product/North-Finder-prod-19690-188928.html

EDDS - Stuttgart

I am not sure all of these systems are suitable for onboard use. The operation of a north-seeking gyro is far from instantaneous, it takes several minutes to produce a single reading, and the system should remain still with respect to the Earth throughout the measurement cycle. Alternatively, its motion should be compensated, which will be implemented in an onboard IRS, but not necessarily (and not likely, either) in a simple “north finder” device.

Last Edited by Ultranomad at 10 Jan 14:40
LKBU (near Prague), Czech Republic

What next – I think those systems just use a magnetometer compass – just like mobile phones. Or maybe a fluxgate, or something using Hall effect sensors.

That isn’t the same thing as characterising the curved trajectory of the device (being located at a point on the earth’s rotating sphere) and yielding the true north from that by projecting that to the rotational axis (apologies for sloppy terminology).

This has been possible for many years using high grade mechanical gyros, and later FOG gyros – as that PPT presentation I link to shows. That shows a Russian FOG device which does it pretty well. I used to know an airline pilot who said their IRS would take about 10 mins to work it out. They had to do it on the ground.

I think your point about having to keep it still for a few minutes must be valid for a device using MEMs gyros – otherwise it is a very tall order! The acceleration involved is miniscule. I bet they are using some narrowband filters, to pick out just the specific motion they are looking for.

Last Edited by Peter at 10 Jan 14:53
Administrator
Shoreham EGKA, United Kingdom
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