need a undesturbed compass

Please correct me if I'm wrong, anil, but that's what I thought you were inventing?

[Edit: please also see: http://en.wikipedia.org/wiki/Celestial_navigation]

A compass unaffected by magnetic fields is a tall order.

I suppose two intercommunicating GPS receivers with a distance between could work as a compass. One moving GPS receiver can also work as a sort of compass, but when the movement stops the compass effect is lost (in other words the typical GPS will not give you a direction is you stand and turn in place.

A gyro compass does not use the earth's magnetic field to indicate North. In fact it points due North in the direction of the planet's spin axis, and not towards the North magnetic pole which is over 1000 km away from the geographic pole.

Thanks. Had I not looked up gyrocompass on Wikipedia, I would have assumed that a gyrocompass was the same as the heading indicator (directional gyro) on an aircraft. These are periodically reset to magnetic north, to compensate for precession (magnetic north being important in flight because the flight level rules are referenced to magnetic north).

I've decided I need a gyrocompass on my sail boat. I'd guess I could buy one for not too much more than 10 times the boat's value.

I don't understand why a gyrocompass has to cost so much. I have seen it in use on a theodolite for geodetic survey. Of course I have not enquired how much it cost, but I am pretty certain it would not be 10 times the cost of your boat --- or is it a toy boat ?

My boat is only worth about $1200. Elnav mentions cheap gyrocompasses being about $20,000. The small directional gyros in small aircraft are about $1000, but they are not sensitive enough to align with the earth's spin access -- they are simply gyro stabilized and need to be reset evey 15 minutes or so -- they are one step up from carrying around a bicycle wheel, spun by hand .

$20,000 is indeed a lot of money for a flywheel whose axis of spin is held horizontal but allowed freedom to point in any horizontal direction, with perhaps a damper thrown in to stabilize the precession so that the spin axis settles down in the N-S direction. Much of the $20,000 may be for sensing the orientation of the spin axis and the electronics for repeater dials on the bridge etc.

Yanthram, have you ever dismantled a north seeking gyro compass or an air craft compass like the one Blink is talking about? The amount of precision machining is incredible. Your over simplification doesn't do the equipment justice. Not to mention which, the description is slightly inaccurate.

By comparison the fluxgate compasses are much cheaper to manufacturer. But then the software get involved. And that as you well know is also expensive.

You still haven't answered my question. Where and in what sort of application did you see a gyro mounted on a theodolite?

OK sir I am no expert on gyros. I have seen a gyro theodolite, which though more bulky than the usual thing was still portable. I have never dismantled a gyro, but I have seen a dismantled aircraft type gyro at an aerospace exhibition. That of course was not a North seeking gyro. Perhaps I have not appreciated the machine as I should. Thank you for correcting me. But reverting to the original problem, I thought a gyro compass is the thing for finding North in the circumstance stated by the original query. Further, I don't think an optical gyro can give true North without drift. I have not had occassion to ponder this, I jumped in on the strength of my knowledge of classical dynamics. Now I must study this a bit more.

Yanthram wrote:

I thought a gyrocompass is the thing for finding North in the circumstance stated by the original query. Further, I don't think an optical gyro can give true North without drift.

REPLY

"Gyro compass" covers a multitude of devices that use a gyroscope as the reference but only the kind that uses the U-tube filled with mercury is North seeking by itself. In that sense you are quite correct. However a number of characteristics make it awkward to use in some applications. Not the least of it being the problems encountered in high latitude use. Transpolar flights not to mention increased sea travel under the polar ice caps quickly revealed the need for something better.

Because the U-tube has to be acted upon by the rotation of the earth to force a precession effect, which ultimately causes the gyro to align itself with the axis of the earth; it takes time for the gyro compass to stabilize. This precession force is greatest at the equator and least at the pole. Even when you help by pre-setting it in the approximate direction it has to be stabilized before you can trust it.

Aircraft gyro compasses are dialed in by using the alignment with the runway and then frequently resetting with reference to a magnetic compass. Commercial systems became pretty sophisticated and used a magnetic coil to sense the position of the floating card magnetic compass. This eliminated the need for the pilot to be re-setting the aircraft gyro manually.

Gyros as a general use device have been used for a long time (WW2) to stabilize instruments against motion. The device on board ship for taking bearings consist of a gyro repeater with a pelorus ring mounted on top. The prism allows the navigator to read the gyrocompass heading directly from the pelorus. However the so-called gyro was simply a repeater that was slaved to the main gyro located somewhere deep inside the ship. For extremely accurate navigation work a theodolite was sometimes mounted on top of the pelorus ring. Sailors often referred to this repeater as the "gyro" as a shorthand term since everyone knew it was just a repeater. Might this be what you are referring to? In the days before Raydyist, Loran C, and GPS this was about the best and most accurate method for doing hydrographic survey work.

The venerable Norton bomb sight used a couple of gyros to stabilize the optics during the bomb run. These gyros were vacuum driven instead of electrically driven. Later on, three axis gyro stabilized platforms were used for navigating. Electronic repeaters allowed an instantaneous readout of heading. These platforms became known as inertial navigation instruments since they were self-contained and could indicate true north.

The three axis gyros underwent continual refinements as exemplified by Litton Industries products . One source of error was the inertia of the gyros itself. By eliminating any mass in an optical gyro that particular error was eliminated.

In the early Nineties KVH married a very sensitive fluxgate compass to a gyro reference system plus a sophisticated micro processor system. The result was a self correcting true north indicating replacement for the venerable mechanical north seeking gyroscope that ships had been using since WW2. Although the primary sensor was a magnetic device it had the ability to eliminate the stray magnetic influence of a steel ship.

The microprocessor contained a look up table to give the declination (also called deviation) and thus it was able to indicate true north at latitudes below where the magnetic north pole was roaming around. For most civilian surface vessel use this was adequate. KVH subsequently received type approval for the use of this product instead of the standard US Navy compass used on small surface vessels. Thousands of fishing boats also adopted this compass because it had a true north indication and a NMEA 183 output port for driving an autopilot. This KVH Azimuth compass cost less than one quarter as much as the cheapest Sperry mechanical gyrocompass. And it di dno tneed annual recalibration. As early as 1992 ,New Brunswick University was experimenting with multiple GPS receivers for use as a compass substitute. In the early days a base line of 10 feet between receiver antennas was required due to the use of SA. Removal of SA and the widespread implementation of differential GPS allowed the development of much smaller antenna arrays. Consumer models have been offered for sale for a couple of years now. COMNAV is giving a manufacturers suggested list price for their model of around $1995 which is only fifteen inches long. Unfortunately it's essentially a black box since you need some form of ECDIS navigation display in order to read the direction data output.

All of the above is commercial grade non -military equipment. Detailed explanation of theory, mode of operation, and construction details, are readily available and has been for years.

Even the venerable flux gate compass for autopilot use has not escaped refinement. Most north -up radar displays and many of the better quality autopilots now require the use of rate gyros in conjunction with flux gate compasses. The rate gyro stabilizes the image against wave induced apparent motion on the display.

In today's world of navigation "gyro compass" is pretty much a generic term and needs clarification to detail exactly how its used and the exact construction method for that model. Apart from reading the US Navy text "BOWDITCH" on gyroscope principles of operation, a great deal can be learned by dumpster diving for scrapped units and of course getting them from surplus equipment dealers. At one time I must have owned and dismantled nearly every kind of gyro compass there was. Sigh! I wasn't allowed to keep the inertial unit from Litton industry. Oh well!

Elnav:

This has been a great and most informative series of posts! Thanks, Ken

Yamthram wrote: I don't understand why a gyrocompass has to cost so much. I have seen it in use on a theodolite for geodetic survey.

REPLY: I suspect you are confusing terminology. Theodolites of the old type did not have gyroscopes. Newer total stations may use reference gyros like the kind Blink is talking about.

That is what I used for short term reference when doing my reciprocals while compass adjustment.s Although the newer types tend to be optical models that measure the doppler effect on two light beams circulating counter to each other. The only time I have seen gyro stabilized theodolites were on ship based surveys but that has pretty much been supplanted by newer equipment. Where did you see it used.

A true north seeking gyro depends on lots of MASS of many kilograms to acquire the correct orientation relative to the planetary spin axis. these weigh at least 20 kilos and the older Sperry weighed in at several hundred Kg. stood a meter tall and required so much power it needed a genset. The other reason I doubt a north seeking gyro was used is that they take many hours to stabilize from time it is turned on. In this day and age there are many more precise methods for setting up a station. Even one that is being used as the bench mark for geodetic surveys.

north seeking gyro was used is that they take many hours to stabilize from time it is turned on.

From memory, we always used to flash our gyros up 24 hours before proceeding to sea.

"The gyros' toppled"............... that's about the only time deck officers used to move in a hurry,or drop their bundle. There would aways be a pipe......... "Chief EA to the gyro room"

I used to look after the Gyros and the stabilizers on HMS Puma many years ago, but with good PM, we never had a fallout!!

I can honestly say that on none of the ships I served on, did we ever lose a gyro. We also had dual systems, each with dual power suppliesand magnetic as backup.

That sort of thing was something we trained for but never actually needed!

Many years ago there were "solid state" compasses for around €200 each if I remember correctly, surely they are still available and much cheaper......I see they are magnetic, so that may not help much......

Check this link out:-

Solid_state_compasses

Yes they are available and still cost relatively the same. These "solid state" compasses are fluxgates. And they are pretty good. BUT they are still subject to magnetic influences so its possible to screw them up by placing somethting containing a magnet close to them. Case in point purses with magnetic clasps and steel drink cans. I have bought cheaper one for as litte as $50 USD but these are junk. Little more than toys. I used them for class room display as a take apart model. .

Good grief! Where have you been hiding? That product was invented mor than 10 years ago and is now standard equipment on all survey vessels. Recreational boaters could buy such a unit from about two years ago for only $1500.

GPS compasses can be found advertised in many trade publications dealing wit GIS, survey work, hydrographic developments etc. Consumer GPS compasses are also available.

You might also want to check into KVH gyro supported flux gate compasses. These units are so good they can compensate for the iron in ship's hulls etc. I was testing them 12 years ago.

Hi elnav,

It is mandatory that all commercial fishing vessels in Australia have to have a compass swing every 2 years which has to be carried out by a suitably qualified person.

I know. Same here in Canada. In fact I did work as a compass adjustor for a while. I even developed a computerized system for data logging the steering compass heading and recording this against a table. I used a miniature fluxgate sensor placed on top of the magnetic steering compass. The card magnets were stronger than the earth's magnetic field so the fluxgate was sensing the card position, not the earth's magnetic field. I also had a KVH automatic compensated compass placed in a location free of variable field due to wiring, moving machinery etc. Then I used a gyro for reference when doing my reciprocals for cancelling out the gross deviation errors. Smaller compasses have adjustment screws. Larger commercial vessels have a slightly different setup. In addition, steel hulls have Kelvin spheres mounted either side of the binnacle and flinders bars hung below the compass bowl to compensate for heeling errors.

Sadly the availability of GPS convinced a lot of people they did not require a magnetic steering compass. So now except for those vessels required by the coast guard to carry mandatory compasses and which must be adjusted at periodic intervals there is no more adjustor work .

Incidentally true north seeking gyros require yearly calibration and mechanical checks that must be done at the factory. Being a mechanical device it is subject to wear both in the gimbals and also in the rotor. Cost of such a compass is around $20,000 for the cheap ones. The COMNAV GPS compass cost $1500 and does not require calibration. It is self calibrated.

Thanks for that elnav, being an engineer, I am not into the "upper deck" stuff, however I am not completely lost. I got a little..............little being the operative word..........when I spent time on patrol boats, because one would spend quite a bit of time in the wheelhouse. It wasn't all lost on me, I even recognise the majority of terms you used................however give me the engineering!!!!!!

Anyone sailing, even just coastal voyages, not having a simple magnetic compass as a standby to his GPS (and not knowing how to use it either probably!!), is a sheer lunatic in my book and yes, it should be swung once a year......as should any "sailor" without one!!!

It is possible that minor changes and differences to the ships electrics or things made of metal have been added that could cause a minor error or three on simple compasses....

Quite a few times I hear "deck" lecturers giving students (fishermen that are supposed to have had a minimum of 2 years sea time, as "deckies") a bit of hard time because they have said we don't need a compass, we have GPS, and we have a back up GPS.

I'm afraid they don't live in the real world, and when I get most of them doing engineering..............probably why I have grey hair!!!!!!!

They have probably never navigated without GPS, it would do them good to have it fall over occasionally to keep them on their toes!!!!

Never a truer word was spoken Andy..........most think that if you don't use it why bother learning about it.

One of the biggest problems I have is students saying, "Just teach us what is in the examination" or, "Is this going to be in the exam?"

The only thing I say is "It doesn't matter everything that I am teaching you is in the syllabus, therefore could be examinable either in the written or oral examinations"

Oh! sorry I shouldn't say orals.............the stupid blond bimbo in The Government Transport Organisation, who is in charge of quals said they will in future be called verbals................all I said was you call them what you want, they are and always will be to me "orals."

Hi;

I am no good at these kind of stuff but i think the ones we use while diving like the compass of an electronical suunto dive computer are these kind of compasses; otherwise we would not be able to navigate underwater under big ships or diving to wrecks meaning tons of metal.

just an assumption.

basak

Suunto had an excellent fluxgate product. It was small lenough to fit in a wrist watch sized case. In most cases you only need to get 50 - 75 feet away from a steel hull to avoid most of the effects.

Dear Guest, GPS systems are so cheap nowadays, I cannot imagine that your system, no matter how good, can compete on price.

I can buy a GPS Navi with all of Europe on a memory card for about €100. Let that be $135 or so, that is cheap!!!!

There is not much space in that price for a compass unaffected by any stray magnetic fields I am afraid!!!

Anil,

Three letters - RLG

(That's ring laser gyroscope)

Amazing little devices. Several years ago, I inspected a facility where a DoD contractor was assembiing these devices in a Class 100 Clean room. I'd never even heard of an RLG until that day...

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Just my $0.02...

JMAN, unfortunately RIGs do not seek true north. They are used as stabilizing references in conjuntion with Inertial navigation systems using accelerometers or with flux gate sensors or some other navigation system. These laser gyros are the ultimate in low mass frictionless gyros.

True north seeking gyros rely on the precession force created from the spinning mass and the U tube of mercury.

elnav,

I believe that you are overlooking the North finding ring laser gyros that have been developed (for defense & aerospace)...

No rotating masses, no precession forces, no spin-up time, and no errors due to torque. This translates into dead-on accuracy. I believe that the "compass error" in such devices' finding North is less than 5 arcsec.

At least on this planet...

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Just my $0.02...

Got a link? Evidently I have fallen behind the curve.