GPS Vs. IRS

Gyrocompass spin on board the Enterprise involved a marked point from port to a known landmark to get relative to True North. Spin up was always done in port.

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Gyrocompass spin on board the Enterprise involved a marked point from port to a known landmark to get relative to True North. Spin up was always done in port.

And how do you know that the movement of the gyrocompass was caused by the rotation of the earth and not the rotation of the celestial bodies above the earth; attracting the weights of a gyrocompass through gravitational vectors?

I didn't say they had to be slaved to electrical systems. 

Inertial guidance systems are prone to integration drift.  They must be slaved to other systems, such as GPS, to maintain positional accuracy.

Quote from: TheEngineer on July 23, 2007, 07:15:15 PM

I didn't say they had to be slaved to electrical systems. 

Quote from: TheEngineer on July 20, 2007, 12:58:36 AM

Inertial guidance systems are prone to integration drift.  They must be slaved to other systems, such as GPS, to maintain positional accuracy.

Mathematically.

The drift was very small and predictable from what I was told.

Quote from: JackASCII on July 24, 2007, 06:48:44 AM

Mathematically.

I was genuinely hoping for a more...complete answer. 

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The drift was very small and predictable from what I was told.

The drift is small, over short periods of time.  The error arises from the signal itself, which is then integrated, twice, providing an exponentially increasing position error. 

Take for example, a good quality single axis accelerometer.  It has an error of about a milli-g.  Now let's assume the government has extreme quality accelerometers and has done other, tricky stuff, to get the error down three orders of magnitude, to a micro-g.  If you were to turn this simple, one axis INS on and leave it alone in a ship in dry dock, it would begin to integrate the noise in the system.  After one minute, the error in position is about .0018m.  After an hour, it's grown to 6.48m.  After a day, it's become 3.732km.  In one year the error would grow to 297,260km.  If the system was left alone for the four and a half years, it would be able to place the ship to somewhere out towards Venus, as the error is now at over 10 million kilometers!

Sorry, I'll have to look it up. I tried to find something online but I think the old tech stuff isn't going to be easy to find. Damn, I wish I had my old navy pubs. They are in a storage shed in another town.

I sort of see what you are saying, but is that a linear increase in error or exponential? Looks exponential they way you are describing it.

Either way, the error is predictable.

I'm not sure how length measurements would fit into this unless the ship was in motion.

As far as I remember the 3 axis gimbal used was not resynced until port. That could be 6 months or longer.

Quote from: JackASCII on July 25, 2007, 08:34:37 AM

Sorry, I'll have to look it up. I tried to find something online but I think the old tech stuff isn't going to be easy to find. Damn, I wish I had my old navy pubs. They are in a storage shed in another town.

All my experience in the matter is with aircraft, work and personally.  I just wanted to see if my naval counterparts had come up with something tricky to remove error.

I've been looking online, but haven't found much. I'll have to wait until I get into my shed again... hopefully in a couple of weeks when I visit North Alabama.

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As far as I remember the 3 axis gimbal used was not resynced until port. That could be 6 months or longer.

It's a different can of worms if you are talking about a gyroscope.

Quote from: rentakow (New age Martin Luther King Jr.) on July 24, 2007, 05:03:00 PM

Quote from: TheEngineer on July 23, 2007, 07:15:15 PM

I didn't say they had to be slaved to electrical systems. 

Quote from: TheEngineer on July 20, 2007, 12:58:36 AM

Inertial guidance systems are prone to integration drift.  They must be slaved to other systems, such as GPS, to maintain positional accuracy.

I don't see why those statements are grouped together like that...

Uh, no.  That's backwards.

GPS is an electrical system,thus IN is slaved to it.

Quote from: The Kommunist on August 23, 2007, 08:03:00 AM

GPS is an electrical system,thus IN is slaved to it.

INS-slaved to->GPS

That's how it should read.

Nice.  I guess I can't read very well at 8 in the morning. 

The question is, how kan a mekanikal navigation system konkur with a digital one.