A lot of rubbish has been said about the schuler oscillation... Much of it found in pilot literature...
It is actually very simple.
If you have a platform that is horizontal when you are in London, it will not be horizontal anymore when you reach Australia, if you keep it in a fixed position (relative to space).
And gyros provide fixed positions relative to space only. So we need some sort of mechanism that will tie the platform relative to the round earth.
In INS systems this is done by measuring acceleration and thereby the speed that we move with.
So if you are moving with XX kts direct south, a tie-mechanism will tilt the platform xx deg/hour so it is kept horizontal with regards to the earth.
So far so good... The problem is then, that this tie mechanism has no clue weather an acceleration is actually caused by the airplane moving, or because the platform is not horizontal.
If the airplane is parked, and the platform for some reason is not horizontal, you will pick up an acceleration due to earth gravity.. This will be interpreted as if the plane was actually moving, and the tie-mechanism will start tilting the platform. The tilting will continue until the platform passes the horizontal, and starts tilting the other way. This is then interpreted as a deceleration, and eventually an acceleration in the opposite direction, causing the tie-mechanism to now tilt the platform back again...And we then have an oscillation....
The frequency of this is quite slow (84 minutes), and unless you dampen this the INS/IRS wold be worthless.
Schuler was a German engineer who claimed that INS/IRS system would be impossible to build, because an accelerometer, is unable to figure out if it is measuring actual acceleration or "false" acceleration due to a tilted platform. He actually published a science paper about this :-D
Last edited by lasseb; 6th Aug 2012 at 11:14.