Just read about some gyro stuff. What I said before about the north and south limitations: That has very little to do with the magnetic field of the planet. When you turn on an INS, it enters the alignment phase. It is supposed to be sitting on the ramp without movement. It still detects movement though; the movement of the earth since the gyro/laser gyro measures all movements in space. It take this movement/ acceleration and calculates at what latitude the unit is. Rotation speed is higher at the equator than further north and that's how the computer figures out the current latitude. This gets more difficult the closer you get to the poles because the rotaional speed differences get smaller and it gets more difficult to exacly determine the current latitude. The INS looks at this angular acceleration and since it knows where the rotation is from, it knows that the true north must be in a 90 degree angle to that. In flight, the computers constantly correct the sensed data for the rotation of the earth at any latitude and also for the aircraft following the curved surface of the planet which, when followed at a certain altitude, is sensed as a continous change of direction downward in space. Still couldn't find any of the magnetic stuff, but one thing is for sure: Either there is magnetic variation data in the data base which the system uses to interpolate magnetic variation at any give time or there must be some sensing input. So if you enter the current position during Alignment, the unit not only checks your entered position with the one it stored upon previous shutdown (reasonableness test), it also compares the latitude you entered with the one calculates to be on (system performance test). Before i go back to looking for more info, a little note from the manual: If there is a fault in the system where I can still use the IRS for attitude and heading info, I have to enter the magnetic heading (from the old Whiskey Compass) via entry in the overhead panel and redo this about every 10 minutes, just like the good old gyro compass.