Its a fairly simple thing, and something a pilot does not need to worry about as they have absolutly no control over it.
Small correction to 18's reply, its an 84 minutes on the surface of the earth.
How this error effects an INS I will try any outline as simply as possible.
The basis for an INS is the inertial frame of referance, for an INS to "show" no motion it must be "falling" towards earth accelerating with gravity at 9.81 m/s or 32.2 ft/s however you want to look at it so that in its frame of referance it is not accelerating, a stationary INS is accelerating towards earth at a rate of gravity.
If the INS is not "falling" then to get a "zero" change in the inertial referance frame the INS must deduct the "gravity" out.
Accelerometers measure acceleration, some of the acceleration the accelerometer "sees" is gravity.
The magnitude of gravity changes with height and position on earth, and even the composition of the ground below. (if you remember newtons law of gravity, gravity reduces with height from the centre of mass "earth" by an inverse square, and earth is not a perfect sphere, it has a flattening factor, the more dense the material, the higher the mass, the higher the gravity attraction)
This image shows the variation of gravity across earth.
Now we have established that the INS need to know what the gravity is at its present position to, but where am I, and what is my gravity now ?
So the poor old INS needs to go away and do loop using the acceleration->velocity (vector)->position-> calculate gravity loop back at the new position, this is a feedback loop which will oscillate, at the Schuler frequency, and is dependant on the accuracy of the
calculated gravity at the position.
How much will it vary ...
Altitude : for commercial aircraft less than 1%
Latitude : 0.0001->0.001 %
What I also find amazing is that the weather can effect the earths
actual gravity field, if you look at this
GEOS page you will see evidence of a "gravity wave" moving at 20-30 knots as a result of a tropical depression.
From the
published conclusions of a Simulation study of INS navigation you can see some typical errors, they state
The incurred drifts from the satellite fields are quite small, 0.1-0.2 nm/hr, largest over the Alps due to the more rough gravity field. This is because the INS errors are to a large degree caused by the higher-wavelength gravity field variations, not taken into account in these simulations. The overall performance for a normal gravity field compensated INS will be very small after the gravity field is improved after GOCE mission, because the dominant error of the integrated anomalous field itself. For INS systems which incorporate gravity compensation, the simulated errors of GOCE versus EGM shows that an imporvement of about a factor 3 may be obtained for a perfectly compensated system. Therefore GOCE would be beneficial to aid navigation accuracy for the most accurate INS sytems, especially at higher (supersonic) speeds.
NB If you are a CAA/JAR examiner this is not the sort of thing a pilot will need to be examined on, from what I hear there is already enough useless crap you guys write questions on.