The earth wobbles, at a frequency of about 1 wobble per 78 minutes.
An INS doesn't naturally wobble, so therefore it appears to wobble relative to the earth.
This wobble is called the Schuler cycle.
A Schuler loop is a resonant circuit set to the period and amplitude, but in antiphase, to the Schuler cycle.
Adding the output of the Schuler loop to the INS output makes the output correct, relative to the surface of the earth.
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