It was a theory discovered by a German of that name early last century, and it's an 88 minute period. That's the period that a pendulum that's the length of from the Earth's surface to the centre of the Earth.
I'm not sure how that relates to an INS, but he was ahead of his time as it was about 50 years before the technology was good enough to build something that needed his equations to correct themselves.
I may be wrong in all this, but it's what I read or heard some time ago.