Here's a start...
The major effect imposed by the earth's spherical
shape is somewhat similar to that caused by the
earth's rotation. As an aircraft travels across the
surface of the earth, its path becomes an arc due to
the shape of the earth. Consequently, the gyros -
particularly the pitch axis gyro - measure a rotational
rate, because traveling in a curved path always
involves rotation. This rate, called the transport
rate, does not describe rotational motion of the
aircraft with respect to the earth's surface. Therefore,
the IRS must calculate how much transport
rate is being measured by the gyros, and subtract
that value from their measurements.
INS erors continue to diverge with average amplitude
increasing exponentially