If an earth quake can accelerate the rotation with a certain movement or change in placement of mass, isn't it equally likely to slow the rotation with an opposing change from a different shift in tectonics? And how the heck does NASA measure this change so deep inside earth's crust? I'm a bit skeptical.
Is GPS satellite data precise to the microsecond? I don't think so.
I think they measure the rotation against the fixed stars. It wouldn’t be hard to get it to below microsecond precision.
averaging the time a signal takes to transmit from one ‘fixed’ point to one in space can certainly be averaged when that signal travels at 187,000 miles per second
the delay in response is averaged between signals all the time. If that average changes to any measurable degree, you either have moving ‘fixed’ points or moving satellites, yet satellites also transmit to each other.
The assumption is that the circumference decreased slightly, thus accelerating the rotation much as a figure skater accelerates her spin by pulling her arms inward. The angular momentum must remain constant.
Once upon a time, I made fiber optic gyroscopes for a living. The FOGs we made were for tactical applications, not high-accuracy navigation. However, we could still easily measure the Earth’s rotation with our gyros. A high accuracy gyro would be more than able to measure such a small change in angular momentum.