QUESTION:
Would a clock orbiting in the range from 100 to 400 miles above the Earth's surface be slow relative to clocks on Earth? Does the Space Shuttle carry clocks with enough precision to detect this?

ANSWER from Melissa Bodeau on April 15, 1997:
The short answer is no - the Shuttle's onboard Master Timing Units (MTUs) only go down to millisecond granularity, which has proven to be sufficient for our purposes. We would have to be in orbit for a while before the drift you're postulating accumulated enough to have any effect. In addition, the MTUs can be updated by the crew or from the ground by inputting a delta to the incorrect time through the General Purpose Computers (GPCs). Nominally, our procedures call for a time update to our Greenwich Mean Time (GMT) clock within the MTU when an error exceeds 100 milliseconds, or when the MTUs go through a year-end rollover.