This precise:

To tap the F1's full accuracy, scientists have to know their precise relative position to the clock, and account for weather, altitude and other externalities. An optical cable that links the F1 to a lab at the University of Colorado, for example, can vary in length as much as 10 mm on a hot day -- something that researchers need to continually track and take into account. At F1's level of precision, even general relativity introduces problems; when technicians recently moved F1 from the third floor to the second, they had to re-tune the system to compensate for the 11-and-a-half foot drop in altitude.