Universal Time Coordinated (UTC)

Local solar mean time at Greenwich Meridian. A uniform atomic time system maintained by the U.S. Naval Observatory and kept very close to UT2 by offsets. GPS time is directly relatable to UTC: UTC – GPS = seconds (the constant changes: it was 5 seconds in 1988, 8 seconds in 1993, and is 13 seconds in 2002).

Some commonly used versions of UTC are:

• UT0: Universal Time as deduced directly from observations of stars and the fixed numerical relationship between Universal and Sidereal Time, 3 minutes 56.555 seconds.
• UT1: UT0 corrected for polar motion.
• UT2: UT1 corrected for seasonal variations in the earth's rotation rate.

See also GPS time.