Before any notion of standard time existed, villages and cities simply kept track of their local time, determined from the position of the Sun in the sky. When trains became an important means of transportation, these local time systems became problematic as train scheduling required a single time system. Such a time system called for the definition of time zones: typically 24 geographic strips bounded by longitudes that are multiples of 15∘. This and navigational demands gave rise to Greenwich Mean Time (GMT), based on the mean solar time at the meridian passing through Greenwich, United Kingdom, which is the conventional 0-meridian in geography. GMT became the world time standard of choice.
GMT was later replaced by Universal Time (UT), a system still based on meridian crossings of stars, albeit distant quasars, as this approach provides more accuracy than that based on the Sun. It is still the case that the rotational velocity of our planet is not constant and the length of a solar day is increasing. So UT is not a perfect system either. It continues to be used for civilian clock time, but it has now officially been replaced by International Atomic Time (TAI).
UT actually has various versions, among them UT0, UT1 and UTC. UT0 is the Earth’s rotational time observed at some location. Because the Earth experiences polar motion as well, UT0 differs between locations. If we correct for polar motion, we obtain UT1, which is identical everywhere. Nevertheless, UT1 is still a somewhat erratic clock system because of the varying rotational velocity of the planet, as mentioned above. The degree of uncertainty is about 3 ms per day.
Coordinated Universal Time (UTC) is used in satellite positioning and is maintained with atomic clocks. By convention, it is always within a margin of 0.9 s of UT1, and twice annually it may be shifted to stay within that margin. This occasional shift of a leap second is applied at the end of 30 June or, preferably, at the end of 31 December. The last minute of such a day is then either 59 or 61 seconds long. So far, adjustments have always entailed adding a second. UTC time can only be determined to the highest precision after the fact, as atomic time is determined by the reconciliation of the observed differences between a number of atomic clocks maintained by different national time bureaus.
In recent years, we have learned to measure distance, and therefore also position, with clocks, by using satellite signals, the conversion factor being the speed of light, approximately 3 × 108 m s-1 in a vacuum. As a consequence, multiple seconds of clock bias could no longer be accepted, and this is where atomic clocks are at an advantage. They are very accurate time keepers, based on the exact frequencies at which specific atoms (Cesium, Rubidium and Hydrogen) make discrete energy-state jumps. Positioning satellites usually have multiple clocks on board; ground control stations have even better quality atomic clocks.
Atomic clocks are not flawless, however: their timing tends to drift from true time and they, too, need to be corrected. The drift, and the change in drift over time, are monitored and included in the satellite’s navigation message, so that these discrepancies can be corrected for.