USING UTC TO DETERMINE THE EARTH'S ROTATION ANGLE

The Earth's rotation angle is a critical component of the suite of five Earth orientation parameters used to transform between terrestrial and celestial reference systems. This angle is defined mathematically using an adopted conventional relationship between UT1 and the mathematical quantity known as "Earth Rotation Angle" (ERA). For practical purposes, then, UT1 UTC provides a convenient means to obtain UT1, knowing UTC, and thus the ERA. Because the Earth's rotational speed is variable, it is not practical to model UT1 as a function of time with the accuracy needed for many applications. Consequently astronomical and geodetic institutions from around the world share observations of the Earth's rotation angle and these data are then used to provide users the latest observations of UT1 UTC as well as predicted estimates with accuracy that depends on the prediction interval. This process can provide users with daily updates of UT1 UTC with accuracy of the order of tens of microseconds and predictions with accuracy better than 1 millisecond up to ten days in advance. The International Earth Rotation and Reference Systems Service (IERS) was established in 1987 by the International Astronomical Union and the International Union of Geodesy and Geophysics to provide this information operationally. In addition to the services routinely providing UT1 with sub-millisecond accuracy, UTC is currently adjusted to keep [UT1-UTC] < 0.9 seconds, and this definition provides a means to access UT1 automatically with accuracy of the order of one second. Should UTC be defined without the restriction keeping [UT1-UTC] < 0.9 seconds, the low accuracy estimate of UT1 (+/- 1 second) would no longer be assured. However the existing national and international services can be expected to provide the current products as they do now via paper bulletin and electronic means. It is assumed that the accuracy of those products will always reflect the state of the art. In the future, high-speed transfer of high-quality observational astronomical, meteorological, oceanic and geophysical data promise to decrease the latency of the observations and provide UT1 UTC at sub-daily intervals with increasingly improving accuracy. In addition to the current means of distribution, increasing access to electronic communication services has the potential to provide near real-time, state of the art UT1-UTC to users when and wherever it is needed. If there were sufficient demand, we might even envision a UT1-UTC application being made available for future hand-held devices.