The latest Leap Seconds and Earth Motion files used by the SDP Toolkit are now available. The files linked to it are updated frequently - check the file headers for the exact updates and times. To view a file, click. To download, shift-click.
The Leap Seconds file ($PGSDAT/TD/leapsec.dat) contains a record of leap seconds which are designated by the International Earth Rotation Service (IERS). The Toolkit requires leap second information for accurate time conversions.
The Earth Motion file ($PGSDAT/CSC/utcpole.dat) contains a record of the Earth's variable and slowing rotation with respect to Coordinated Universal Time (UTC). The Toolkit requires this information for accurate transformations between inertial and Earth-fixed coordinates and for determining Greenwich Sidereal Time.
As a reminder, the Toolkit contains scripts to update these data files weekly. While we will post the latest files here, we encourage you to run the update scripts at your own site:
Instructions appear in the Toolkit User Guide, and as comments in the scripts themselves.
Predicted Leap Seconds and Stale File Limits :
Toolkit 5.2.X, like its predecessor, does NOT support "predicted" leap seconds. Therefore, users who wish to run simulations for far future dates will have to use some artifice, such as ignoring the error messages or temporarily altering the files. Any such activity is at the user's risk; we strongly recommend simply running your simulation for the current epoch, recent times, or the near future. If you have up-to-date data files, you should be able to run simulations as long as about 80 days in the future with the new Toolkit. We have shortened this time as a protection to the user against having stale files. The actual period during which the files can be used is 83 days from the last update, but if the local files are updated only weekly, the period can be a week less. As real time processing draws nigh, the risk of processing real science data with stale Earth motion data files dictates that we require up to date files either from running the scripts or from our web site.
While almost everyone knows about leap seconds, for those users wishing more information on the second file (Earth rotation data) a brief explanation follows:
Appendix on Earth Rotation
UTC (Coordinated Universal Time) clocks all run based on the SI second, whose length, now defined by vibrations of the Cesium atom, was set to allow 86400 seconds in a mean solar day as of ~1900. This value for the second can be expected to persist indefinitely, in order to provide continuity in physical standards, although it may be slightly refined as clocks improve.
Because the rotation of the Earth is slower than in 1900, and variable, we cannot find the Earth's axial (West to East) rotation angle, i.e. Greenwich Sidereal Time, from the time (UTC) alone. The file "utcpole.dat", updated weekly from the U.S. Naval Observatory ftp server "maia.usno.navy.mil", brings in top quality historical data and future estimates on Earth rotation. When this file is updated weekly, Earth aspect is valid to better than 1 meter accuracy. Three more detailed reports are available on the EDHS Web Page, under Toolkits, (click on "Validation of the SDP Toolkit Earth Motion Software and its Effect on Geolocation".) Greenwich Sidereal Time is actually represented in the file as "UT1-UTC"; the Toolkit transforms these values as needed. The values represent Earth rotation equivalent to up to 450 m linear surface motion.
The Earth motion data file also contains data on polar motion, a smaller correction, of order 5 to 15 m, used in the Toolkit.
Occasionally, cumulative Earth rotation falls so far behind the mean rate of ~1900 that the correction factor amounts to nearly a whole second. When the IERS (International Earth Rotation Service) in Paris estimates that this situation will occur in the next few months, it announces, in advance, a leap second. That announcement, normally 5 or 6 months ahead, is definitive and final. In unusual circumstances, however, the IERS might define a new leap second on as little as 90 days notice, which results in our choosing 83 days file life. When the leap second takes place, the UTC clock is adjusted by one second AND the data stream in "utcpole.dat" acquires a compensating jump at precisely the same time! (If we failed, you'd see the Earth rotation jump by ~15 arc seconds, or 464 m). The Toolkit is set up to meld the two changes, the leap second and the jump in UT1-UTC, seamlessly, but you need your files to be current to ensure good performance.
Peter D. Noerdlinger
Revised Jan 2, 1998