Scenario Earth Orientation Parameters (EOP)
Specify the following to update the Earth Orientation Parameters (EOP).
| Option | Description |
|---|---|
| Start Time | Displays a read-only date based on the selected EOP file. |
| Stop Time | Displays a read-only date based on the selected EOP file. |
| Inherit EOP Source from Application Preferences | If you select this option, whatever choice was made on the Edit Preferences - Data Sources page is used here. You can learn more about the Data Services page here. Nothing is loaded into the scenario until you click the Reload EOP file button. |
| Filename |
Specifies an EOP file (.txt) provided by CelesTrak, which you can download from http://celestrak.com/SpaceData (default), or an EOP file (.dat) located in the DynamicEarthData directory. Use the browse button to browse to a file in a different directory. The newly selected file will be read when you click Apply. If you use a script to update the contents of the EOP file after you bring up STK, click Reload EOP file to force the EOP data to be reloaded. The source of the observed data in the CelesTrak-provided EOP file is IERS. The values are slightly different from the USNO data that the standard EOP.dat file contains. For more information, see: Using EOP and Space Weather Data for Real-Time Operations (PDF), by David A. Vallado of the Center for Space Standards & Innovation (CSSI). |
When Earth Orientation Parameters are required at times past the end of the tabulated data in the selected file, the EOP parameter values are decayed to zero over a one-day interval following the final time in the file. To change this behavior to retain the final values in the file, the last line in the file can be duplicated and the Modified Julian Date field in the new final line changed to a large value (such as 75000) to extend the applicability of those values far out into the future.
DAT files
These files contain Earth Orientation Parameters that include the pole wander values and the UT1-UTC time corrections. STK uses these parameters when transforming between Earth Fixed and Earth Mean J2000 coordinate frames. All the EOP files have identical formats. The data for these files comes from the USNO series 7 / IERS Bulletin A. The format for the data is as seen in the IERS rapid service section of Bulletin A:
| EOP Data Format | |
|---|---|
| Element | Unit |
| MJD | N/A |
| x | " (arc second) |
| error | " (arc second) |
| y | " (arc second) |
| error | " (arc second) |
| UT1-UTC | s (second) |
| error | s (second) |
The total number of data records is the first entry in the file. The EOP.dat file contains recently observed data (about five years worth) and about one year of predict data. This file is updated with every release of STK, and so the actual dates spanned by this file have changed with every release. Older versions of the EOP.dat file have also been included in this directory to maintain backward compatibility with previous versions. Version identifiers are included in the filename. If you use the more recent file (EOP.dat), you may notice small differences over the previous version of STK. Position differences are generally less than a few meters. You can find timely updates of the EOP.dat file on AGI's ftp site:
https://ftp.agi.com/pub/STKData/CentralBodies/Earth/
The file EOP.dat.all provides more historical data; this file contains the more recent data (just as in the EOP.dat file) but in addition contains observed data from as far back as about January 2,1973. Updates of this file are also available on the AGI website.
The Data Update Utility enables you to update the EOP data files.
True of Date to Earth Centered Pseudo-Fixed Transformation (Pseudo ECF)
The transformation from TOD to Earth Centered Pseudo-Fixed (Pseudo ECF) involves a single rotation about the z-axis by the apparent Greenwich hour angle. The angle is computed as the sum of the mean Greenwich hour angle and the equation of equinox. The former is also computed as the sum of the mean Greenwich hour angle at zero hour UT1 and the offset angle. The mean Greenwich hour angle is computed using a cubic polynomial in Julian Date (JD) UT1 time past the J2000 epoch. The coefficients of the polynomial are converted from US Naval Observatory circular No. 163, the Document CG-SCF-225C Code Ident 23892 and from the Explanatory supplement to the Astronomical Almanac. The offset angle is based on the Earth rotation rate, which is updated linearly as a function of zero hour JD past the J2000 epoch. The computation of the zero hour UT1 also requires tabulated values of UT1-UTC from the Earth Orientation Parameter (EOP) table.
Pole Wander Transformation
Transformation from the Pseudo ECF reference frame to the Earth Centered Fixed reference frame is based on two small angles taking into account continental drift. The angles are obtained from the Earth Orientation Parameters (EOP) table, which is constructed based on data from the US Naval Observatory. This transformation is the motion of the rotational pole.