Office of Geomatics

Ephemeris

The NGA GPS Precise Ephemeris is the calculated satellite positions over the period of a day. This product serves as the DoD "Truth" standard for the GPS satellite's orbits and clock states. These ephemerides compile with the IERS Technical Note 36/IERS Conventions (2010) (TN36) for all tidal corrections, ocean loading, and tropospheric corrections with Global Mapping Function for its GPS orbit and clock estimates. The Predicted Ephemeris is also available and provides estimated future orbits. Nine days of predictions and one thirty day prediction file are provided daily. The Precise Ephemeris is provided in either Center of Mass (COM) or Antennae Phase Center (APC) positions. The COM file references all values to a satellite's center of mass, while the APC file references to the phase center of a satellite's transmitter. All ephemeris data can be found under the Data tab.

The NGA Precise Ephemeris COM and APC files are ASCII files which conform to the industry standard SP3 enhanced format. The naming convention is ngawwwwd for the COM daily files and apcwwwwd for the APC daily files, where wwww is the GPS week and d is the day (Sunday being 0 through Saturday being 6). The NGA Predicted Ephemeris has the same SP3 enhanced format but only provided as COM. The files are named p#NGAwwwwd1, where # is (0-8) for the day of prediction and wwww is the GPS week and d is the day (Sunday being 0 and Saturday being 6) starting the day after the last updated Precise Ephemeris. The thirty day NGA Predicted Ephemeris is the same naming convention but always uses # as 0 and ends in _30 for the first predicted day with thirty days of predictions, p#NGAwwwwd1_30.

Earth Orientation Parameter Predictions (EOPP)

NGA extrapolates the current and future offset of Earth's inertial orientation to the WGS 84 reference frame's orientation by fitting 435 days of polar X and Y axes locations and 32 days of UT1-UTC differences from USNO and a zonal Earth tide model. In accordance with the IERS Technical Note 36/IERS Conventions (2010) (TN36), NGA applies the full 62 Gross zonal tide model. This coordinate frame translation is part of NGA's maintenance of the WGS 84 reference frame. The EOPP input file is located on the USNO website located in Resources > External Links.

The NGA EOPP files are in ASCII format. The naming convention is EOPPyddd.TXT. Where y is the last digit of the year and ddd is the calendar day of the year. This is also the effectivity day of the coefficients/predictions. Please see the NGA GNSS EOPP Parameters under Resources > Documentation for details on constants used and product format. Files can be accessed from the Data tab.

Clock State Files

An ASCII text file containing a sequence of ascending time-ordered data records of redundant SV clock estimates in the NGA SV State and Covariance and NGA Clock SV/MS state datasets.

The naming convention is CLKyddd.txt. Where y is the last digit of the year and ddd is the calendar day of the year. Please see the NGA GNSS Clock State Specifications under Resources > Documentation for details on content and product format. Files can be accessed from the Data tab.

IGS Comparisons

Excel tables and graphs of NGA vs. IGS Helmert Transformation Coefficients and Orbit Comparisons.

External Links

Product Documentation