Office of Geomatics

Enabling GEOINT accuracy through geodetic, geophysical and photogrammetric sciences and content.

"Geodesy remains our life blood"

-- Robert Cardillo, NGA Director
The Director's Intent, 10/3/14
Products and/or services described may only be available to authorized customers.

Global Navigation Satellite System (GNSS)

Welcome to the NGA GNSS Division home page. This web site contains information related to the NGA GPS Precise Ephemeris (PE) and Earth Orientation Parameter Prediction (EOPP) products as well as other pertinent documentation including document definitions, constants used, and station/antenna information.



Notices
  • As of 01-Dec-17 NGA will discontinue the production of self-extracting zip files.
  • Current Leap Second Information: No leap second at end of December 2017.
    • 01 January 2017 0h UTC: TAI-UTC = 37 sec, GPS-UTC = 18 sec
  • EPOCHA products to be changed from .EXE to ASCII format in the immediate future. Date TBD.
  • On 06 April 2019, the GPS week number will roll over.
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. NGA produces the precise ephemeris and defines the World Geodetic System (WGS 84) reference frame from GPS data at the sixteen combined USAF and NGA monitor stations around the world. The Predicted Ephemeris are also available and provide estimated future orbits. Only nine days of predictions, updated daily, are provided. 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.

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 EOPP Product Description under Resources > Documentation for details on constants used and product format. Files can be accessed from the Data tab.

Current GPS Week: 19792

Download Ephemeris & EOPP Data

To download ephemeris and EOPP data, select the data type from the drop-down box below and then select the date or date range. Use this link to download directly from the FTP site: ftp://ftp.nga.mil/pub2/gps

to
GPS Calendar Data
Coming soon.

POINT OF CONTACT
NGA Public Affairs
publicaffairs@nga.mil
Phone: 571-557-5400
Last modified: 07-Dec-17