NGA Geomatics - GNSS (2024)

Notices

WGS 84 Terrestrial Reference Frame (TRF)

NGA implemented a new realization of the WGS 84 TRF, named WGS 84 (G2296) in its orbit and clock products beginning on January 7, 2024. Please see new WGS84 documentation in Resources > Product Documentation > WGS 84 (G2296) or reach out to geomatics@nga.mil if there is an issue.

This update ensures that the WGS 84 TRF remains consistent with the latest realization of the International Terrestrial Reference Frame, ITRF2020. WGS 84 (G2296) represents a shift of approximately 6.3 mm from the previous realization, WGS 84 (G2139), and has been demonstrated to provide a 0.7 cm improvement in accuracy relative to station positions calculated by the International GNSS Service (IGS).

Antenna Offset

With the release of WGS 84 (G2296) NGA orbit and clock products are produced using the IGS20 ANTEX Antenna Offset values. Click this link for a copy of the IGS file used for implementation. For products released after January 7, 2024, NGA recommends all users of its orbit and clock products also use the IGS20 Antenna Offset values for all GPS satellites. Please see the new WGS 84 documentation for details or reach out to geomatics@nga.mil if there is an issue.

Leap Second

• Current Leap Second: No leap second through June 2024
• Last Leap Second: December 2016
• 01 January 2017 0h UTC: TAI-UTC = 37 sec, GPS-UTC = 18 sec
• Website: https://maia.usno.navy.mil/products/leap-second

Ephemeris

The NGA GPS Precise Ephemeris is the calculated satellite positionsover the period of a day. This product serves as the DoD "Truth"standard for the GPS satellite's orbits and clock states. Theseephemerides compile with the IERS Technical Note 36/IERSConventions (2010) (TN36) for all tidal corrections, ocean loading,and tropospheric corrections with Global Mapping Function for its GPSorbit and clock estimates. The Predicted Ephemeris is also availableand provides estimated future orbits. Nine days of predictions andone thirty day prediction file are provided daily. The PreciseEphemeris is provided in either Center of Mass (COM) or AntennaePhase Center (APC) positions. The COM file references all values to asatellite's center of mass, while the APC file references to thephase center of a satellite's transmitter. All ephemeris data can befound under the Data tab.

The NGA Precise Ephemeris COM and APC files are ASCII files whichconform to the industry standard SP3 enhanced format. The namingconvention is ngawwwwd for theCOM daily files and apcwwwwd forthe APC daily files, where wwwwis the GPS week and d is the day(Sunday being 0 through Saturday being 6). The NGA Predicted Ephemerishas the same SP3 enhanced format but only provided as COM. The filesare named p#NGAwwwwd1, where # is(0-8) for the day of prediction andwwww is the GPS week andd is the day (Sundaybeing 0 and Saturday being 6) starting the day after the lastupdated Precise Ephemeris. The thirty day NGA Predicted Ephemeris isthe same naming convention but always uses # as 0 and ends in _30 for thefirst 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 inertialorientation to the WGS 84 reference frame's orientation by fitting 435days of polar X and Y axes locations and 32 days of UT1-UTCdifferences from USNO and a zonal Earth tide model. In accordance withthe IERS Technical Note 36/IERS Conventions (2010) (TN36), NGAapplies the full 62 Gross zonal tide model. This coordinate frametranslation is part of NGA's maintenance of the WGS 84 referenceframe. 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 dddis 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. Filescan 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