Analysis of 7-Helmert Transformation Parameters between ITRF and WGS84 Following a New Realization of WGS84

Nam-Woo Noh*, Joonseong Gim, Kwan-Dong Park

In the past, the use of independent GPS Satellite Antenna Phase Center (PCO) models by the National Geospatial-Intelligence Agency (NGA) and the IGS was the primary cause of a Z-axis systematic error (T_z) of approximately 5 cm during coordinate transformation between the two reference frames. To resolve this error and minimize the differences between ITRF and WGS84, the NGA carried out a major update in 2021, unifying the GPS PCO model with the IGS and reducing the scale factor of the WGS84 coordinate system by -4.4 ppb. Accordingly, this study conducts a quantitative analysis of the 7-Helmert transformation parameters between ITRF and WGS84 following the new realization of WGS84. For the analysis, satellite positions were calculated using one year of GFZ precise ephemerides and broadcast ephemerides (BRDC) from 2024, followed by PCO corrections using IGS20.atx and the estimation of transformation parameters utilizing Least Squares Estimation (LSE). The results show that T_z, which previously reached 5.02 cm, decreased to -1.89 cm, and the scale factor shifted from -0.26 ppb to 3.59 ppb as a compensatory effect of the scale reduction. Finally, the Root-Sum-Square calculated from the 7 transformation parameters decreased from 6.25 cm to 3.24 cm, representing a reduction of approximately 50%. These results demonstrate that the systematic error between ITRF and WGS84 has been successfully eliminated, significantly improving the alignment accuracy between the two reference frames.
Keywords: ITRF, WGS84, 7-Helmert transformation parameter

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