Higher‐order ionospheric effects on the GPS reference frame and velocities

Abstract

We describe how GPS time series are influenced by higher‐order ionospheric effects over the last solar cycle (1995–2008) and examine implications for geophysical studies. Using 14 years of globally reprocessed solutions, we demonstrate the effect on the reference frame. Including second‐ and third‐order ionospheric terms causes up to 10 mm difference in the smoothed transformation to the International Terrestrial Reference Frame (ITRF) 2005, with the Z translation term dominant. Scale is also slightly affected, with a change of up to ∼0.05 ppb. After transformation to ITRF2005, residual effects on vertical site velocities are as high as 0.34 mm yr−1. We assess the effect of the magnetic field model on the second‐order term and find a time‐varying difference of 0–2 mm in the Z translation. We also assess the effect of omitting the third‐order term. We find that while the second‐order term is responsible for almost all the Z translation effects, it is the combination of the second‐ and third‐order terms that causes the effect on scale. Comparison of our GPS reprocessing with ITRF2005 suggests that GPS origin rates may vary with time period. For example, we find Z translation rates of −0.82 ± 0.17 mm yr−1 for 1995–2008 and 0.17 ± 0.24 mm yr−1 for 1995–2005. If GPS were to contribute to origin rate definition for future ITRFs, higher‐order ionospheric corrections would need to be applied due to their effect on translation parameters during solar maximum.