Comparison of time transfer of IF-PPP, GIM-PPP, and RIM-PPP

Global navigation satellite system precise point positioning (GNSS PPP) is a technology widely used in precise time and frequency transfer. In this study, we compare PPP time transfer based on ionospheric-free PPP (IF-PPP), undifferenced and uncombined PPP with global ionospheric map constraint (GIM-PPP), and PPP with regional ionospheric map constraint (RIM-PPP). The receiver code bias effect on receiver clock estimates is derived for IF-PPP and GIM-PPP. In the experiment, six GNSS receivers of EUREF Permanent Network (EPN) equipped with the external hydrogen masers (H-MASER) are tested for the day of year (DOY) 009 to 365, 2020, and the regional ionospheric delay modeling estimation is performed with 202 EPN stations for the corresponding period. Then, all the PPP timing solutions are carried out in forward-filtering mode. The results show that the GIM-PPP model and the RIM-PPP mode can be used for PPP one-way timing, with accuracy identical compared to the traditional IF-PPP model. Modified Allan deviation is used to evaluate the time transfer stability. The results show that the short-term frequency stability of the GIM-PPP and RIM-PPP models is significantly better than that of the IF-PPP model, which improves by 20 and 50%, respectively.