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[SGD01-10] How does station position modelling affect the VLBI scale in ITRF2020?
Keywords:ITRF2020, VLBI
Introduction
The International Terrestrial Reference Frame (ITRF) is a global geodetic reference which integrates multiple space geodetic techniques, i.e. VLBI, SLR, GNSS and DORIS. The ITRF comprises transformation parameters (Translation vector, Scale factor and Rotation matrix) and station positions and velocities. In the most recent realization of the ITRF, the ITRF2020, it was found that the VLBI scale parameter has a positive drift after 2013.75. While several possible reasons for this apparent VLBI scale drift are being discussed in the IVS community, a clear explanation for the issue has not been identified yet.
In this study, we investigate the cause of the apparent VLBI scale drift in the ITRF2020 using the same approach as used for the ITRF2020 production.
Method
We compare the discontinuities and post-seismic deformation (PSD) models used for VLBI station positions and velocities with those for co-located GNSS stations in the ITRF2020, and estimate the impact of these model differences on the VLBI scale drift. We use the CATREF software for the analysis of the VLBI data. CATREF has been developed to generate ITRF solutions based on SINEX data of VLBI, SLR, GNSS and/or DORIS and other a-priori files. We also check the consistency between the non-linear behavior of the Ny-Ålesund station and a present-day ice-melting model, which could be one of the main factors causing the scale drift, derived from the analysis of CryoSat data.
Results
We added all the discontinuities denoted as “non-linearity” in the GNSS discontinuity file of CATREF in the ITRF2020 to the corresponding VLBI models and checked how large the impact is on the scale drift. The results show that non-linearities in the evolution of the Ny-Ålesund station has a great impact on the scale, and non-linearities of all other stations have almost no impact (Table 1). For more stringent constraints, we applied the velocity change of up-component in the GNSS model of Ny-Ålesund to the VLBI model. It is consistent with the results of adding discontinuities to Ny-Ålesund, and it also alleviates the scale drift (Table 1).
Other stations were also checked whether they have any non-linearities not listed in the discontinuity list of GNSS stations. We found that the GNSS stations in Wettzell have a velocity change around 2016, though it is not listed in the discontinuity list. We added a discontinuity in 2016 in the VLBI model of Wettzell. It also has a large impact on the scale drift (Table 2).
We also checked PSD models used in ITRF2020. We applied the model of Tsukuba station to Ishioka and Sejong stations which are affected by PSD of the 2011 off the Pacific coast of Tohoku Earthquake, but models for these two stations were not applied in the IRF2020 production. Including PSD models for Ishioka and Sejong improves the fitting result of their up component but has no impact on the scale parameters.
In conclusion, the scale drift can be partly explained by the uplift of the Ny-Ålesund and Wettzell stations. It is found that the Svalbard area has experienced uplift due to glacial melting (Kierulf et al. 2022). The cause of uplift in Wettzell is uncertain but it could be caused by a localized movement around the area.
The rest of the causes of the scale drift could be explained by discontinuities caused by station events. The details of this study were presented by Le Bail in the last IVS General Meeting (proceeding in prep).
Reference
Kierulf, H.P., Jack, K., Boy, J., Geyman, E.C., Mémin A., Omang, O.C.D., Steffen, H., and Steffen, R., 2022, Geophysical Journal International, 231, 1518-1534
The International Terrestrial Reference Frame (ITRF) is a global geodetic reference which integrates multiple space geodetic techniques, i.e. VLBI, SLR, GNSS and DORIS. The ITRF comprises transformation parameters (Translation vector, Scale factor and Rotation matrix) and station positions and velocities. In the most recent realization of the ITRF, the ITRF2020, it was found that the VLBI scale parameter has a positive drift after 2013.75. While several possible reasons for this apparent VLBI scale drift are being discussed in the IVS community, a clear explanation for the issue has not been identified yet.
In this study, we investigate the cause of the apparent VLBI scale drift in the ITRF2020 using the same approach as used for the ITRF2020 production.
Method
We compare the discontinuities and post-seismic deformation (PSD) models used for VLBI station positions and velocities with those for co-located GNSS stations in the ITRF2020, and estimate the impact of these model differences on the VLBI scale drift. We use the CATREF software for the analysis of the VLBI data. CATREF has been developed to generate ITRF solutions based on SINEX data of VLBI, SLR, GNSS and/or DORIS and other a-priori files. We also check the consistency between the non-linear behavior of the Ny-Ålesund station and a present-day ice-melting model, which could be one of the main factors causing the scale drift, derived from the analysis of CryoSat data.
Results
We added all the discontinuities denoted as “non-linearity” in the GNSS discontinuity file of CATREF in the ITRF2020 to the corresponding VLBI models and checked how large the impact is on the scale drift. The results show that non-linearities in the evolution of the Ny-Ålesund station has a great impact on the scale, and non-linearities of all other stations have almost no impact (Table 1). For more stringent constraints, we applied the velocity change of up-component in the GNSS model of Ny-Ålesund to the VLBI model. It is consistent with the results of adding discontinuities to Ny-Ålesund, and it also alleviates the scale drift (Table 1).
Other stations were also checked whether they have any non-linearities not listed in the discontinuity list of GNSS stations. We found that the GNSS stations in Wettzell have a velocity change around 2016, though it is not listed in the discontinuity list. We added a discontinuity in 2016 in the VLBI model of Wettzell. It also has a large impact on the scale drift (Table 2).
We also checked PSD models used in ITRF2020. We applied the model of Tsukuba station to Ishioka and Sejong stations which are affected by PSD of the 2011 off the Pacific coast of Tohoku Earthquake, but models for these two stations were not applied in the IRF2020 production. Including PSD models for Ishioka and Sejong improves the fitting result of their up component but has no impact on the scale parameters.
In conclusion, the scale drift can be partly explained by the uplift of the Ny-Ålesund and Wettzell stations. It is found that the Svalbard area has experienced uplift due to glacial melting (Kierulf et al. 2022). The cause of uplift in Wettzell is uncertain but it could be caused by a localized movement around the area.
The rest of the causes of the scale drift could be explained by discontinuities caused by station events. The details of this study were presented by Le Bail in the last IVS General Meeting (proceeding in prep).
Reference
Kierulf, H.P., Jack, K., Boy, J., Geyman, E.C., Mémin A., Omang, O.C.D., Steffen, H., and Steffen, R., 2022, Geophysical Journal International, 231, 1518-1534