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Cheng et al. Satell Navig (2021) 2:2 Page 5 of 12

Fig. 3 Distribution of 10 selected regional reference control stations for SDCORS

Fig. 4 Diferences between P DRF and P ERF at the epoch 2015.412 at 10 reference stations (unit: cm)

Maintenance of the CGCS2000 with a linear enhanced model based on the assumption of nonlinear
velocity model station motions, including seasonal (annual and semian-
All the ITRFs, except for the ITRF2014, were established nual) signals (Davis et al. 2012) and post-seismic defor-
based on a linear model ftting the coordinates of geo- mations for those sites subject to major earthquakes. Due
detic reference sites (Altamimi et al. 2002, 2012). Te to the longer time series of the inputs and more accurate
linear assumption is signifcant for tectonic interpreta- mathematical model, the ITRF2014 has demonstrated its
tions. However, the stations that have non-linear motions superiority to its previous ITRF releases (Altamimi et al.
have their residuals up to a few centimeters, especially 2016).
when loading efects are neglected (Bennett 2008; Ble- Tere are two ways to maintain a reference frame with
witt and Lavalée 2002; Collilieux et al. 2010). Te latest linear movement. One is using an improved plate motion
release of an ITRF, the ITRF2014, was generated with an model to construct a linear velocity model (Gan 2007)