## 1 Introduction  
The tidal force of the Sun and Moon can cause deformation of the solid Earth, which is named \"body tide\", and it also causes periodic rise and fall of the ocean mass, which is called \"ocean tide\". The response of the solid Earth to this mass distribution is known as \"ocean tide loading\" (OTL). Modern high-precision geodetic technology can determine large-scale deformation at the sub-millimeter level, but the effect of tidal displacement must be considered [1; 2; 3; 4]. Body tide can now be modeled with an accuracy of usually less than 1% [5]. OTL displacement can be obtained by convolution integral of an ocean tide model and the Green's function calculated from an Earth model [6], which we call an OTL model. [PERSON] et al. [7] reported that the height error of M2 OTL displacement can reach around 20% (~8 mm), depending on the ocean tide model adopted and the handling strategy for the grid cells in coastal areas. The change of the vertical OTL displacement in some areas when using various Earth models can reach 2-3 mm [8]. Therefore, OTL is still the main uncertain factor in tidal displacement analysis. Since 2000, the Global Positioning System (GPS) has been widely used to estimate OTL displacement, including eight semidiurnal and diurnal tidal constituents. The methods for estimatingOTL displacement can be divided into static and kinematic methods. Adding OTL displacements (amplitude and phase) to the static (usually one-day) GPS solution model and estimating them together with other parameters such as coordinates is called a static method. [PERSON] et al. (2019) first determined the vertical OTL displacements by a static method, and reported that the amplitude differences of the M2 height component between the GPS estimates and the OTL model were less than 5 mm at 90% of the GPS reference stations. Over the next 10 years, many scholars ([PERSON] and [PERSON], 2016; [PERSON] et al., 2017; [PERSON] and [PERSON], 2018; [PERSON] and [PERSON], 2019) used static methods to estimate OTL displacements in different areas, for which the M2 vector difference between the GPS estimates and the OTL model can reach the sub-millimeter level. [PERSON] et al. (2017) estimated global OTL displacements, which indicated that the vertical root-mean-square (RMS) residuals of M2 were 0.6-1.2 mm between the GPS estimates and the different OTL models. Estimating OTL displacement from a high-sampling GPS coordinate series is called the kinematic method. [PERSON] and [PERSON] (2018), [PERSON] and [PERSON] (2018), [PERSON] et al. (2017), [PERSON] et al. (2018), and [PERSON] et al. (2019) used a GPS (1-4 h) relative coordinate series to estimate relative OTL displacements in different regions, for which the M2 vector difference between the GPS estimates and OTL models was usually 1-5 mm. [PERSON] et al. (2019) were the first to estimate absolute OTL displacements in Antarctica using a GPS kinematic precise point positioning (KPPP) coordinate series every 300 s. Since then, [PERSON] et al. (2018), [PERSON] and [PERSON] (2019), [PERSON] et al. (2019), and [PERSON] et al. (2019) have used KPPP to estimate the tidal deformation in different areas. These authors reported that the vertical M2 vector difference between the GPS estimates and OTL models can reach the sub-millimeter level. [PERSON] et al. (2019) used an 18.6-year KPPP series to estimate OTL displacements at Shanghai Astronomical Observatory (SHAO) station in China, and reported an M2 RMS agreement of around 0.6 mm in the vertical component compared with OTL models. In previous studies, the accuracy of GPS-estimated OTL displacement has been evaluated by comparing the results with OTL models. However, this is not accurate enough, because the difference between GPS estimates and OTL models also includes the uncertainties of the OTL models, the unmodeled body tide, the numerical calculation error, and so on. [PERSON] et al.