The ocean pole tide loading and its effect on GPS position time-series

Abstract

SUMMARY The changes in the centrifugal force induced by polar motion perturb the ocean, causing an ocean pole tide. The induced displacements due to the ocean pole tide, namely ocean pole tide loading (OPTL), previously ignored, raises the concerns in the GPS data processing with the increasing accuracy of Global Position System (GPS) technique. Though the amplitude of this effect is small, it is worth to demonstrate the magnitude and its impact on GPS solutions of the processing choices for correcting it from observations or onto estimated coordinates. For OPTL modelling, subdaily polar motion can introduce annual variations in OPTL with the small magnitudes in micrometres. The new secular pole model can cause OPTL vertical velocity difference up to 0.03 mm yr−1 compared with the mean pole model of International Earth Rotation and Reference System Service Conventions (2010). The OPTL deformation is dominated by the annual (∼365.25 d) and Chandler (∼433 d) periods, and the largest peak-to-peak variations can reach 0.70, 0.84 and 2.38 mm for East, North and Up components, respectively. We then investigate the effects of OPTL correction on GPS daily positions of 133 stations from 2014 to 2018. The root mean square of OPTL induced GPS daily displacements can reach submillimetre level. In most cases, a posteriori OPTL correction with daily averaged values applied onto the coordinates is acceptable considering the small deviation between the solution removing OPTL from coordinates and the solution correcting OPTL from observations. However, this does not hold when GPS coordinates have been aligned to a secular reference frame, as the annual/Chandler variations of OPTL would be biased. When reference frame alignment is required, either OPTL correction at observation level or a posteriori OPTL correction before reference frame alignment is recommended. Furthermore, we have demonstrated that the effects of OPTL on the annual variations of GPS position time-series can reach as much as 0.4 mm in the verticals. It has limited effect on the linear velocity and reduction of the residuals scatter (annual variations excluded), within Global Geodetic Observing System requirements for the future terrestrial reference frame at millimetre level.