Determination of Ocean Tide Loading Displacements Using a Dense Continuous GPS/BDS Network

The Global Positioning System (GPS) and the Global Navigation Satellite System (GLONASS) have been used to estimate ocean tide loading displacement (OTLD) with high accuracy; however, GPS has been problematic at K2 and K1 frequencies because they coincide with the GPS orbital period and revisit period, and GLONASS with an ambiguous float performs best for the two aforementioned constituents in the north and up directions. Here, we investigated the potential of using the BeiDou Navigation Satellite System (BDS) constellation [the medium Earth orbit (MEO) satellite revisit period of seven sidereal days, distinct from K1] to improve the accuracy of the eight major ocean tide loading constituents. BDS and combined GPS+BDS OTLD estimations are improved based on the algorithm of GPS kinematic precise point positioning (PPP), and all of them resolve ambiguity. Data from 72 continuously operating Global Navigation Satellite System (GNSS) reference stations distributed in Fujian Province from 2017 to 2020 were collected and processed to produce OTLD parameters for each of the three modes: GPS, BDS, and GPS+BDS. To investigate whether considering the effect of second-order ionosphere (Ion2) delays improved the tidal constituents, the Ion2 delays were used to calculate the OTLD parameters, and the results demonstrated that Ion2 delays improved four solar/sidereal constituents (S2, K2, K1, and P1). By comparing the root-mean-square misfits between the reference values and the GNSS OTLD estimates, the results showed that the reference/GPS+BDS misfits of most constituents were the lowest among the three components, except for K1 and P1. Meanwhile, reference/BDS misfits for the K1 and P1 constituents are all less than 0.6 mm, demonstrating that BDS estimation can improve the accuracy for K1 and P1 in three directions, particularly for the east component. The phasor plots also show that GPS, BDS, and GPS+BDS show poor agreement for the K2 constituent. BDS agrees with the reference model at the 95% confidence level, whereas GPS and GPS+BDS do not agree with the reference model for the K1 constituent. We then propose the use of a combination of constellation modes to determine the OTLD parameters: GPS+BDS for five constituents (M2, N2, O1, Q1, S2) and BDS-only solutions for the K1 and P1 constituents. The K2 constituent, which is problematic in GPS+BDS solutions and requires the help of GLONASS or Galileo, avoids orbital errors over a 12 h period.