LAUSR

High-density GPS monitoring networks have been built in seismically active areas to obtain high-precision real-time seismic displacements with high spatial resolution, which is necessary for rapid source inversion and decreasing the blind zone for earthquake early warning. To reduce the costs for large regions such as China, single-frequency (SF) receivers are preferred to densify the current sparse dual-frequency (DF) GPS network. We developed a new approach for SF precise point positioning ambiguity resolution (PPP-AR) with mixed SF and DF receivers. The ionospheric corrections, the main issue in processing SF data, were derived directly from geometry-free carrier-phase observations with fixed ambiguities, and then interpolated to the SF stations using the Kriging interpolator. The uncalibrated phase-delay (UPD) at L1 frequency was also estimated based on a regional network to enable PPP-AR. A dataset of 30 days from SAPOS network with different station densities was utilized to validate the performance of SF PPP-AR. The experimental results show that the accuracy of original ionospheric estimates is about 0.016 m, which is better than 0.035 m obtained from interpolation for an average station separation of less than 56 km. The L1 UPDs can be estimated hourly due to its high temporal stability; all of the phase residuals are within 0.3 cycle. Compared with the post-processed daily solutions, the positioning accuracies of SF kinematic PPP-AR can reach 1–2 cm and 2–3.5 cm for the horizontal and vertical components, respectively. Convergence time can be achieved within 10 min when the station spacing is less than 52 km, but it will increase to more than 20 min when the station spacing is larger than 99 km since most ambiguities cannot be fixed correctly. Finally, we applied this approach to the 2018 Mw7.9 Alaska earthquake and successfully retrieved the seismic displacement waveforms at a comparable accuracy level when compared to the ionospheric-free (IF) PPP solutions. The accuracy of the coseismic offset estimates relative to the final solutions based on the combination of GPS and accelerometer daily solutions, and is about 5–7 mm which demonstrates the capability for capturing the coseismic displacements.