Rongxin Fang

Real‐time capture of seismic waves using high‐rate multi‐GNSS observations: Application to the 2015 Mw 7.8 Nepal earthquake

The variometric approach is investigated to measure real-time seismic waves induced by the 2015 Mw 7.8 Nepal earthquake with high-rate multi-GNSS observations, especially with the contribution of newly available BDS. The velocity estimation using GPS + BDS shows an additional improvement of around 20% with respect to GPS-only solutions. We also reconstruct displacements by integrating GNSS-derived velocities after a linear trend removal (IGV). The displacement waveforms with accuracy of better than 5 cm are derived when postprocessed GPS precise point positioning results are used as ground truth, even if those stations have strong ground motions and static offsets of up to 1–2 m. GNSS-derived velocity and displacement waveforms with the variometric approach are in good agreement with results from strong motion data. We therefore conclude that it is feasible to capture real-time seismic waves with multi-GNSS observations using the IGV-enhanced variometric approach, which has critical implications for earthquake early warning, tsunami forecasting, and rapid hazard assessment.

Real-time high-precision earthquake monitoring using single-frequency GPS receivers

Abstract We present a new method for high-precision real-time earthquake monitoring that focuses on station velocity using single-frequency GPS receivers. By means of GPS absolute velocity determination (AVD), real-time station velocity of millimeter per second precision can be obtained epoch by epoch using the GPS broadcast ephemeris. While the monitoring stations are considered stationary during aseismic time, the coseismic displacement can be instantaneously recorded by the velocity changes. The 1-Hz GPS data at selected stations during the Mw 8.0 Wenchuan earthquake, the Mw 9.0 Tohoku-Oki earthquake and the Mw 6.6 Lushan earthquake are processed with the AVD approach. The results indicate that the phase and amplitude of AVD-derived velocity time series induced by the earthquakes coincide with those derived from post-mission kinematic PPP approach. Even when coseismic displacements at some stations are occasionally not visible in the position time series, the velocity series can still clearly manifest the earthquake displacement signal. With the AVD approach, the precision and reliability of the real-time monitoring can be improved significantly. It is also indicated the velocity series can be used for fast post-earthquake assessment such as rapid determination of earthquake magnitude, locating epicenter, determining seismic wave propagation velocity and other earthquake studies.

BDS Precise Point Positioning for Seismic Displacements Monitoring: Benefit from the High-Rate Satellite Clock Corrections

In order to satisfy the requirement of high-rate high-precision applications, 1 Hz BeiDou Navigation Satellite System (BDS) satellite clock corrections are generated based on precise orbit products, and the quality of the generated clock products is assessed by comparing with those from the other analysis centers. The comparisons show that the root mean square (RMS) of clock errors of geostationary Earth orbits (GEO) is about 0.63 ns, whereas those of inclined geosynchronous orbits (IGSO) and medium Earth orbits (MEO) are about 0.2–0.3 ns and 0.1 ns, respectively. Then, the 1 Hz clock products are used for BDS precise point positioning (PPP) to retrieve seismic displacements of the 2015 Mw 7.8 Gorkha, Nepal, earthquake. The derived seismic displacements from BDS PPP are consistent with those from the Global Positioning System (GPS) PPP, with RMS of 0.29, 0.38, and 1.08 cm in east, north, and vertical components, respectively. In addition, the BDS PPP solutions with different clock intervals of 1 s, 5 s, 30 s, and 300 s are processed and compared with each other. The results demonstrate that PPP with 300 s clock intervals is the worst and that with 1 s clock interval is the best. For the scenario of 5 s clock intervals, the precision of PPP solutions is almost the same to 1 s results. Considering the time consumption of clock estimates, we suggest that 5 s clock interval is competent for high-rate BDS solutions.

Epicenter and magnitude of large earthquake determined from high-rate GPS observations: A case study of the 2008 M8.0 Wenchuan earthquake

For earthquake and tsunami early warning and emergency response, the earthquake epicenter and magnitude should be determined rapidly and correctly. Using high-rate GPS observations, we can readily obtain precise and high resolution displacement time series and the seismic waveforms during the earthquake. In this paper, a new algorithm is proposed for estimating the earthquake epicenter and magnitude with the seismic waveforms derived from high-rate GPS data during the earthquake. A case study of the 2008 Wenchuan earthquake is conducted from 1 Hz GPS data and the epicenter and magnitude are determined. Compared with the results issued by the China Seismological Bureau, the estimation error of the epicenter and the magnitude is about 12 km and 0.1 magnitude unit, respectively. It has shown that high-rate GPS could be a new tool feasible for estimating the earthquake epicenter and magnitude, independent of or combined with seismometers.

Real-time cycle-slip detection of GPS undifferenced carrier-phase measurements

Real-time high-precision GPS positioning and navigation requires that cycle-slip in the undifferenced carrier-phase measurements can be detected instantaneously. A slip of only a few cycles can bias measurements enough to make centimeter-level positioning or navigation difficult. Over the past decade a number of methods have been developed to detect and repair cycle slips. The majority of methods invariably are used in the post-processing cycle-slip detection. A method has been developed from various exiting techniques, that provides real-time cycle-slip detection (i.e., using only current epochs GPS carrier-phase measurements). The approach utilizes two linear combinations, the Geometry-free and the Melbourne-Wübbena combination. The low degree polynomial fitting and running-average filter are used to detect cycle slips. Simulation tests are conducted to the kinematic data. Results indicate that single-cycle slips can be reliably detected instantaneously.

Stochastic Models of Very High-Rate (50 Hz) GPS/BeiDou Code and Phase Observations

In recent years, very high-rate (10–50 Hz) Global Navigation Satellite System (GNSS) has gained a rapid development and has been widely applied in seismology, natural hazard early warning system and structural monitoring. However, existing studies on stochastic models of GNSS observations are limited to sampling rates not higher than 1 Hz. To support very high-rate GNSS applications, we assess the precisions, cross correlations and time correlations of very high-rate (50 Hz) Global Positioning System (GPS)/BeiDou code and phase observations. The method of least-squares variance component estimation is applied with the geometry-based functional model using the GNSS single-differenced observations. The real-data experimental results show that the precisions are elevation-dependent at satellite elevation angles below 40° and nearly constant at satellite elevation angles above 40°. The precisions of undifferenced observations are presented, exhibiting different patterns for different observation types and satellites, especially for BeiDou because different types of satellites are involved. GPS and BeiDou have comparable precisions at high satellite elevation angles, reaching 0.91–1.26 mm and 0.13–0.17 m for phase and code, respectively, while, at low satellite elevation angles, GPS precisions are generally lower than BeiDou ones. The cross correlation between dual-frequency phase is very significant, with the coefficients of 0.773 and 0.927 for GPS and BeiDou, respectively. The cross correlation between dual-frequency code is much less significant, and no correlation can be found between phase and code. Time correlations exist for GPS/BeiDou phase and code at time lags within 1 s. At very small time lags of 0.02–0.12 s, time correlations of 0.041–0.293 and 0.858–0.945 can be observed for phase and code observations, respectively, indicating that the correlations in time should be taken into account in very high-rate applications.

GPS and BeiDou Differential Code Bias Estimation Using Fengyun-3C Satellite Onboard GNSS Observations

Differential code biases (DCBs) are important parameters in GNSS (Global Navigation Satellite System) applications such as positioning as well as ionosphere remote sensing. In comparison to the conventional approach, which utilizes ground-based observations and parameterizes global ionosphere maps together with DCBs, a method is presented for GPS and BeiDou system (BDS) satellite DCB estimation using onboard observations from the Chinese Fengyun-3C (FY3C) satellite. One month worth of GPS and BDS data during March 2015 was exploited and the GPS C1C-C2W and BDS C2I-C7I DCBs were explored. To improve DCB estimation precision, the dual frequency carrier phase measurements leveled by code measurements were used to form basic observation equation. Code multipath errors of the FY3C onboard GPS/BDS observations were assessed and modeled as grid maps, and their impact on DCB estimation was analyzed. By correcting code multipath errors, the stability of DCB estimates was improved by 5.0%, 3.1%, 16.2% and 13.6% for GPS, and BDS geosynchronous orbit satellites (GEOs), inclined geosynchronous satellite orbit satellites (IGSOs) and medium Earth orbit satellites (MEOs), respectively. The monthly stability of FY3C-based DCBs was at the order of 0.1 ns for GPS satellites, 0.2 ns for BDS GEOs and 0.1 ns for BDS IGSOs and MEOs. By comparison to the ground-based DCB products issued by other institutions, FY3C-based DCBs showed stability degradation for BDS C02 and C05 satellites, while, for other satellites, the stability reached a similar or even superior level. The estimated FY3C receiver DCB stability was at the order of 0.2 ns for both GPS and BDS. In addition to the DCB estimates, the obtained vertical total electron content above the FY3C satellite orbit was also investigated and its realism was examined in physical and numerical aspects.

The Impact of Tohoku-Oki Earthquake on “CMONOC” Stations

In order to analyze the impact of the Mw 9.0 Tohoku-Oki earthquake occurring in 11th, March, 2011 upon basic stations of “Crustal Movement Observation Network of China”, the paper selects 6-day long pre-seismic GPS observation data and 3-day long post-seismic GPS data, processing them by using PANDA software’s Precise Point Positioning module, thus pre-seismic and post-seismic combined coordinate solutions of the stations are obtained, and co-seismic displacement caused by the earthquake is analyzed.