BDS and ultra-high rate GNSS in monitoring coseismic dynamic motions of the 2022 Luding M_W6.8 and 2025 Dingri M_W7.1 earthquakes in China

High-rate Global Navigation Satellite Systems (hrGNSS) provide clip-free, geodetically referenced displacement time series that are valuable for resolving near-field earthquake dynamics. Here, we analyze ultra-high-rate (50 Hz) and conventional (1 Hz) hrGNSS data from the 2022 Luding M_W6.8 and the 2025 Dingri M_W7.1 earthquakes. We process the data using both differential positioning and precise point positioning (PPP), and evaluate single-GPS, GPS-GLONASS-Galileo (GRE), and GPS-GLONASS-Galileo-BeiDou-2 (GREC2) solutions. The GREC2 combination significantly reduces positioning noise relative to GPS-only, with improvements of 2%-23% and 49%-83% on the east- and north-components, respectively, while GRE shows smaller but systematic gains (east 2%-24%, north 30%-72%). Differential positioning shows slightly lower noise than PPP, i.e., about 14% east, about 6% north on average, despite a sub-centimeter precision for both approaches. The near-fault 50 Hz waveforms in the Luding event feature the resolvable energy within the 0.3-0.5 Hz band, implying potential aliasing in 1 Hz records. These results demonstrate that integrating BeiDou Navigation Satellite System (BDS) can improve the precision of dynamic displacements. However, the 1 Hz sampling rate may be susceptible to aliasing effects when the site is located near the fault.