# Precise Positioning of BDS, BDS/GPS: Implications for Tsunami Early Warning in South China Sea
## 1 Introduction
Hence, it is considered to be a more trustworthy tool, especially for the near field tsunami early warning. Several previous studies have discussed or demonstrated the potential of using GPS data for tsunami early warnings. For example, [PERSON] _et al._[16] showed that the magnitude, mechanism, and spatial extent of rupturing of the 26 December 2004 Sumatra earthquake might have been accurately determined using only 15 min of GPS data following an earthquake initiation. Simultaneously, [PERSON] _et al._[17] pointed out that the reliable prediction of tsunami waves on the Indonesian coast can be issued within less than 5 min of an earthquake by incorporating special types of near-field GPS arrays (\"GPS-Shield\" concept for Indonesia). They also proposed deployment of such arrays for other tsunamigenic active regions. Following this idea, GITEWS (German Indonesian Tsunami Early Warning System) became the first TEWS to implement real-time GPS for a tsunami early warning [18, 19]. Recently, [PERSON] _et al._[20] replayed the Great Tohoku 2011 event in that they presented a complete processing chain starting from actual raw GPS data and fully simulated the situation as it would be in a warning center ending up with a very fast and qualified tsunami early warning. Now using real-time GPS for earthquake and tsunami early warnings is an active area of research (e.g., [18, 19, 20, 21, 22, 23, 24, 25, 26]). To date, geo-hazard monitoring with GNSS is restricted to the oldest, largest and most-used GPS system. Around the end of 2012, China has completed the regional constellation of the BeiDou Satellite System (BDS), and the system has been running routinely since then. Even at its second-phase stage, numerous studies (e.g., [27, 28, 29]) have demonstrated that precise positioning performance of BDS is equal in match against that of GPS in the Asia-Pacific region. Compared with a single satellite navigation system, the fusion of GPS, GLONASS, GALILEO and BDS can significantly increase the number of observed satellites, reduce the position dilution of precision (PDOP) [29] and, thus, increase robustness and the reliability of observations. What is more, BDS, due to its similarity to GPS signal structure and frequencies, provides a much better chance for investigating the impact of multi-GNSS capacity on real-time precise positioning in terms of accuracy and reliability. In the present study, we investigate the potential of the BDS system to contribute to the tsunami early warning system in the South China Sea region. In particular, we assess the accuracy of the real-time BDS processing and corresponding source inversion due to a hypothetical tsunamigenic earthquake at the Manila trench. As shown in previous studies (e.g., [30; 31; 32; 33]), the Manila subduction zone (Figure 1) has been identified as a zone of potential tsunami hazard in the SCS region. Despite the absence of clear historical evidence on large tsunamis generated at the Manila trench, tsunami deposits found at the Xischa Islands (~1024 AD) may be attributed to a large event in this source region [34]. According to the recently compiled historical tsunami database for the northeastern South China Sea [35], annual probability of a tsunami in this region from any source is very high (~33%). However, the likelihood of a damaging tsunami per year is much smaller (1%-2%). In the absence of large historical events from the Manila trench, damaging tsunamis in the South China Sea have been previously studied by means of numerical modeling. Simulations [30; 36] suggest that an earthquake with a magnitude of Mw = 8.0 or larger at the Manila trench may cause significant tsunami damage along the whole south-eastern coast of China, Southwestern Taiwan and West Philippines. Taking the potential large-scale disaster into account, the tsunami early warning system in the SCS region was suggested by [30; 37; 38] based on DART-type deep ocean buoys. In fact, China has installed two buoys in SCS in 2014.