# Application of Multi-System Combination Precise Point Positioning in Landslide Monitoring
## 1 Introduction  
As a sudden geological disaster, landslides are highly destructive and severely harmful to people's lives, property and safety. As the source of disasters is usually located in the middle and upper part of mountains and the vegetation coverage is high in most areas, it is often difficult for geological survey personnel to reach them, and thus these areas are characterized by strong concealment and difficult investigation [1, 2]. The questions of how to strengthen the monitoring of the hidden danger of geological disaster, make timely discoveries and effectively identify the potential landslide disaster have become the focus and difficulty of geological disaster prevention and control. Traditional landslide deformation monitoring uses a total station and level to monitor the displacement changes in the slope development process, which has some shortcomings, such as long monitoring periods, the significant influence of terrain, difficult measurement, high labor intensity and poor real-time performance [3, 4, 5]. Since the 1990s, differential GNSS positioning technology has been widely used in geological hazard monitoring [6, 7]. By using two GNSS receivers to form double difference observation values, common errors such as the receiver clock error and satellite clock error can be eliminated, and strong correlation errors such as tropospheric delay and ionospheric delay can be lessened. The positioning accuracy can reach centimeter level or even millimeter level [8, 9]. However,differential GNSS positioning technology requires at least one receiver to be placed on a known station for observation. When the number of visible satellites is limited, the distance between the known point and the monitoring point is too far, the environment is complex, and the height difference is too large, then the positioning accuracy is not high [10]. In 1997, American Jet Power Laboratory (JPL) [11] presented a high-precision, non-differential GPS single-point absolute positioning technology called Precise Point Positioning (PPP). It is made of a single GNSS receiver, using an IGS (International GNSS Services) center to provide the precision of satellite orbit and clock products, for a variety of model to intensive model error corrections. It uses the acquisition of pseudo-range and phase observation values to obtain high precision for coordinates of the designated position according to the earth's international frame of reference. Without the support of the reference station, the accuracy of PPP can reach a decimeter or centimeter scale. Under the condition of long-time static observation, it can achieve the same accuracy as traditional differential GPS positioning [12]. Because of the rapid development of the multi-system GNSS and the growing maturity of precision satellite orbit and precision clock calibration products provided by IGS, the application of multi-system GNSS PPP technology is the current research hotspot. Domestic and foreign scholars have studied GPS/BDS [13; 14; 15; 16], GPS/GLONASS [17; 18; 19], BDS/GPS/GLONASS [20; 21], GPS/BDS/Galileo/GLONASS [22; 23; 24; 25] and other combination modes. A large number of studies have shown that the PPP results of different combinations are different in terms of convergence time and positioning accuracy. In the case of fewer visible satellites and a complex surrounding environment, multi-system combination PPP has obvious advantages. In the application of PPP technology in landslide monitoring, [PERSON] [26] used GIPSY's PPP module to calculate the GPS data of peristaltic landslides for two years in a row. The study showed that the resolution accuracy of horizontal and elevation directions with an observation duration of 24 h was 2-3 mm and 8 mm, respectively. When there was no extreme weather in the observation period, the accuracy of the horizontal direction was 5 mm for the observation period of 4 h, and the accuracy of the elevation direction was 10 mm for the observation period of 8 h. [PERSON] et al.