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For real-time precise point positioning (PPP) users, the State Space Representation (SSR) messages from the server are usually transmitted by wireless data link. Due to unstable communication links in practical applications, it is common that the real-time orbit and clock streams are delivered to the users at certain time delays. For multi-GNSS combined positioning, the satellite orbit and clock types for GPS, GLONASS, BDS, and Galileo are different, the ability to maintain orbit and clock accuracy will be of different characteristics when SSR delay occurs. In this condition, it is difficult to maintain the real-time precision positioning accuracy and availability with conventional models. In this study, the orbit and clock extrapolating errors are evaluated based on GPS, GLONASS, BDS-2, BDS-3, and Galileo SSR products from CNES; and the performance of real-time positioning during outages of SSR correction are also analyzed for these four systems. The experiment results show that the accuracy of the orbital clock extrapolation decreases with the increase of the SSR delay time. However, significant differences in the orbital clock extrapolation errors are observed for different GNSS satellites under the same SSR delay. For 10 minutes SSR delay, the GLONASS signal-in-space range errors (SISREs) caused by orbit and clock offset extrapolation can reach 20.9 and 7.3 cm, respectively, while the BDS and Galileo SISREs caused by orbit and clock offset extrapolation can be kept within 2.2 cm, the clock offset has been converted to distance by the light speed. The performance of BDS and Galileo real-time positioning under SSR delays are significantly better than that of GLONASS and GPS. Compared with the traditional model, the four-system combined real-time positioning performance under SSR delay situation is significantly improved when ultra-rapid precise orbits are used and the clock extrapolation error characteristics for different satellites are considered. Using this new model, the positioning accuracy in the horizontal and vertical directions can maintain at 1.7 and 2.1 cm under 10 minutes SSR delay. This research has important reference value for improving the robustness of real-time positioning and optimizing the SSR update rate.

Aiming at the lack of high accurate antenna phase correction (PCC) model for the BDS-2 IGSO and MEO satellites, in the BDS-2 and BDS-3 satellites joint precision orbit determination (POD) and precise point positioning (PPP), in this paper, an improved method for estimating PCV and z-offset parameters is used to refine the B1I/B3I ionosphere-free linear combination PCC model for the BDS-2 IGSO and MEO satellites. Compared with the results obtained using the ground-calibrated PCO model published by BeiDou official, the STD of SLR residuals obtained by the POD using the refined PCC model is reduced by 0.6~2.4 cm, and the improvement percentage is about 8.6%~33.3%. The floating PPP solution results, based on the refined BDS-2 satellite PCC model in this paper and the existing BDS-3 PCC model, have been significantly improved by 9.5 mm (37.2%) in the elevation direction.

Three-dimensional water vapor distribution information with high spatial and temporal sampling can be obtained all-weather by using GNSS water vapor tomography technology, which has become a major research hotspot in GNSS meteorology. At present, the traditional tomography method based on single navigation system has some shortcomings, such as low utilization rate of observation data and uneven distribution of observation. This paper designs and implements a multi-GNSS water vapor tomography method which takes into account the boundary incident signal. The accuracy of the method is verified by using the observation data of Hong Kong CORS network and radio sounding products. The improvement of the results of water vapor tomography by introducing boundary incident signal and multi-GNSS signal is analyzed in detail. The results show that compared with the traditional GPS tomography method, the proposed water vapor tomography method, which combines the advantages of boundary incident signal and multi-GNSS signal, can obtain more accurate and reliable three-dimensional water vapor information.

Implementing the high-precision, real-time and three-dimensional deformation monitoring for the landslide area, which is the prerequisite for the accurate warning of landslide disasters. GNSS technology is currently the only way to directly obtain the three-dimensional vector deformation of landslide disaster surface, but GNSS has two problems of high-cost and low-reliability in large-scale landslide monitoring. The ideas of Internet of Things and the concept of "cloud platform plus monitoring terminal" are proposed in this article. Thus, we develop a real-time BeiDou/GNSS monitoring equipment with thousands RMB cost. The millimeter monitoring and warning cloud platform also are developed independently. This equipment successfully applied to real-time monitoring and early warning of Heifangtai landslide in Gansu Province. Cooperated with early warning system of the Chengdu University of Technology, we issued an accurate warning signal 40 minutes in advance to avoid casualties and property losses. The remote video surveillance installed on the landslide body recorded the whole process of landslide disaster for the first time.

Since the 21st century, with the deterioration of the world environment and the intense human activities, geological disasters have been occurred more frequently. Precision space observation technology is an important means for geological disasters monitoring and early warning. In this paper, the characteristics and monitoring methods of common geological disasters are introduced. The technical characteristics and application of InSAR, LiDAR, high resolution remote sensing and GNSS are discussed. The integration of high precision spatial monitoring technology are reviewed and prospected. Finally, the future trends of the geological disaster monitoring and early warning technology are summarized.

The ground monitoring stations of BeiDou Navigation Satellite System (BDS) are regional distribution and the number of these stations is small. The more global ground stations cannot be built in the short term. The ground regional monitoring stations are difficult to observe the global Medium Earth Orbit Satellite (MEO) continuously, which leads to low precision of orbits in BDS real-time broadcast ephemeris. Based on the above problems, in view of real time satellite GNSS data of low earth orbit satellite can effectively make up the lack of regional monitoring stations in space overlay, a method is proposed that the GNSS receivers of LEO satellites used as high dynamic space-based monitoring stations combining with the data of the ground monitoring stations to calculate and forecast the MEO satellite orbits. The numeral results show that, using the data of seven regional monitoring stations add 1 to 3 LEO satellites, the precision of the MEO orbit determination can be increased by about 21%, 34% and 55% respectively. It also shows that, the ground regional monitoring stations combined with the data of LEO satellites can effectively improve the orbit precision of MEO satellite. It is suggested that using the data combined with ground stations and LEO satellites to improve the precision of broadcast ephemeris of MEO in BDS.