Satellite availability and point positioning accuracy evaluation on a global scale for integration of GPS, GLONASS, BeiDou and Galileo
Introduction
The satellite-based positioning technology has been extensively applied in several areas, including air, sea, and land navigation, low-earth orbit (LEO) satellite orbit determination, static and kinematic positioning, flight-state monitoring, crustal movement monitoring, and surveying (Xu, 2007, Vilayev et al., 2017). For a long period, the satellite-based positioning technology mainly relies on GPS system. In recent years, GLONASS, BeiDou and Galileo systems have boomed. A full GLONASS constellation consisting of 24 operational satellites has been completely revitalized since 2012. BeiDou was declared to provide navigation and position services over the Asia-Pacific region with a constellation of 14 operational satellites on December 27, 2012. Five new generation BeiDou satellites were successfully launched on March 30, July 25 and September 30, 2015, and February 1, 2016, respectively, which marks the start of the BeiDou system expansion from regional to global scale. The Galileo constellation has had 4 In-Orbit Validation (IOV) satellites and 14 Full Operational Capability (FOC) satellites since November 17, 2016. The Global Navigation Satellite System (GNSS) of the future will likely feature a combination of the GPS, GLONASS, BeiDou and Galileo systems (Li et al., 2015a, Li et al., 2015c, Lu et al., 2016, Montenbruck et al., 2017).
The joint use of multi-constellation signals has many advantages. Multi-constellation integrated positioning has the potential to significantly improve the positioning accuracy due to the increased number of visible satellites and the improved satellite sky distribution (Yang and Xu, 2016), especially when positioning is performed in areas with GNSS signal blockages. The increased satellite visibility can also reduce the blind areas for GNSS services, which are caused by terrain, building and tree shadowing as well as satellite failure. In addition, the reliability of position solutions can be enhanced because of the higher measurement redundancy (Yang et al., 2011, Li et al., 2015b). Thus, many efforts have been made to investigate the benefits from the multi-constellation integration, such as multi-GNSS real-time kinematic (RTK) positioning (Teunissen et al., 2014, Odolinski et al., 2015), precise point positioning (PPP) (Cai and Gao, 2013, Cai et al., 2015, Li et al., 2015), and single point positioning (SPP) (Pan et al., 2017a, Santerre et al., 2014). Most of the research work were based on the datasets from tens of or even several stations, which cover limited latitude and longitude areas. A further characterization and understanding of the positioning and navigation performance under the current GNSS constellations on a global scale is required. For a satellite-based positioning technology, the satellite visibility and Position Dilution of Precision (PDOP) can be used as important indexes to evaluate the performance (Gao et al., 2017).
We focus on the evaluation of availability and point positioning for four-constellation integration with GPS, GLONASS, BeiDou and Galileo. The availability indicates whether the position solutions can be achieved for users with a specific location, the time span that the position service can last, and the conditions of positioning accuracy during the service time. The presentation starts with a description of the current space segment status for the four GNSS systems. Subsequently, the PDOP definition for multi-GNSS constellations is presented. Next, the availability on a global scale is evaluated in terms of the number of visible satellites, the PDOP and the percentage of time span over which the position determination is feasible over the total time span during approximately a orbital repeat period. Also, the effects of compatibility between different satellite systems on availability are analyzed. Furthermore, the performance of four-constellation integrated dual-frequency PPP, single-frequency PPP and SPP is assessed using the datasets from 59 globally distributed four-system stations spanning 30 days. Finally, the main points and the conclusions are summarized.
Section snippets
Space segment status
Each satellite in the GPS constellation flies in a nearly circular orbit at an altitude of approximately 20,200 km, and circles the earth twice a day. GPS satellites are arranged into six equally-spaced orbital planes surrounding the earth, and each plane contains four slots occupied by baseline satellites. The orbital planes are inclined at 55°, and their ascending nodes are equally spaced 60° apart. The 24-slot arrangement ensures that users can view at least four GPS satellites from virtually
PDOP definition for multi-GNSS constellations
In view that the PDOP describes the propagation of random errors in measurements into the noise levels of the unknown parameters, it can be used as an important index to analyze the accuracy of positioning and navigation and the geometric strength of observations (Milbert, 2009). The GNSS positioning is based on the measurement of pseudorange which is an estimation of the distance between the satellite and the receiver. When the positioning is based on one single constellation, for example,
Availability evaluation
For the purpose of comparison, the datasets are processed in six different constellation combinations, i.e., GPS-only, GLONASS-only, BeiDou-only, Galileo-only, GPS/GLONASS and GPS/GLONASS/BeiDou/Galileo. The ground tracks of the GPS, GLONASS and Galileo satellites repeat every one day, eight days and ten days, respectively. The repeat cycle is one day for BeiDou GEO and IGSO satellites, while the orbital repeat period for BeiDou MEO satellites is seven days. In addition, the repeat cycle of the
Effects of compatibility on availability
For the purpose of investigating the effects of compatibility between different GNSS systems on the availability, the average PDOP values at a cut-off elevation angle of 40° for the four-constellation integrated case shown in Fig. 7 are re-computed without consideration of the inter-system time scale offset. The results indicate significantly decreased PDOP values. Fig. 8 provides the improvement rates of PDOP values without ITSO consideration over the PDOP values with ITSO consideration. The
Point positioning
The above results can only be used for indirectly indicating the positioning and navigation performance under the current GNSS constellations. For further analysis, datasets collected at 43 Multi-GNSS Experiment (MGEX) stations and 16 international GNSS Monitoring and Assessment System (iGMAS) stations on August 16–September 14, 2016, are employed to assess the performance of four-constellation integrated point positioning, including dual-frequency PPP, single-frequency PPP and SPP. The
Conclusions
The BeiDou and Galileo have already begun to transmit real signals for navigation and positioning applications, which has extended the GNSS family to four constellations. In this study, the availability of the current four-constellation integration with GPS, GLONASS, BeiDou and Galileo on a global scale is evaluated in terms of the number of visible satellites, the PDOP and the percentage of time span over which the position solutions can be acquired over the total time span during
Acknowledgments
This study was supported by National Natural Science Foundation of China (Grant No. 41474025) and Key Laboratory of Geospace Environment and Geodesy, Ministry of Education, Wuhan University (Grant No. 15-02-06).
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