Originallypublished as:
Li, X., Ge, M., Dai, X., Ren, X., Fritsche, M., Wickert, J., Schuh, H. (2015): Accuracy and reliability of multi-GNSS
real-time precise positioning: GPS, GLONASS, BeiDou, and Galileo. - Journal of Geodesy, 89, 6, p. 607-635.
DOI: http://doi.org/10.1007/s00190-015-0802-8
Accuracy and reliability of Multi-GNSS real-time precise 1
positioning: GPS, GLONASS, BeiDou, and Galileo 2
3
Xingxing Li
1,2
, Maorong Ge
1
, Xiaolei Dai
1,2
, Xiaodong Ren
2
, Mathias Fritsche
1
, Jens Wickert
1
, and Harald 4
Schuh
1
5
6
1. German Research Centre for Geosciences (GFZ), Telegrafenberg, 14473 Potsdam, Germany; email: lixin@gfz-potsdam.de 7
2. Wuhan University, 129 Luoyu Road, 430079, Wuhan, Hubei, China;8
9
Abstract: In this contribution, we present a GPS+GLONASS+BeiDou+Galileo four-system model to 10
fully exploit the observations of all these four navigation satellite systems for real-time precise orbit 11
determination, clock estimation and positioning. A rigorous multi-GNSS analysis is performed to achieve 12
the best possible consistency by processing the observations from different GNSS together in one 13
common parameter estimation procedure. Meanwhile, an efficient multi-GNSS real-time precise 14
positioning service system is designed and demonstrated by using the Multi-GNSS Experiment (MGEX), 15
BeiDou Experimental Tracking Network (BETN), and International GNSS Service (IGS) networks 16
including stations all over the world. The statistical analysis of the 6 h predicted orbits show that the 17
radial and cross root mean square (RMS) values are smaller than 10 cm for BeiDou and Galileo, and 18
smaller than 5 cm for both GLONASS and GPS satellites, respectively. The RMS values of the clock 19
differences between real-time and batch-processed solutions for GPS satellites are about 0.10 ns, while 20
the RMS values for BeiDou, Galileo and GLONASS are 0.13, 0.13 and 0.14 ns, respectively. The 21
addition of the BeiDou, Galileo and GLONASS systems to the standard GPS-only processing, reduces the 22
convergence time almost by 70% , while the positioning accuracy is improved by about 25%. Some 23
outliers in the GPS-only solutions vanish when multi-GNSS observations are processed simultaneous. 24
The availability and reliability of GPS precise positioning decrease dramatically as the elevation cutoff 25
increases. However, the accuracy of multi-GNSS precise point positioning (PPP) is hardly decreased and 26
few centimeter are still achievable in the horizontal components even with 40° elevation cutoff. At 30° 27
and 40° elevation cutoffs, the availability rates of GPS-only solution drop significantly to only around 28
70% and 40% respectively. However, multi-GNSS PPP can provide precise position estimates 29
continuously (availability rate is more than 99.5%) even up to 40° elevation cutoff (e.g., in urban 30
canyons). 31
32
Keywords: Multi-GNSS constellation; Real-time Precise Point Positioning; Precise Orbit and Clock 33
Determination; GPS, GLONASS, BeiDou and Galileo 34
35
1 Introduction 36
Besides the already longer time operational Global Navigation Satellite Systems (GNSS) GPS and 37
GLONASS, two additional systems have recently emerged: Galileo and BeiDou. GPS is currently 38
operating at full capability and GLONASS has been revitalized and is also fully operational. Furthermore, 39
both GPS and GLONASS are being modernized (Cai and Gao, 2013). The European Galileo, is the third 40
GNSS, aiming to offer a continuous, more flexible and precise positioning service with a whole set of 41
related parameters and sub-services with importance for broad spectrum of applicants. Four In-Orbit 42
Validation (IOV) satellites have been successfully launched and are in orbit. Currently the IOV phase is 43
closed and it is a transition phase to Full Operational Capability (FOC). The full Galileo constellation will 44
consist of 30 satellites in three orbital planes, including three in-orbit spare ones (Montenbruck et al., 45
2014). China’s BeiDou Navigation Satellite System has been providing continuous positioning, 46
navigation and timing (PNT) services since December 27, 2012, covering the whole Asia-Pacific region. 47
The current BeiDou constellation consists of 5 Geostationary Earth Orbit (GEO), 5 Inclined 48
Geo-Synchronous Orbit (IGSO) and 4 Medium Earth Orbit (MEO) satellites available for PNT services. 49
The next installation phase will complete the constellation, which comprises 5 GEO, 3 IGSO, and 27 50
MEO satellites by the end of 2020 (China Satellite Navigation Office (CSNO), 2012). Once all four 51
systems are fully deployed, more than 100 satellites will be available for high precision PNT applications. 52
With the two new and emerging constellations BeiDou and Galileo as well as the ongoing 53
modernization of GPS and GLONASS, the world of satellite navigation is undergoing dramatic changes. 54
The next generation GNSS have the potential to enable a better and wider range of applications for PNT. 55
Already nowadays, much more satellites are in view, transmitting navigation data at more frequencies as 56
during the past years with the dual-frequency system GPS only. The accuracy, reliability and availability 57
of precise positioning will be improved significantly as compared to GPS-only solutions, provided that a 58
combination of the satellite systems is used (Ge et al., 2012, Li et al., 2015). This will also allow for the 59
important shortening of the initialization time in real-time kinematic applications. 60
In the past the data processing of multi-GNSS was focused on the fusion of GPS and GLONASS 61
(Dach et al., 2006; Cai and Gao, 2013). Thanks to the completion of the constellation of the BeiDou 62
regional system and the establishment of several ground tracking networks, BeiDou precise orbit 63
determination (POD) (Ge et al., 2012; He et al., 2013b; Zhao et al., 2013), GPS/BeiDou combined POD 64
(Shi et al. 2012, Steigenberger et al. 2011; Hauschild et al., 2012; Montenbruck et al., 2012), BeiDou 65
precise point positioning (PPP, Zumberge et al., 1997) (Li et al., 2013a, 2015) and relative positioning 66
(He et al., 2013a; Teunissen et al., 2014) have been investigated recently. Galileo satellite orbits and 67
clocks were determined using ground tracking data from the COoperative Network for GIOVE 68
Observations (CONGO) network as well as the Multi-GNSS Experiment (MGEX) network (e.g. 69
Steigenberger et al., 2011). Initial results on combined GPS/Galileo single-baseline real-time kinematic 70
(RTK) were presented by Odijk and Teunissen (2013). 71
The International GNSS Service (IGS) has initiated the MGEX since 2012 to enable an early 72
experimentation and familiarization with the emerging new signals and systems as well as to prepare a 73
future, full-featured multi-GNSS service for the scientific community (Montenbruck et al., 2014). Table 1 74
shows the MGEX analysis centers and related data products (http://www.igs.org/mgex/products). Most of 75
the above mentioned research work and MGEX products are based on single-system or dual-system (e.g., 76
GPS/GLONASS, GPS/Beidou, GPS/Galileo) modes. There is very little research and development on the 77
full exploitation of all the four navigation satellite system, except some commercial advertisement (Chen 78
et al., 2013). Meanwhile, all of the current MGEX products are generated in post-processing mode and 79
only available with a latency of several days or even longer, which cannot satisfy the requirements for 80
time-critical or real-time applications. 81
Table 1. MGEX Analysis Centers and Products 82
Institution ID Products
CNES/CLS，France grm GAL
CODE（AIUB），Switzerland com GPS+GLO+GAL
GFZ, Germany gfm GPS+GAL, GPS+BDS
ESA/ESOC，Germany esm GPS+GAL
JAXA，Japan qzf GPS+QZS
TUM，Germany tum GAL+QZS
Wuhan Univ.，China wum BDS
83
GFZ, as one of the IGS (International GNSS Service, Dow et al., 2009) real-time data analysis 84
centers, is operationally running its EPOS-RT software (Earth Parameter and Orbit determination System 85
