The Multi-GNSS Experiment (MGEX) of the International GNSS Service (IGS) - Achievements, prospects and challenges

In this contribution, we present a GPS+GLONASS+BeiDou+Galileo four-system model to fully exploit the observations of all these four navigation satellite systems for real-time precise orbit determination, clock estimation and positioning. A rigorous multi-GNSS analysis is performed to achieve the best possible consistency by processing the observations from different GNSS together in one common parameter estimation procedure. Meanwhile, an efficient multi-GNSS real-time precise positioning service system is designed and demonstrated by using the multi-GNSS Experiment, BeiDou Experimental Tracking Network, and International GNSS Service networks including stations all over the world. The statistical analysis of the 6-h predicted orbits show that the radial and cross root mean square (RMS) values are smaller than 10 cm for BeiDou and Galileo, and smaller than 5 cm for both GLONASS and GPS satellites, respectively. The RMS values of the clock differences between real-time and batch-processed solutions for GPS satellites are about 0.10 ns, while the RMS values for BeiDou, Galileo and GLONASS are 0.13, 0.13 and 0.14 ns, respectively. The addition of the BeiDou, Galileo and GLONASS systems to the standard GPS-only processing, reduces the convergence time almost by 70 %, while the positioning accuracy is improved by about 25 %. Some outliers in the GPS-only solutions vanish when multi-GNSS observations are processed simultaneous. The availability and reliability of GPS precise positioning decrease dramatically as the elevation cutoff increases. However, the accuracy of multi-GNSS precise point positioning (PPP) is hardly decreased and few centimeter are still achievable in the horizontal components even with 40
 $$^{\circ }$$
 elevation cutoff. At 30
 $$^{\circ }$$
 and 40
 $$^{\circ }$$
 elevation cutoffs, the availability rates of GPS-only solution drop significantly to only around 70 and 40 %, respectively. However, multi-GNSS PPP can provide precise position estimates continuously (availability rate is more than 99.5 %) even up to 40
 $$^{\circ }$$
 elevation cutoff (e.g., in urban canyons).

/pdf/accuracy-and-reliability-of-multi-gnss-real-time-precise-4m191yhy3t.pdf

Accuracy and reliability of multi-GNSS real-time precise positioning: GPS, GLONASS, BeiDou, and Galileo

Many applications, such as marine navigation, land vehicles location, etc., require real time precise positioning under medium or long baseline conditions. In this contribution, we develop a model of real-time kinematic decimeter-level positioning with BeiDou Navigation Satellite System (BDS) triple-frequency signals over medium distances. The ambiguities of two extra-wide-lane (EWL) combinations are fixed first, and then a wide lane (WL) combination is reformed based on the two EWL combinations for positioning. Theoretical analysis and empirical analysis is given of the ambiguity fixing rate and the positioning accuracy of the presented method. The results indicate that the ambiguity fixing rate can be up to more than 98% when using BDS medium baseline observations, which is much higher than that of dual-frequency Hatch-Melbourne-Wubbena (HMW) method. As for positioning accuracy, decimeter level accuracy can be achieved with this method, which is comparable to that of carrier-smoothed code differential positioning method. Signal interruption simulation experiment indicates that the proposed method can realize fast high-precision positioning whereas the carrier-smoothed code differential positioning method needs several hundreds of seconds for obtaining high precision results. We can conclude that a relatively high accuracy and high fixing rate can be achieved for triple-frequency WL method with single-epoch observations, displaying significant advantage comparing to traditional carrier-smoothed code differential positioning method.

https://www.mdpi.com/1424-8220/16/1/1/pdf

Instantaneous Real-Time Kinematic Decimeter-Level Positioning with BeiDou Triple-Frequency Signals over Medium Baselines

The world of satellite navigation is undergoing dramatic changes with the rapid development of multi-constellation Global Navigation Satellite Systems (GNSSs). At the moment more than 70 satellites are already in view, and about 120 satellites will be available once all four systems (BeiDou + Galileo + GLONASS + GPS) are fully deployed in the next few years. This will bring great opportunities and challenges for both scientific and engineering applications. In this paper we develop a four-system positioning model to make full use of all available observations from different GNSSs. The significant improvement of satellite visibility, spatial geometry, dilution of precision, convergence, accuracy, continuity and reliability that a combining utilization of multi-GNSS brings to precise positioning are carefully analyzed and evaluated, especially in constrained environments.

/pdf/precise-positioning-with-current-multi-constellation-global-xmpkmzkhp8.pdf

Precise positioning with current multi-constellation Global Navigation Satellite Systems: GPS, GLONASS, Galileo and BeiDou

We will focus on single-frequency single-baseline real-time kinematic (RTK) combining four Code Division Multiple Access (CDMA) satellite systems. We will combine observations from the Chinese BeiDou Navigation Satellite System (BDS), European Galileo, American Global Positioning System (GPS) and the Japanese Quasi-Zenith Satellite System (QZSS). To further strengthen the underlying model, attention will be given to overlapping frequencies between the systems. If one can calibrate the inter-system biases, a common pivot satellite between the respective systems can be used to parameterize double-differenced ambiguities. The LAMBDA method is used for ambiguity resolution. The instantaneous (single-epoch) single-frequency RTK performance is evaluated by a formal as well as an empirical analysis, consisting of ambiguity dilution of precision (ADOP), bootstrapped and integer least-squares success rates and positioning precisions. The time-to-correct-fix in some particular cases when instantaneous RTK is not possible will also be analyzed. To simulate conditions with obstructed satellite visibility or when low-elevation multipath is present, various elevation cut-off angles between 10 and 40° will be used. Four days of real data are collected in Perth, Western Australia. It will be shown that the four-system RTK model allows for improved integer ambiguity resolution and positioning performance over the single-, dual- or triple-systems, particularly for higher cut-off angles.

Combined BDS, Galileo, QZSS and GPS single-frequency RTK

Chinese Beidou satellite navigation system constellation currently consists of eight Beidou satellites and can provide preliminary service of navigation and positioning in the Asia-Pacific Region. Based on the self-developed software Position And Navigation Data Analysis(PANDA) and Beidou Experimental Tracking Stations (BETS), which are built by Wuhan University, the study of Beidou precise orbit determination, static precise point positioning (PPP), and high precision relative positioning, and differential positioning are carried out comprehensively. Results show that the radial precision of the Beidou satellite orbit determination is better than 10 centimeters. The RMS of static PPP can reach several centimeters to even millimeters for baseline relative positioning. The precision of kinematic pseudo-range differential positioning and RTK mode positioning are 2–4 m and 5–10 cm respectively, which are close to the level of GPS precise positioning. Research in this paper verifies that, with support of ground reference station network, Beidou satellite navigation system can provide precise positioning from several decimeters to meters in the wide area and several centimeters in the regional area. These promising results would be helpful for the implementation and applications of Beidou satellite navigation system.

Precise orbit determination of Beidou Satellites with precise positioning

GNSS single-epoch real-time kinematic (RTK) positioning depends on correct ambiguity resolution. If the number of observed satellites in a single epoch is insufficient, which often happens with a standalone GNSS system, the ambiguity resolution is difficult to achieve. China's BeiDou Navigation Satellite System has been providing continuous passive positioning, navigation and timing services since December 27, 2012, covering China and the surrounding area. This new system will increase the number of satellites in view and will have a significant effect on successful ambiguity resolution. Since the BeiDou system is similar to GPS, the procedure of data processing is easier than that for the Russian GLONASS system. We briefly introduce the time and the coordinate system of BeiDou and also the BeiDou satellite visibility in China, followed by the discussion on the combined GPS/BeiDou single-epoch algorithm. Experiments were conducted and are presented here, in which the GPS/BeiDou dual-frequency static data were collected in Wuhan with the baseline distance varying from 5 to 13 km, and processed in separate and combined modes. The results indicate that, compared to a standalone GPS or BeiDou system, the combined GNSS system can increase the successful ambiguity fixing rate for single epochs and can also improve the precision of short baselines determination.

https://www.tpbin.com/Uploads/Subjects/b484a2d9-7dba-4300-9363-02bd8c34bb88.pdf

Reliable single-epoch ambiguity resolution for short baselines using combined GPS/BeiDou system

The Chinese Beidou system, also known as Compass, has entered its trial operational stage and can already provide services for triple-frequency users. Using triple-frequency signals is expected to be of great benefit for ambiguity resolution. Based on error characteristic analysis of the Beidou frequencies, we introduce the procedure of selecting the best combinations of triple-frequency signals. The geometry-based model and geometry-free model of triple-frequency signals are presented. Three triple-frequency carrier ambiguity resolution (TCAR) methods are described, which include the cascading rounding method, the stepwise AR method and the modified stepwise AR method. In order to evaluate the performance of these methods, observations from baselines of various lengths were collected using Beidou triple-frequency receivers and were processed epoch-by-epoch using the three methods. The same observation data were also processed in a dual-frequency mode for comparison. The results show that, compared to the dual-frequency based solution, the single epoch ambiguity resolution success rate with triple frequency improved nearly 30 % for the short baselines (<20 km) and 100 % for the mid-length baselines (20–50 km) using the proposed modified stepwise AR method.

https://www.tpbin.com/Uploads/Subjects/5c4180b9-aca5-46e8-8bed-75d5c49729d5.pdf

Triple-frequency carrier ambiguity resolution for Beidou navigation satellite system

As is true of GPS, multipath disturbance is a limiting factor for further improvements in BDS high precision positioning applications. We investigate the characteristics of multipath errors and multipath mitigation for BDS geostationary earth orbit, inclined geosynchronous orbit and medium earth orbit satellites according to their orbital repeat periods. We argue that multipath errors can be mitigated using models extracted from a previous period since there is a strong agreement correlation in the multipath errors between subsequent periods. A multipath mitigation method, which is based on observation domain filtering and considers the repeat period of every satellite independently, was applied. The application of the proposed method shows that BDS positioning precision was improved by about 56, 48, and 48 % to 1.90, 1.28, and 4.37 mm in the north, east, and up components. After resolving BDS carrier phase multipath error, the positioning precision of the combined GPS&BDS with multipath correction reached to 0.95, 0.92, and 2.31 mm in the north, east, and up components that were more precise than either the GPS or BDS system alone. The more accurate and reliable precision obtained from the combined system after applying multipath mitigation method, would greatly contribute to high-performance precision deformation monitoring and seismological studies.

Carrier phase multipath mitigation for BeiDou navigation satellite system

The difficulty to detect and repair cycle slip of carrier phase measurements is a key limit for continuously high accuracy of GNSS positioning and navigation services. We propose an automated cycle slip detection and repair method for data preprocessing of a CORS network. The method jointly uses double-differenced (DD) geometry-free (GF) combination and ionospheric-free observation corrected for the computed geometrical distance (IF-OMC) to estimate the cycle slips in dual-frequency observations. The DD GF combination, which is only affected by the ionospheric residual, can be used to detect cycle slips with high reliability except for special pairs such as (77, 60) on GPS L1/L2 frequencies. The detection principle of the IF-OMC observable is such that there is a large discontinuity related to the previous epoch when cycle slips occur at the present epoch. The disadvantages of these two combinations can be overcome employing the proposed detection method. The cycle slip pair (77, 60) has no effect on the GF combination, while a change of 14.65 m is derived from GPS L1/L2 observations using the IF-OMC algorithm. Using pre-determined station coordinates as precise values, we found that the accuracy of the DD IF-OMC combination was 18 mm for a 200-km CORS baseline. Therefore, cycle slips in dual-frequency observations can be correctly and uniquely determined using DD GF and IF-OMC equations. The proposed method was verified by adding simulated cycle slips in observations collected from the CORS network under a quiet ionosphere and shown to be effective. Moreover, the method was assessed with observations made during intense ionospheric activity, which generated extensive cycle slips. The results show that the algorithm can detect and repair all cycle slips apart from two exceptions relating to long data gaps.

Weiming Tang

Papers

Precise orbit determination of Beidou Satellites with precise positioning

Reliable single-epoch ambiguity resolution for short baselines using combined GPS/BeiDou system

Triple-frequency carrier ambiguity resolution for Beidou navigation satellite system

Carrier phase multipath mitigation for BeiDou navigation satellite system

A double-differenced cycle slip detection and repair method for GNSS CORS network