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

The International global navigation satellite system (GNSS ) Service (IGS ) is an organization devoted to the generation of high-precision GNSS data and products; a service that benefits science and society. It is a voluntary federation of over 200 self-funding agencies, universities, and research institutions in more than 100 countries. Established in 1992 and formally launched on 1st January 1994, the IGS has delivered an uninterrupted time series of products that are utilized by a broad spectrum of users. IGS products have evolved over time, including the provision of GNSS data for constellations other than GPS , and the addition of real-time GNSS data and products.

The International GNSS Service

In order to better understand the differential code biases (DCBs) of global navigation satellite system, the IGGDCB method is extended to estimate the intra- and inter-frequency biases of the global positioning system (GPS), GLONASS, BeiDou navigation satellite system (BDS), and Galileo based on observations collected by the multi-GNSS experiment (MGEX) of the international GNSS service (IGS). In the approach of IGGDCB, the local ionospheric total electronic content is modeled with generalized triangular series (GTS) function rather than using a global ionosphere model or a priori ionospheric information. The DCB estimated by the IGGDCB method is compared with the DCB products from the Center for Orbit Determination in Europe (CODE) and German Aerospace Center (DLR), as well as the broadcast timing group delay (TGD) parameters over a 2-year span (2013 and 2014). The results indicate that GPS and GLONASS intra-frequency biases obtained in this work show the same precision levels as those estimated by DLR (about 0.1 and 0.2–0.4 ns for the two constellations, respectively, with respect to the products of CODE). The precision levels of IGGDCB-based inter-frequency biases estimated over the 24-month period are about 0.29 ns for GPS, 0.56 ns for GLONASS, 0.36 ns for BDS, and 0.24 ns for Galileo, respectively. Here, the accuracies of GPS and GLONASS biases are assessed relative to the products of CODE, while those of BDS and Galileo are compared with the estimates of DLR. In addition, the monthly stability indices of IGGDCB-based DCBs are 0.11 (GPS), 0.18 (GLONASS), 0.17 (BDS), and 0.14 (Galileo) ns for the individual constellation.

Determination of differential code biases with multi-GNSS observations

Integer ambiguity fixing can significantly shorten the initialization time and improve the accuracy of precise point positioning (PPP), but it still takes approximate 15 min of time to achieve reliable integer ambiguity solutions. In this contribution, we present a new strategy to augment PPP estimation with a regional reference network, so that instantaneous ambiguity fixing is achievable for users within the network coverage. In the proposed method, precise zero-differenced atmospheric delays are derived from the PPP fixed solution of the reference stations, which are disseminated to, and interpolated at user stations to correct for L1, L2 phase observations or their combinations. With the corrected observations, instantaneous ambiguity resolution can be carried out within the user PPP software, thus achieving the position solutions equivalent to the network real-time kinematic positioning (NRTK). The strategy is validated experimentally. The derived atmospheric delays and the interpolated corrections are investigated. The ambiguity fixing performance and the resulted position accuracy are assessed. The validation confirms that the new strategy can provide comparable service with NRTK. Therefore, with this new processing strategy, it is possible to integrate PPP and NRTK into a seamless positioning service, which can provide an accuracy of about 10 cm anywhere, and upgrade to a few centimeters within a regional network.

Regional reference network augmented precise point positioning for instantaneous ambiguity resolution

The focus is on the quality assessment of precise orbit and clock products for the emerging Galileo, BeiDou, and QZSS systems. Products provided by Multi-GNSS Experiment (MGEX) over 2 years are used for evaluation. First, the products are assessed by orbit and clock comparisons among individual analysis centers (ACs), which give us an objective impression of their consistency. In addition, the precise orbits are verified by satellite laser ranging (SLR) residuals, which can be regarded as indicators of orbit accuracy. Moreover, precise point positioning (PPP) tests are conducted to further verify the quality of MGEX precise orbits and clocks. Orbit comparisons show agreements of about 0.1---0.25 m for Galileo, 0.1---0.2 m for BeiDou MEOs, 0.2---0.3 m for BeiDou IGSOs, and 0.2---0.4 m for QZSS. The BeiDou GEO orbits, however, have the worst agreements having a few meters differences. Clock comparisons of individual ACs have a consistency of 0.2---0.4 ns for Galileo, 0.2---0.3 ns for BeiDou IGSOs, 0.15---0.2 ns for BeiDou MEOs, 0.5---0.8 ns for BeiDou GEOs, and 0.4---0.8 ns for QZSS in general. The SLR validations demonstrate an accuracy of about 0.1 m for the current Galileo, BeiDou IGSO/MEO orbits, and about 0.2 m for QZSS orbits. However, the SLR residuals of BeiDou GEO orbits show a systematic bias of about ź0.5 m together with a standard deviation of 0.3 m. Solutions of PPP with different products mostly agree well with each other, which further confirms the good consistency of orbits and clocks among ACs. After convergence, an accuracy of 1 mm to 1 cm for static PPP and a few centimeters for kinematic PPP is achieved using multi-GNSS observations and MGEX orbit and clock products. However, it should be noted that a few exceptions may exist throughout the evaluations due to the insufficient models, different processing strategies, and ongoing updates applied by individual ACs.

Assessment of precise orbit and clock products for Galileo, BeiDou, and QZSS from IGS Multi-GNSS Experiment (MGEX)

Realtime kinematic precise point positioning (PPP) requires 1 Hz GPS satellite clock corrections. An efficient clock estimation approach is presented. It applies a combined dual-thread algorithm consisting of an undifferenced (UD) and epoch-differenced (ED) engine. The UD engine produces absolute clock values every 5 s, and the ED engine produces relative clock values between neighboring epochs at 1-s interval. A final 1-Hz satellite clock can be generated by combining the UD absolute clock and ED relative clock efficiently and accurately. Forty stations from a global tracking network are used to estimate the realtime 1-Hz clock with the proposed method. Both the efficiency and accuracy of the resultant clock corrections are validated. Efficiency test shows that the UD processing thread requires an average time of 1.88 s on a 1-GHz CPU PC for one epoch of data, while ED processing requires only 0.25 s. Accuracy validation test shows that the estimated 1-Hz clock agrees with IGS final clock accurately. The RMS values of all the available GPS satellite clock bias are less than 0.2 ns (6 cm), and most of them are less than 0.1 ns (3 cm). All the RMS values of Signal in Space Range Error (SISRE) are at centimeter level. Applying the accurate and realtime clock to realtime PPP, an accuracy of 10 cm in the horizontal and 20 cm in the vertical is achieved after a short period of initialization.

Satellite clock estimation at 1 Hz for realtime kinematic PPP applications

The latest generation of GNSS satellites such as GPS BLOCK-IIF, Galileo and BDS are transmitting signals on three or more frequencies, thus having more choices in practice. At the same time, new challenges arise for integrating the new signals. This paper contributes to the modeling and assessment of triple-frequency PPP with BDS data. First, three triple-frequency PPP models are developed. The observation model and stochastic model are designed and extended to accommodate the third frequency. In particular, new biases such as differential code biases and inter-frequency biases as well as the parameterizations are addressed. Then, the relationships between different PPP models are discussed. To verify the triple-frequency PPP models, PPP tests with real triple-frequency data were performed in both static and kinematic scenarios. Results show that the three triple-frequency PPP models agree well with each other. Additional frequency has a marginal effect on the positioning accuracy in static PPP tests. However, the benefits of third frequency are significant in situations of where there is poor tracking and contaminated observations on frequencies B1 and B2 in kinematic PPP tests.

Modeling and assessment of triple-frequency BDS precise point positioning

This article first clearly figures out the relationship between parameters of timing group delay (TGD) and differential code bias (DCB) for BDS, and demonstrates the equivalence of TGD and DCB correction models combining theory with practice. The TGD/DCB correction models have been extended to various occasions for BDS positioning, and such models have been evaluated by real triple-frequency datasets. To test the effectiveness of broadcast TGDs in the navigation message and DCBs provided by the Multi-GNSS Experiment (MGEX), both standard point positioning (SPP) and precise point positioning (PPP) tests are carried out for BDS signals with different schemes. Furthermore, the influence of differential code biases on BDS positioning estimates such as coordinates, receiver clock biases, tropospheric delays and carrier phase ambiguities is investigated comprehensively. Comparative analysis show that the unmodeled differential code biases degrade the performance of BDS SPP by a factor of two or more, whereas the estimates of PPP are subject to varying degrees of influences. For SPP, the accuracy of dual-frequency combinations is slightly worse than that of single-frequency, and they are much more sensitive to the differential code biases, particularly for the B2B3 combination. For PPP, the uncorrected differential code biases are mostly absorbed into the receiver clock bias and carrier phase ambiguities and thus resulting in a much longer convergence time. Even though the influence of the differential code biases could be mitigated over time and comparable positioning accuracy could be achieved after convergence, it is suggested to properly handle with the differential code biases since it is vital for PPP convergence and integer ambiguity resolution.

Fei Guo

Papers

Assessment of precise orbit and clock products for Galileo, BeiDou, and QZSS from IGS Multi-GNSS Experiment (MGEX)

Satellite clock estimation at 1 Hz for realtime kinematic PPP applications

Modeling and assessment of triple-frequency BDS precise point positioning

Timing group delay and differential code bias corrections for BeiDou positioning

The contribution of Multi-GNSS Experiment (MGEX) to precise point positioning