The core performance elements of global navigation satellite system include availability, continuity, integrity and accuracy, all of which are particularly important for the developing BeiDou global navigation satellite system (BDS-3). This paper describes the basic performance of BDS-3 and suggests some methods to improve the positioning, navigation and timing (PNT) service. The precision of the BDS-3 post-processing orbit can reach centimeter level, the average satellite clock offset uncertainty of 18 medium circular orbit satellites is 1.55 ns and the average signal-in-space ranging error is approximately 0.474 m. The future possible improvements for the BeiDou navigation system are also discussed. It is suggested to increase the orbital inclination of the inclined geostationary orbit (IGSO) satellites to improve the PNT service in the Arctic region. The IGSO satellite can perform part of the geostationary orbit (GEO) satellite’s functions to solve the southern occlusion problem of the GEO satellite service in the northern hemisphere (namely the “south wall effect”). The space-borne inertial navigation system could be used to realize continuous orbit determination during satellite maneuver. In addition, high-accuracy space-borne hydrogen clock or cesium clock can be used to maintain the time system in the autonomous navigation mode, and stability of spatial datum. Furthermore, the ionospheric delay correction model of BDS-3 for all signals should be unified to avoid user confusion and improve positioning accuracy. Finally, to overcome the vulnerability of satellite navigation system, the comprehensive and resilient PNT infrastructures are proposed for the future seamless PNT services.

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Basic performance and future developments of BeiDou global navigation satellite system

Nowadays, China BeiDou Navigation Satellite System (BDS) has been developed well and provided global services with highly precise positioning, navigation and timing (PNT) as well as unique short-message communication, particularly global system (BDS-3) with higher precision multi-frequency signals. The precise point positioning (PPP) can provide the precise position, receiver clock, and zenith tropospheric delay (ZTD) with a stand-alone receiver compared to the traditional double differenced relative positioning mode, which has been widely used in PNT, geodesy, meteorology and so on. However, it has a lot of challenges for multi-frequency BDS PPP with different strategies and more unknown parameters. In this paper, the detailed PPP models using the single-, dual-, triple-, and quad-frequency BDS observations are presented and evaluated. Firstly, BDS system and PPP method are introduced. Secondly, the stochastic models of time delay bias in BDS-2/BDS-3 PPP including the neglection, random constant, random walk and white noise are presented. Then, three single-frequency, four dual-frequency, four triple-frequency and four quad-frequency BDS PPP models are provided. Finally, the BDS PPP models progress and performances including theoretical comparison of the models, positioning performances, precise time and frequency transfer, ZTD, inter-frequency bias (IFB) and differential code bias (DCB) are presented and evaluated as well as future challenges. The results show that the multi-frequency BDS observations will greatly improve the PPP performances.

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PPP models and performances from single- to quad-frequency BDS observations

China has recently established the primary constellation of BeiDou-3 global navigation satellite system (BDS-3). It is necessary to conduct a comprehensive assessment about its performance. The signal quality, ambiguity resolution efficiency and real-time kinematic (RTK) performance are assessed based on the datasets collected with two Trimble Alloy receivers that can track all open signals of BDS-3. Then, the precise point positioning (PPP) using combined BDS-2 and BDS-3 measurements is compared with the PPP using BDS-2 only, where the precise satellite orbits and clocks are determined using 116 globally distributed monitor stations. The results show that the signal quality of BDS-3 is generally better than that of BDS-2. Also, the ambiguity resolution efficiency of RTK is improved by incorporating the BDS-3 measurements with success rate improving from 88.5 to 91.4%. Regarding PPP, the convergence time is shortened from about an hour to less than half an hour, while the positioning accuracy is also improved significantly.

Initial assessment of BeiDou-3 global navigation satellite system: signal quality, RTK and PPP

The BeiDou Navigation Satellite System (BDS) has completed the constellation deployment and started to provide global services. After achieving the capabilities of global coverage, global first-class accuracy for Positioning, Navigation and Timing (PNT), global Inter-Satellite Links (ISL) networking, and global featured services, BDS will promote the construction of the comprehensive PNT infrastructure in the new era and play a more active role in international cooperation with other Global Navigation Satellite System (GNSS) providers to better serve humankind and the world.

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Global capabilities of BeiDou Navigation Satellite System

The BDS-3 preliminary system was declared functional on December 27, 2018. It enables positioning service on a global scale. This study conducts a comprehensive investigation of the BDS-3 global service from the perspective of positioning, including satellite ephemeris, benefits of the new BDS-3 satellites, and the new signal effect on three positioning modes: single-point positioning (SPP), precise-point positioning (PPP) and real-time kinematic (RTK). First, the broadcast and precise ephemeris availabilities of 18 BDS-3 Medium Earth orbit (MEO) satellites from January to June 2019 are investigated. Due to the shorter operational timespans of BDS-3 satellites, their broadcast and precise ephemeris files retrieved from Multi-GNSS Experiment (MGEX) have 1.2–19.4% and 10.9–13.77% discontinuities, respectively, while the discontinuities of BDS-2 satellite broadcast and precise ephemeris are only 0.1 and 2.1%. Second, the BDS-3 satellite orbit accuracies and clock precisions are significantly improved, i.e., below 0.5 m and 1.82 ns compared to 2 m and 2.91 ns for BDS-2 because of the inter-satellite links of BDS-3 satellites. Third, an average of 4.4–5.7 BDS satellites can be observed at stations located in North/South America, and 8.9 for the European station FFMJ located in Germany after the BDS-3 global system announcement. As a result, continuous positioning is feasible in these regions. As to four Asia–Pacific stations, their SPP accuracies are improved by 12.1–60.2% compared to those of the BDS-2-only solution. Meanwhile, the PPP and RTK convergence times of Asia–Pacific stations are also shortened. Last but not least, the new B1C signal does not bring convincing improvement to PPP and RTK positioning accuracies in this study, due to the limited number of available B1C new signals at this time.

Assessment of BDS-3 global positioning service: ephemeris, SPP, PPP, RTK, and new signal

China's BeiDou navigation system (BDS) has evolved from the demonstration navigation satellite system (BDS-1) to the regional navigation satellite system (BDS-2). Now, the global BeiDou navigation system (BDS-3) is in construction and is proceeding well. The design and functions of BDS-3 are quite different from those of both BDS-1 and BDS-2. In this paper, the general design, the coordinate reference system, and the system time basis of BDS-3 are introduced. Several new payloads designed to accomplish different objectives are described as well as the platforms on which they are hosted. Since BDS-3 consists of several different constellations, the general service capabilities and special service functions provided by these different constellations are described. The performances of the initial BDS-3 platforms are evaluated based on the available eight-medium Earth orbit (MEO) satellite configuration. The results of satellite orbit determination and prediction with and without the BDS-3 inter-satellite links (ISL) are compared and analyzed.

Introduction to BeiDou-3 navigation satellite system

BeiDou Global Navigation Satellite System (BDS-3) not only performs the normal positioning, navigation and timing (PNT) functions, but also provides featured services, which are divided into geostationary orbit (GEO) and medium earth orbit (MEO) satellite-based featured services in this paper. The former refers to regional services consisting of the regional short message communication service (RSMCS), the radio determination satellite service (RDSS), the BDS satellite-based augmented service (BDSBAS) and the satellite-based precise point positioning service via B2b signal (B2b-PPP). The latter refers to global services consisting of the global short message communication service (GSMCS) and the MEO satellite-based search and rescue (MEOSAR) service. The focus of this paper is to describe these featured services and evaluate their performances. The results show that the inter-satellite link (ISL) contributes a lot to the accuracy improvement of orbit determination and time synchronization for the whole constellation. Compared with some other final products, the root mean squares (RMS) of the BDS-3 precise orbits and broadcast clock are 25.1 cm and 2.01 ns, respectively. The positioning accuracy of single frequency is better than 6 m, and that of the generalized RDSS is usually better than 12 m. For featured services, the success rates of RSMCS and GSMCS are better than 99.9% and 95.6%, respectively; the positioning accuracies of single and dual frequency BDSBAS are better than 3 and 2 m, respectively; the positioning accuracy of B2b-PPP is better than 0.6 m, and the convergence time is usually smaller than 30 min; the single station test shows that the success rate of MEOSAR is better than 99%. Due to the ISL realization in the BDS-3 constellation, the performance and capacities of the global featured services are improved significantly.

Featured services and performance of BDS-3

The key work this study is the power-law noises determination of 18 BeiDou-3 satellites (PRN19-PRN30 and PRN32-PRN37) on-board atomic clocks. First, for the noise determination of white frequency modulation (WFM), random-walk FM (RWFM), and random-run FM (RRFM), which are the basis of the ensemble timescale filter (ETF), we evaluated the Hadamard deviation with a maximum averaging time up to 30 days. To ensure the reliability of the evaluation, the Hadamard deviation was calculated by the relative clock offset determined from two independent techniques: orbit determination and time synchronization (ODTS) and the intersatellite link (ISL). This work shows that the noise characteristics of the clock offset obtained from ODTS and ISL are different, ODTS clocks are conducive to WFM determination because of low observation noise, and the ISL clocks are conducive to RWFM and RRFM determination because of independent to satellite orbit. Second, the lag 1 and $B1$ bias functions were employed to identify different noises, and the intensities of WFM, RWFM, and RRFM were determined by the Hadamard deviations above. The results of this study indicate that: 1) rubidium clocks (Rbs) always suffer from significant RRFM noise, which improves the magnitude component noise when the averaging time achieves several days; 2) the Rbs of BeiDou-3 satellites have distinctive characteristics of WFM, RWFM, and RRFM, different from the passive hydrogen masers (PHMs), which can only discover the WFM and RWFM noises; and 3) because of the RRFM, when the averaging time is greater than 2e5 s, the Hadamard deviation of Rbs is increased, which causes the frequency stability of Rbs to become an order of magnitude lower than PHMs. Last, simulation analyses and natural Kalman timescale (NKT) constructions using the actual ISL data were employed to verify the effectiveness of noises determination. Through this work, we can almost certainly identify the process noise of ETFs, which can become a reference for ETF construction in the future.

Long-Term Frequency Stability Analysis for RRFM and Power-Law Noise Determination of the BeiDou-3 Satellite On-Board Atomic Clocks

Precise point positioning can provide accurate coordinates to users without reference stations, and the high-precision real-time clock offset product is necessary for real-time precise point positioning application. As an integral part of the third generation BeiDou Navigation Satellite System, Precision Point Positioning Service provides dual systems (BDS-3 and GPS) real-time PPP services with centimeter- and decimeter-level accuracy for static and kinematic positioning users around China, respectively. However, there exist inconsistent biases in the clock offset of Precision Point Positioning Service, which will negatively affect the positioning and timing performance of the service. By comparing with the post-processing clock offset, this paper verifies that the broadcast clock offset has smaller and more stable biases in the long term and proposes a regional clock offset estimation strategy using broadcast clock offset for a priori constraint. The results show that the new algorithm can effectively reduce the bias in PPP-B2b clock offset. The new clock offset product could improve convergence speed by 25% and 10% in the horizontal and vertical directions. For positioning accuracy, the improvement is 22% and 17%. The absolute error of timing can also be reduced by 60%.

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The Bias in PPP-B2b Real-Time Clock Offset and the Strategy to Reduce It

Satellite clock offsets are the critical parameters for The Global Navigation Satellite Systems (GNSSs) to provide position and timing (PNT) service. Unlike other GNSSs, BDS-3 uses the two-way superimposition strategy to measure satellite clock offsets. However, affected by some deficiencies of the two-way superimposition strategy, the accuracy of BDS-3 clock offsets parameters is 1.29 ns (RMS), which is the main bottleneck for BDS-3 to improve its space signal accuracy. After analyzing problems in the clock offsets measurement process of BDS-3, the paper proposes a new strategy to real-time estimate high-accuracy satellite clock offsets. The clock offsets estimated by the new strategy show a good consistency with GBM clock offsets. The averaged STD of their differences in MEO is 0.14 ns, and the clock offsets estimated by the new strategy present less fluctuation in the 1-day fitting residuals. Applying the new clock offsets to prediction, BDS-3 can reduce its clock offsets errors from 1.05 ns to 0.29 ns (RMS), about 72%. The above results indicate that the new clock offsets estimated strategy can improve the accuracy of clock offsets parameters of BDS-3 effectively.

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Yufei Yang

Papers

Introduction to BeiDou-3 navigation satellite system

Featured services and performance of BDS-3

Long-Term Frequency Stability Analysis for RRFM and Power-Law Noise Determination of the BeiDou-3 Satellite On-Board Atomic Clocks

The Bias in PPP-B2b Real-Time Clock Offset and the Strategy to Reduce It

High-Accuracy Clock Offsets Estimation Strategy of BDS-3 Using Multi-Source Observations