This paper reviews the status of satellite navigation (as per 11 May 2020)—without claim for completeness—and discusses the various global navigation satellite systems, regional satellite navigation systems and satellite-based augmentation systems. Problems and challenges for delivering nowadays a safe and reliable navigation are discussed. New opportunities, perspectives and megatrends of satellite navigation are outlined. Some remarks are closing this paper emphasizing the great value of satellite navigation at present and in future.

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Status, perspectives and trends of satellite navigation

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

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

Global Navigation Satellites Systems (GNSS) is frequently used for positioning services in various applications, e.g., pedestrian and vehicular navigation. However, it is well-known that GNSS positioning performs unreliably in urban environments. GNSS shadow matching is a method of improving accuracy in the cross-street direction. Initial position and classification of observed satellite visibility between line-of-sight (LOS) and non-line-of-sight (NLOS) are essential for its performance. For the conventional LOS/NLOS classification, the classifiers are based on a single feature, extracted from raw GNSS measurements, such as signal noise ratio, pseudorange, elevation angle, etc. Especially in urban canyons, these measurements are unstable and unreliable due to the signal reflection and refraction from the surrounding buildings. Besides, the conventional least square approach for positioning is insufficient to provide accurate initialization for shadow matching in urban areas. In our study, shadow matching is improved using the initial position from robust estimator and the satellite visibility determined by support vector machine (SVM). The robust estimator has an improved positioning accuracy and the classification rate of SVM classification can reach 91.5% in urban scenarios. An important issue is related to satellites with ultra-high or low elevation angles and satellites near the building boundary that are very likely to be misclassified. By solving this problem, the SVM classification shows the potential of about 90% classification accuracy for various urban cases. With the help of these approaches, the shadow matching has a mean error of 10.27 m with 1.44 m in the cross-street direction; these performances are suitable for urban positioning.

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Machine learning based LOS/NLOS classifier and robust estimator for GNSS shadow matching

A new precise point positioning (PPP) service has been provided by BeiDou global navigation satellite system (BDS-3). The correction parameters, including precision orbit correction, clock offset correction and differential code bias, are transmitted by PPP-B2b signal from BDS-3 geosynchronous orbit satellites. This research conducts a comprehensive evaluation of the accuracy, availability, matching characteristics of PPP-B2b corrections and real-time PPP performance for BDS-3. It is found that PPP-B2b corrections can significantly improve the discontinuous orbit and clock errors caused by broadcast ephemeris updating. The PPP-B2b real-time orbit has slightly better accuracy than the broadcast orbit, and its RMS errors are 6.8 cm, 33.4 cm and 36.6 cm in the radial, along-track and cross-track components for the medium earth orbit (MEO) satellites. However, the accuracy of real-time orbit of the inclined geostationary orbit satellites is about two times poor than that of the MEO satellites. The PPP-B2b real-time clock offset achieves an accuracy of 0.2 ns, improved by about 85.1% compared to the broadcast clock offset. Restricted by the regional tracking network, the accuracies of the PPP-B2b real-time orbit and clock offset are slightly poor than those of the Centre National d’Etudes Spatiales RT products. The availability of PPP-B2b correction in China is better than 80%, and at least 7 visible satellites with available PPP-B2b corrections can be guaranteed. It should be noted that there exists a temporary mismatching state between the orbit and clock offset corrections each time when the Issue of Data for Correction parameter changes. Test results show that an accuracy of 11 cm in horizontal and 17 cm in vertical after convergence can be achieved by BDS-3 dual-frequency real-time kinematic PPP, and the B1C/B2a ionospheric-free (IF) combination shows better positioning accuracy than the B1I/B3I IF combination. The real-time kinematic PPP employing B1C/B2a IF combination can converge to 27.8 cm in horizontal and 36 cm in vertical in 30 min, while the B1I/B3I combination achieves 42.8 cm and 53 cm in the two components at the same positioning time, respectively.

Performance evaluation of BDS-3 PPP-B2b precise point positioning service

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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Yue Mao

Papers

Basic performance and future developments of BeiDou global navigation satellite system