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Basic Performance and Future Developments of Beidou Global Navigation Satellite System Yuanxi Yang* , Yue Mao and Bijiao Sun

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Basic Performance and Future Developments of Beidou Global Navigation Satellite System Yuanxi Yang* , Yue Mao and Bijiao Sun Yang et al. Satell Navig (2020) 1:1 https://doi.org/10.1186/s43020-019-0006-0 Satellite Navigation https://satellite-navigation.springeropen.com/ ORIGINAL ARTICLE Open Access Basic performance and future developments of BeiDou global navigation satellite system Yuanxi Yang* , Yue Mao and Bijiao Sun Abstract 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 ofset 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 efect”). 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. Further- more, the ionospheric delay correction model of BDS-3 for all signals should be unifed 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. Keywords: BDS-3, PNT, Performance, Constellation, Ionospheric delay Introduction By the end of 2019, 28 BDS-3 satellites had been suc- Te BeiDou navigation satellite system (BDS) provides cessfully launched, including 24 medium circular orbit more services compared with other global navigation sat- (MEO) satellites, 3 inclined geostationary orbit (IGSO) ellite systems (GNSSs). Te BeiDou global navigation sat- satellites and 1 geostationary orbit (GEO) satellite. How- ellite system, (BDS-3), is the third step of China’s satellite ever, only 18 MEO satellites can provide services thus far, navigation system construction [1, 2]. In addition to the and other satellites are in test phase. positioning, navigation and timing (PNT) service pro- Te BDS-3 baseline system with 18 MEO satellites vided by all GNSSs, BDS-3 also provides regional mes- has begun providing initial services to global users on sage communication (1000 Chinese characters per time) December 27, 2018, and at least fve satellites are vis- and global short message communication (40 Chinese ible for global users. Many scholars have described the characters per time), global search and rescue service BDS-3 status and the main service function indexes (SAR), regional precise point positioning (PPP) service, [2–6], including the constellation design, service type, embedded satellite-based augmentation service (BDS- navigation signal system, space–time datum and orbit BAS), and space environment monitoring function [3–5]. determination method of BDS-3 system, etc. Te space– time datum, signal-in-space quality, accuracy of satellite broadcast ephemeris, the accuracy of the post-process- *Correspondence: [email protected] ing precise orbit, time synchronization accuracy, sat- State Key Laboratory of Geo-Information Engineering, Xi’an 710054, China ellite clock ofset accuracy and the basic PNT service © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. Yang et al. Satell Navig (2020) 1:1 Page 2 of 8 performance of the BDS-3 baseline system have be cal- Broadcast orbit accuracy: the post precise satellite posi- culated and analyzed [3–5, 7–10]. Because BDS-3 satel- tions are taken as references, and the accuracy of the lites are equipped with inter-satellite links (ISL), more positions calculated by the broadcast ephemeris is the eval- research achievements have been made in the felds of uation object. Te root mean square error (RMSE) of the ISL supported determination of satellite orbit and clock broadcast orbit is evaluated using the diferences (errors of ofset [3–5, 11–13]. According to the preliminary calcu- the broadcast orbit) of the broadcast and precise satellite lation results, the ranging accuracy of the ISLs of BDS-3 orbits in the same coordinate system, i.e. China Geodetic is about 4 cm; if the satellite orbits are determined using Coordinate System 2000 (CGCS2000) [17] and the same only regional station observations, the three-dimensional time scale, i.e. BDS time (BDT) [18]. Te orbit error can be orbit accuracy of the overlapping arc is about 60 cm; with obtained using the following formula [19–22] ISL measurements, the orbit accuracy is about 30 cm, the ⇀ ⇀ ⇀ = + · − 24-h orbit prediction accuracy is also raised from 140 to �R Rbrd A PCObrd Rpre (1) 51 cm [3–5], and the radial accuracy can reach 10 cm, as evaluated through laser observations. ⇀ ⇀ where �R is the broadcast orbit error vector; Rbrd is Te performance of BDS-3 presented by diferent the satellite position vector calculated by the broadcast scholars, using diferent data sources, are highly similar ephemeris with the unit m; A denotes the transforma- to each other. From the aspect of signal, the ratio bias of tion matrix from the satellite-fxed coordinate system to signal component efective power is better than 0.25 dB, the earth fxed system; PCObrd is the correction from the the S curve bias is less than 0.3, the signal-in-space accu- satellite antenna phase center to the satellite mass center racy calculated with post-processing ephemeris and ⇀ with the unit m; Rpre is the precise satellite position vec- broadcast ephemeris is approximately 0.5 m (root mean tor calculated by precise ephemeris with the unit m. And square, RMS), and the signal-in-space continuity and then the broadcast orbit accuracy in a specifc period can availability are approximately 99.99% and 99.78%, respec- be obtained from the statistics of diferences of the satel- tively [7]. Te timing accuracy is better than 19.1 ns lite positions. (95%), which satisfes the system service performance Broadcast clock ofset accuracy: the post precise satellite specifcations. Compared with BDS-2, BDS-3 exhib- clock ofsets are taken as the references, and the accuracy its signifcant improvement in system coverage, spatial of the satellite clock ofsets calculated by the broadcast signal accuracy, availability and continuity [2–5]. At the ephemeris is the evaluation object. Te RMS of the broad- user’s end, service improvements such as the BDS-3 sig- cast clock ofset is evaluated using the diferences between nal design and optimal receiving mode [14], precision the broadcast and precise clock ofsets. Te broadcast orbit determination [3–5, 9, 15], and precise point posi- clock ofset is calculated using the following formula tioning technology [16] have been reported in relevant N t literature and will not be repeated herein. t t k=1 ck c˜ = c − (2) Te main performance evaluation methods and formu- k k N lae are given in the second section. Te current perfor- ˜t mance of BDS-3 by the end of August 2019 is presented where ck is the broadcast clock ofset of the satellite k and analyzed in the third section. Under normal circum- after subtracting the time datum diference at the epoch t stances, the main performance indicators provided by t ; ck is the diference between the broadcast and precise BDS-3 can satisfy or surpass the design indicators, which clock ofsets after time group delay (TGD) and antenna are comparable to the service performance of other phase center corrections; N denotes the number of satel- GNSSs. Te possible improvement strategies for BDS-3 lites; k denotes the satellite number; t denotes the epoch t are presented in the fourth section from the aspects of time. ck is calculated using the following formula system construction, satellite constellation, satellite pay- 2 2 pre brd TGDm1f − TGDm2f e − e load, etc. Conclusions are presented in the fnal section ct = Bt − m1 m2 − k k − C¯ t k k 2 − 2 c k of the paper. fm1 fm2 (3) t Performance evaluation methods where Bk is the clock ofset of the k-th satellite calculated Te spatial signal accuracies of BDS-3 include broadcast by the broadcast clock ofset parameters at the epoch t; C¯ t k orbit accuracy, broadcast clock ofset accuracy, signal-in- k is the precise clock ofset of 2the -th 2satellite at the TGDm1fm1−TGDm2fm2 s 2 2 space ranging error (SISRE), broadcast ionospheric delay epoch t with the unit ; fm1−fm2 is the TGD model accuracy, etc. Specifc evaluation methods are as correction of the satellite clock ofset obtained using the follows: broadcast TGD parameters with the unit s ; TGDm1 is the time delay diference of on-board equipment Yang et al.
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