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Objectives: On July 31, 2020, China's BeiDou-3 satellite navigation system (BDS-3), which operates independently, has been fully completed and officially opened for service. The signal system of BDS-3 has been redesigned to provide public service signals of five frequencies: B1I, B3I, B1C, B2a and B2b. Methods: Based on the observations of international global navigation system monitoring and assessment service and multi-GNSS experiment (MGEX) tracking stations, this paper analyzes the characteristics of pseudoranges multipath error, signal to noise ratio, geometry-free ionosphere-free phase(GFIFP) combined observations of the new signal system, as well as the precise orbit determination (POD) performance evaluation of BDS-3 satellites. Results and Conclusions: The results show that the multipath noise level of BDS-3 satellites signal is better than that of BeiDou-2 navigation satellite system satellites, and there is no systematic deviation related to altitude angle, B1C is more significantly affected by multipath and noise. The GFIFP sequences of different signal combinations of BDS-3 satellites show periodic systematic error related to satellite, and the peak value is about 2 cm. For BDS-3 satellites, the one-step POD method is used. The two frequencies ionospheric free combination of B1I & B3I and B1C & B2a is adopted respectively. The orbit boundary discontinuity and satellite laser ranging carried out an orbit accuracy check, and the results show that when the number of available observations is less than B1I & B3I, the orbit accuracy of B1C & B2a is equivalent to that of B1I & B3I, and the radial internal coincidence accuracy is 6.1 cm and 6.6 cm respectively. [ABSTRACT FROM AUTHOR]

The satellite Fengyun 3C(FY3C) launched in 2013 and carried a global navigation satellite system (GNSS) occultation sounder, which is the first BDS/GPS compatible radio occultation sounder in the world and records both GPS and BDS measurements. It provides an opportunity for investigating the precision realtime orbit determination based on space‐borne GPS/BDS measurements. In order to realize the real‐time orbit determination based on space‐borne GPS/BDS measurements with decimeter‐level accuracy, this paper carries out in‐depth research in the following aspects. Firstly, the real‐time orbit determination mathematical model based on the space‐borne GPS/BDS carrier phase observations is introduced. Then the properties of the errors in the signal’s propagation path that originates from GPS/BDS broadcast ephemeris are analyzed, and the signal‐in ‐propagation path error(SIPPE) statistics of different kinds of GPS/BDS satellites are also presented. Based on the analysis, a pseudo‐ambiguity is modeled as a stochastic process and estimated in the real‐time orbit determmination(RTOD) filter to absorb the SIPPE error, and then the decimeter‐ level accuracy can be realized. The space‐borne GPS/BDS data from China’ s FY3C satellite are em ‐ ployed for the simulative RTOD test using the SATODS software. Results have demonstrated that the position and velocity accuracy of 30 cm and 0.3 mm/s, respectively, are achieved when using GPS/BDS carrierphase data and broadcast ephemeris. Although the accuracy of BDS broadcast ephemeris is obviously worse than that of GPS, orbit results of the GPS/BDS fusion solution with reasonable stochastic models for pseudo‐ambiguities will be slightly superior to that using single GPS data. For example, when the FY3C satellite flew over Asia/Pacific region where more BDS satellites can be tracked, the orbit accuracy can be improved by 10% with the GPS/BDS fusion solution. [ABSTRACT FROM AUTHOR]

• The methodology of PPP-B2b augmented single-frequency precise point positioning with the raw observations is firstly examined. • Better than 10 cm accuracy in horizontal directions is achievable with the PPP-B2b augmented real-time single frequency PPP. • PPP-B2b augmented single-frequency PPP requires 1–2 h' convergence time, which is only slightly longer than the IGS final product solution. The Beidou satellite navigation system (BDS-3) precise point positioning (PPP) B2b services provide freely accessible real-time GPS/BDS-3 precise orbit and clock correction, which is beneficial for high-precision positioning applications in China and its surrounding areas. The performance of the BDS-3 PPP-B2b services for dual-frequency precise point positioning (PPP) has been investigated, while how the PPP-B2b service benefits the single-frequency (SF) users has not been investigated yet. In this paper, the quality of PPP-B2b corrections, including the orbit, clock offset, and signal-in-space ranging error (SISRE) is evaluated first, then the positioning performance of PPP-B2b augmented SF-PPP is examined. By applying the PPP-B2b corrections, both GPS and BDS-3 achieves better than 0.3 m orbit accuracy and 0.2 ns clock accuracy. The performance of PPP-B2b augmented SF-PPP is evaluated with 7 GNSS stations for 15 consecutive days. The results show the PPP-B2b augmented SF-PPP achieves better than 10 cm and 30 cm accuracy for static and kinematic cases respectively. The mean convergence time of the PPP-B2b enabled PPP is close to the final precise product-based PPP solution. [ABSTRACT FROM AUTHOR]

The advancements of Earth observations, remote sensing, communications and navigation augmentation based on low Earth orbit (LEO) platforms present strong requirements for accurate, real-time and autonomous navigation of LEO satellites. Precise onboard real-time orbit determination (RTOD) using the space-borne data of multiple global navigation satellite systems (multi-GNSS) becomes practicable along with the availability of multi-GNSS. We study the onboard RTOD algorithm and experiments by using America's Global Positioning System (GPS) and China's regional BeiDou Navigation Satellite System (BDS-2) space-borne data of the FengYun-3C satellite. A new pseudo-ambiguity parameter, which combines the constant phase ambiguity, the orbit and clock offset error of the GPS/BDS broadcast ephemeris in the line-of-sight (LOS), is defined and estimated in order to reduce the negative effect of the LOS error on onboard RTOD. The analyses on the variation of the LOS error in the GPS/BDS broadcast ephemeris indicate that the pseudo-ambiguity parameter could be modeled as a random walk, and the setting of the power spectral density in the random walk model decides whether the pseudo-ambiguity can be estimated reasonably and the LOS error could be reduced or not. For different types of GPS/BDS satellites, the LOS errors show different variation characteristics, so the power spectral density should be set separately and differently. A numerical search approach is presented in this paper to find the optimal setting of the power spectral density for each type of GPS/BDS satellite by a series of tests. Based on the optimal stochastic model, a 3-dimensional (3D) real-time orbit accuracy of 0.7–2.0 m for position and 0.7–1.7 mm/s for velocity could be achieved only with dual-frequency BDS measurements and the broadcast ephemeris, while a notably superior orbit accuracy of 0.3–0.5 m for position and 0.3–0.5 mm/s for velocity is achievable using dual-frequency GPS/BDS measurements, due to the absorption effect of the estimated pseudo-ambiguity on the LOS error of the GPS/BDS broadcast ephemeris. Compared to using GPS-alone data, the GPS/BDS fusion only marginally improves the onboard RTOD orbit accuracy by about 1–3 cm, but the inclusion of BDS satellites increases the number of the tracked GNSS satellites and thus speeds up the convergence of the filter. Furthermore, the GPS/BDS fusion could help suppress the local orbit errors, ensure the orbit accuracy and improve the reliability and availability of the onboard RTOD when fewer GPS satellites are tracked in some anomalous arcs. [ABSTRACT FROM AUTHOR]

On 12 February 2014, a major earthquake occurred near the Yutian County, China. In this paper, the ionospheric variations around the time of this earthquake were analysed on the basis of the global positioning system vertical total electron contents (VTEC) and computerised ionospheric tomography (CIT) electron density distributions. Three ionospheric anomalies on 2, 17 and 19 February were detected with the global VTEC data from Center for Orbit Determination in Europe before and after the earthquake. Based on the analysis of the geomagnetic indices, we think the anomalies on 17 and 19 February could be related to the geomagnetic disturbances. Through further investigations on the temporal and spatial characteristics of the anomaly on 2 February, we draw the conclusion that the pre-earthquake ionospheric anomaly is probably the seismo-ionospheric precursor of the Yutian earthquake. In addition, the CIT results of the anomaly on 2 February were used to study the vertical distribution of this anomaly. The results showed that the ionospheric anomaly was mainly concentrated at the heights between 200 and 400 km. [ABSTRACT FROM AUTHOR]