BDS-3 Satellites Research Articles

Optimization of ground tracking stations for BDS-3 satellite orbit determination

Abstract A precise satellite orbit is the basis for high-precision services for users, and the determination of high-precision satellite orbits depends on the reasonable distribution and volume of ground tracking stations (GTSs). In July 2020, the BeiDou global navigation satellite system (BDS-3) was completed and began to be operated globally. Based on the principle of dynamic orbit determination, we first developed an algorithm to determine the optimal distribution and number of GTSs for accurate satellite orbit determination. Subsequently, based on 10 GTSs located in China, the optimal distribution and number of GTSs with and without inter-satellite link (ISL) ranging observations were determined. Finally, real measurements of GTS and ISL ranging observation data were used to validate the GTS results yielded by the algorithm. Using 18 BDS-3 satellites, and based on non-ISL observations and 10 Chinese regional GTSs, we found that GTSs distributed near the equator contributed significantly to the accuracy of orbit determination for these satellites compared to other GTSs. Orbit determination results obtained from the measured data of the optimal GTSs were more accurate than those obtained from the measured data of uniformly distributed global GTSs. Upon the addition of 55 uniformly distributed GTSs worldwide, the orbit determination accuracy for BDS-3 satellites tended to plateau. Moreover, following the addition of ISL, although the distribution and number of GTSs was optimized using the proposed algorithm, the orbit determination results from the measured data revealed that a change in the distribution and number of GTSs had a limited effect on the BDS-3 orbit determination results.

Performance assessment of real-time precise point positioning using BDS PPP-B2b service signal

China’s BeiDou Navigation Satellite System (BDS) can provide real-time precise point positioning (PPP) service for users in China and surrounding areas with PPP-B2b signal. In this paper, real-time PPP is assessed and studied based on 8 days of PPP-B2b and BDS/GPS broadcast messages. Real-time precise ephemerides are compared against Multi-GNSS Experiment (MGEX) final products and the positioning performance of real-time PPP is evaluated with MGEX/iGMAS stations. Results show that the real-time orbit accuracy is about 0.0725, 0.2479 and 0.2587 m in directions of radial (R), along (A) and cross (C) for BDS-3 satellites, and 0.0729, 0.3098 and 0.2193 m for GPS satellites. The standard deviation values of the clock offset errors are about 0.05–0.18 ns for BDS-3 satellites, and 0.05–0.15 ns for GPS satellites. In addition, the differential code bias (DCB) parameters of PPP-B2b agree well with parameters of MGEX DCB and time group delay (TGD), with the differences less than 2 and 0.15 ns, respectively. Furthermore, for the positioning experiments, the average positioning accuracies of BDS and GPS integrated real-time PPP are about 1.07, 2.69 and 2.25 cm in north (N), east (E), and up (U) directions for static mode; and about 3.6, 5.9 and 9.4 cm for kinematic mode. Moreover, the convergence times for an accuracy level of 20 cm in N, E and U components are about 8–22 min, and 10–8 min, for static mode and kinematic mode, respectively.

The analysis on BDS-3 carrier phase time transfer under the mode of basic navigation mode

The BeiDou navigation satellite system 3rd generation (BDS-3) has been operating officially since the middle of 2020 which can provide service for the global users. The precise point positioning (PPP) time transfer is the crucial application for the global navigation satellite system. Besides the number of visible satellites, the performance of precise time transfer also strongly depends on the quality of satellite orbit and clock products. The contribution of this work is to comprehensively evaluate the precision of BDS-3 satellite orbit and clock of four post-processed products (GBM, IAC, WUM and SHA) as well as one real-time product (CNT) and analyze the performance of PPP time transfer using these products. The std of clock differences of IAC and WUM showed better agreement respect to GBM, followed by SHA and CNT. The statistics of four links show, of all the averaging time intervals, BDS-3 results estimated by the GBM, IAC and WUM scenarios are comparable, and they are more stable than those of CNT and SHA in the smooth time link in terms of frequency stability. Whereas, the discrepancy on the scenarios can be neglected in the noisy time link. For global position system (GPS) and Galileo PPP time transfer with different products, BDS-3 and Galileo has similar performance which are slightly worse than GPS.

Comparison and assessment of long-term performance of BDS-2/BDS-3 satellite atomic clocks

Recently, the performance of the new-generation BeiDou Navigation Satellite System (BDS) satellite clocks has attracted considerable attention. There are two types of BDS clocks developed in China—namely, the new rubidium (Rb-II) and passive hydrogen maser (PHM)—and these can affect positioning, navigation and timing (PNT) services. In this paper, to comprehensively evaluate and analyze the performance of BDS satellite atomic clocks, the physical characteristics are exhibited, including phase, frequency, frequency drift and noise. The physical characteristics are applied to detect the switching of satellite clocks. The frequency stability of atomic clocks is assessed by overlapping Allan deviation (ADEV) and overlapping Hadamard deviation (HDEV). The periodic characteristics of satellite clock bias data are studied through spectrum analysis. Based on 550 day precision satellite clock bias data provided by Wuhan University, China, experiments were performed, focusing on the performance of the Rb-II and PHM clocks installed on BDS-3. Some valuable conclusions are obtained: (a) the BDS-3 satellite clocks perform better than BDS-2. The root mean square value of the frequency drift of BDS-3 satellite clocks is enhanced by 41% compared to BDS-2. The noise of BDS-3 is 0.216 ns, which is 55% better than BDS-2. (b) The frequency stability of BDS-2 satellite clocks is worse than that of BDS-3. The mean daily stability of BDS-2 and BDS-3 satellite clocks by ADEV is 5.29 × 10−14 and 5.36 × 10−14, respectively, while that with HDEV is 2.35 × 10−14 and 0.70 × 10−14. For PHM clocks, the stability at 10 000 s with ADEV and HDEV is 2.29 × 10−14 and 2.25 × 10−14, respectively, while the daily stability is 0.77 × 10 −14 and 0.67 × 10−14, respectively. (c) BDS geostationary orbit (GEO) and inclined GEO satellite atomic clocks have multiple significant periodic terms while the medium earth orbit satellite clocks only own one or two, which are related to the corresponding satellite orbital periods.

Preliminary assessment of BeiDou Navigation Satellite System satellite orbit determination accuracy and positioning accuracy

Orbit determination accuracy and positioning accuracy are two key indexes to assess the performance of navigation system. With frontal positioning methods put forward, the effect of orbit determination accuracy on positioning accuracy is decreasing. In this paper, three main positioning methods provided by BDS is introduced, and the influence of BDS orbit determination accuracy on the results of single point positioning, differential positioning and precise point positioning is respectively deduced. After that, data from two receivers set at Beijing and Chengdu is analysed to evaluate orbit determination accuracy and positioning accuracy. Apart from several extreme values, BDS satellites radial orbit errors are within 1.5m, tangential orbit errors are within 2m and normal orbit errors are less than 5m. Accuracy of single point positioning differential positioning and precise point positioning is 1.9m 1.2m and 0.3m, respectively. Effect of orbit determination accuracy on single point positioning is significant. On the other hand, orbit determination accuracy has less effect on differential positioning accuracy and precise point positioning.

BDS-3/Galileo Time and Frequency Transfer with Quad-Frequency Precise Point Positioning

In this work, quad-frequency precise point positioning (PPP) time and frequency transfer methods using Galileo E1/E5a/E5b/E5 and BDS-3 B1I/B3I/B1C/B2a observations were proposed with corresponding mathematical models. In addition, the traditional dual-frequency (BDS-3 B1I/B3I and Galileo E1/E5a) ionospheric-free (IF) model was also described and tested for comparison. To assess the proposed method for time transfer, datasets selected from timing labs were utilized and tested. Moreover, the number of Galileo or BDS-3 satellites, pseudorange residuals, positioning accuracy and tropospheric delay at receiver end were all analyzed. The results showed that the proposed quad-frequency BDS-3 or Galileo PPP models could be used to time transfer, due to stability and accuracy identical to that of dual-frequency IF model. Furthermore, the quad-frequency models can provide potential for enhancing the reliability and redundancy compared to the dual-frequency time transfer method.

Assessing IGS GPS/Galileo/BDS-2/BDS-3 phase bias products with PRIDE PPP-AR

Ambiguity Resolution in Precise Point Positioning (PPP-AR) is important to achieving high-precision positioning in wide areas. The International GNSS (Global Navigation Satellite System) Service (IGS) and some other academic organizations have begun to provide phase bias products to enable PPP-AR, such as the integer-clock like products by Centre National d’Etudes Spatials (CNES), Wuhan University (WUM) and the Center for Orbit Determination in Europe (CODE), as well as the Uncalibrated Phase Delay (UPD) products by School of Geodesy and Geomatics (SGG). To evaluate these disparate products, we carry out Global Positioning System (GPS)/Galileo Navigation Satellite System (Galileo) and BeiDou Navigation Satellite System (BDS-only) PPP-AR using 30 days of data in 2019. In general, over 70% and 80% of GPS and Galileo ambiguity residuals after wide-lane phase bias corrections fall in ± 0.1 cycles, in contrast to less than 50% for BeiDou Navigation Satellite (Regional) System (BDS-2); moreover, around 90% of GPS/Galileo narrow-lane ambiguity residuals are within ± 0.1 cycles, while the percentage drops to about 55% in the case of BDS products. GPS/Galileo daily PPP-AR can usually achieve a positioning precision of 2, 2 and 6 mm for the east, north and up components, respectively, for all phase bias products except those based on German Research Centre for Geosciences (GBM) rapid satellite orbits and clocks. Due to the insufficient number of BDS satellites during 2019, the BDS phase bias products perform worse than the GPS/Galileo products in terms of ambiguity fixing rates and daily positioning precisions. BDS-2 daily positions can only reach a precision of about 10 mm in the horizontal and 20 mm in the vertical components, which can be slightly improved after PPP-AR. However, for the year of 2020, BDS-2/BDS-3 (BDS-3 Navigation Satellite System) PPP-AR achieves about 50% better precisions for all three coordinate components.

Initial Assessment of BDS PPP-B2b Service: Precision of Orbit and Clock Corrections, and PPP Performance

On 31 July 2020, the Beidou global navigation satellite system (BDS-3) was officially announced as being commissioned. In addition to offering global positioning, navigation, and timing (PNT) services, BDS-3 also provides precise point positioning (PPP) augmentation services. The satellite orbit correction, clock correction and code bias correction of BDS-3 and other global navigation satellite systems (GNSS) are broadcast by the BDS-3 geostationary earth orbit (GEO) satellites through the PPP-B2b signal. The PPP-B2b service is available for users in China and the surrounding area. In this study, an initial assessment of the PPP-B2b service is presented, with collected 3-day PPP-B2b messages. Based on broadcast ephemeris and PPP-B2b messages, the precise satellite orbits and clock offsets can be recovered. This precision is evaluated with the precise ephemeris from the GeoForschungsZentrum Potsdam (GFZ) analysis center as references. The results indicate that the accuracy of BDS-3 satellite orbits in the direction of radial, along-track, and cross-track is 0.138, 0.131, and 0.145 m, respectively, and for GPS a corresponding accuracy of 0.104, 0.160, and 0.134 m, respectively, could be obtained. The precision of clock offsets can reach a level of several centimeters for both GPS and BDS-3. Both the performance of static PPP and kinematic PPP are evaluated using the observations from four international GNSS monitoring assessment service (iGMAS) stations. Regarding static PPP, the average convergence time is 17.7 min to achieve a horizontal positioning accuracy of better than 0.3 m, and a vertical positioning accuracy of better than 0.6 m. The average positioning accuracy in the direction of east, north, and up-directions are 2.4, 1.6, and 2.3 cm. As to kinematic PPP, the average RMS values of positioning errors in the direction of east, north, and up are 8.1 cm, 3.6 cm, and 8.0 cm after full convergence.

An efficient clock offset datum switching compensation method for BDS real-time satellite clock offset estimation

The quality of real-time satellite clock offsets products is one of the key elements for real-time satellite clock performance monitoring and real-time precise point positioning (PPP). However, impacted by the station location, environment, communication equipment, receiver performance, network delay, temporary modem failure or other reasons, the observation data of clock offset datum receiver occasionally outage, resulting in all satellite clocks offset cannot be estimated. In this contribution, an efficient clock offset datum switching compensation method for BDS real-time satellite clock offset estimation is proposed. One hundred stations distributed around the world, from day of year (DOY) 074 to 075 in 2019 are used to estimate BDS real-time satellite clock offset. Four schemes: clock offset datum switching without compensation (S1), clock offset datum switching with satellite clock offset jump compensation (S2), clock offset datum switching with satellite clock offset jump and receiver clock drift compensation (S3, the proposed method), clock offset datum without switching (S4), are used to validate the proposed method. By using the proposed method, the satellite clock offset jump can be eliminated and the satellite clock offset consistency can be maintained. The satellite clock offsets STD for S1, S2, S3 and S4 are 0.184, 0.183, 0.183 and 0.150 ns, respectively. The fitting precision of S1, S2, S3 and S4 are 70907.344, 0.731, 0.502 and 0.543 ns, respectively. The frequency stability of S3 shows the best performance among S2, S3 and S4 when the averaging interval time is 10,000 and 20,000 s, which can be attributed to the effectiveness of proposed method and frequency stability difference of clock offset datum receiver. The fitting precision and frequency stability cannot be reflected correctly by clock offset due to the satellite clock offset jump in S1. For precise point positioning (PPP), the coordinates jump and re-convergence can be eliminated by using the proposed method. Thus, this method can provide a reference for obtaining real-time, continuous and reliable BDS satellite clock offset products.

Precise orbit determination of BDS-3 satellites using B1C and B2a dual-frequency measurements

Compared to the BeiDou regional navigation satellite system (BDS-2), the BeiDou global navigation satellite system (BDS-3) includes the newly designed B1C and B2a signals, which are compatible with the L1 and L5 frequencies of the global positioning system (GPS). Considering that the precise orbit determination (POD) of the BDS-3 constellation is currently restricted to the legacy B1I and B3I signals, we reported the POD performance of BDS-3 satellites using B1C and B2a dual-frequency measurements. Nine globally distributed Multi-GNSS Experiment (MGEX) stations were selected to determine the orbits of BDS-3 satellites during the period of July 2019. The results show that B1C/B2a-based POD enables an average three-dimensional root-mean-square error (3D RMS) of 24.2 cm, and the precision is better than 6 cm in the radial component in a comparison of two-day overlapping arcs. Satellite laser ranging (SLR) validation achieves an overall precision of 6.8 cm in RMS differences. Compared to the B1I/B3I-based POD results, the quality of B1C/B2a-based orbits is improved by approximately 9% across the whole BDS-3 constellation, indicating that the new B1C/B2a signals can be employed for the superior POD performance of BDS-3 satellites. Moreover, we investigated the behavior of the solar radiation pressure (SRP), which is generally considered one of the primary error sources in BDS-3 medium earth orbit (MEO) dynamic orbit determination. The ECOM7 SRP model has a better POD performance in continuous yaw steering (CYS) mode than the ECOM5 and ECOM9 SRP models. The results also show that there is no degradation in the orbit precision of BDS-3 MEO satellites when the elevation angle of the sun above the orbital plane (β angle) varies within the range of − 4° to + 4°.

The Performance Evaluation of BeiDou-3 System Time Based on CGGTTS

According to the Common GNSS Generic Time Transfer Standard Version2E (CGGTTS_V2E) developed by the GNSS (Global Navigation Satellite System) Working Group of the International Consultative Committee for Time and Frequency (CCTF), the data processing software is developed by using the pseudorange signal measured by the GNSS receiver, which is used to generate the CGGTTS files in the standard format, and its reliability is verified. The results show that, compared with the CGGTTS files generated by sbf2cggtts software, the offset between the GNSS system time and local time scale calculated by the same GPS and BDS satellite observations in the same epoch is identical, and the difference with the absolute value of the difference less than 0.5 ns accounts for 96% and 94% of the total, respectively. Taking Chinese standard time UTC(NTSC) (Coordinated Universal Time (National Time Service Center)) as the reference time scale, the data processing software is used to process the observations of the B1I and B3I dual-frequency ionospheric combination of BeiDou-2 and BeiDou-3 satellites, and generate the CGGTTS files in the standard format, and the performance of BeiDou system time is evaluated by analyzing the parameter of offset between the GNSS system time and local time scale. The results show that, compared with BeiDou-2, the internal precision of BeiDou-3 system time is increased by about 28%, and the frequency stability of medium and long-term is obviously better than BeiDou-2 after one day.

Calibration and analysis of BDS receiver-dependent code biases

Different designs of receiver correlators and front ends will cause different biases in terms of the pseudorange. In this paper, we present an algorithm to derive GNSS receiver-dependent code biases by estimating an additional code bias for each satellite based on different orbit and clock products and apply the calibrated biases in user positioning. One-month data from 117 globally distributed multi-GNSS experiment stations are selected to analyze the characteristics of BeiDou Navigation Satellite System (BDS) code biases, including BDS-2 and BDS-3 satellites. It is shown that the BDS code biases on B1I and B3I signals differ by receiver type, receiver model, antenna type, or even unknown factors. The code biases on BDS-2 satellites can reach up to 4 ns from peak to peak, while they are within 2 ns for BDS-3 satellites. The receiver-dependent code biases are calibrated based on the broadcast ephemeris and the precise products. With a correlation coefficient of approximately 0.9, it is demonstrated that the calibrated code biases are in agreement not only among different products, but also with previous result of BDS-2. To validate the effect of the calibrated code biases, we apply them in single point positioning (SPP), precise point positioning (PPP), and real-time kinematic (RTK) positioning. It is proven that with corrected BDS code biases, SPP users will benefit from an overall improvement of 9.5%, 9.9%, and 27.4% at B1I, B3I, and B1I/B3I ionosphere-free frequencies. For the ionosphere-free based positioning using BDS-3 only, the impact of code biases cannot be ignored. The corrected code biases will accelerate the convergence performance in PPP. For RTK applications, especially for single-frequency users, the fixing rate and ratio values will also improve when the code biases are corrected.

Considering inter-receiver pseudorange biases for BDS-2 precise orbit determination

The inter-receiver pseudorange biases exist between receivers equipping with different front-end designs and correlator types. It is distinguishable from differential code biases and would affect Global Navigation Satellite System (GNSS) estimations if it is not taken into account in data processing. In this contribution, the effects of inter-receiver pseudorange biases on the regional BeiDou navigation satellite system (BDS-2) satellite precise orbit determination (POD) will be investigated. For validation purposes, 35 globally distributed Multi-GNSS Experiment ground stations for which the receivers are from three manufacturers were used to determine the precise orbits of BDS-2 satellites from day of year 154 to 163, 2017. The accuracy of orbit overlap and satellite laser ranging (SLR) residual validation comparisons show that by implementing inter-receiver pseudorange bias corrections, the POD accuracies of the BDS-2 satellites can be effectively improved. The root mean square errors of 24-h precise orbit determination overlap corresponding to the radial, cross-track, and along-track components are improved by 1.4%, 2.7%, and 12.7%, respectively, after correcting the inter-receiver pseudorange biases. The standard deviations of the SLR residuals of satellite C01, C13, and C11 are reduced from 30.7, 7.1, and 3.5 cm to 30.2, 6.8, and 2.9 cm, respectively. The results also showed that inter-receiver pseudorange biases will cause an along-track bias in Geostationary Earth Orbit (GEO) satellite orbit determination. The performances of GEO POD in along-track component can be improved by considering the inter-receiver pseudorange biases.

First results of BDS positioning for LBS applications in the UK

The last satellite of BeiDou Navigation Satellite System with Global Coverage (BDS-3) constellation was successfully launched on June 23rd, 2020, and the entire system began to provide Positioning, Navigation, and Timing (PNT) services worldwide. We evaluated the performance of location services using BDS with a smartphone that can track the Global Navigation Satellite System (GNSS) satellites in Nottingham, UK. The static and kinematic experiments were conducted in an open meadow and a lakeside route covered by trees, respectively. Experimental results show that BDS has good visibility, and its overall signal carrier-to-noise density ratio (C/N0) is comparable to that of Global Positioning System (GPS). The average C/N0 of BDS-3 satellites with elevation angles above 45° on B1 band is the highest among all systems, reaching 40.0 dB·Hz. The noise level of the BDS pseudorange measurements is within 0.5 m, and it has a good consistency among satellites. In the static experiment, the standard deviations of BDS positioning in the east, north and up directions are 1.09, 1.16, and 3.02 m, respectively, and the R95 value of the horizontal position is 2.88 m. In harsh environments, the number of BDS satellites tracked by the smartphone is susceptible to environmental factors. The bias Root Mean Squares (RMS) in the three directions of the whole kinematic positioning are 6.83, 6.68, 11.67 m, in which the positioning bias RMS values in a semi-open environment are only 2.81, 1.11, 3.29 m. Furthermore, the inclusion of BDS in multiple GNSS systems can significantly improve the positioning precision. This study intends to provide a reference for the further improvements of BDS global PNT services, particularly for Location-Based Services (LBS).

Orbit determination of BDS-3 satellite based on regional ground tracking station and inter-satellite link observations

• Under regional GTS, ISL significantly improves BDS-3 orbit determination accuracy. • The initial weight ratio has little effect on the result when VCE algorithm is used. • VCE algorithm has contribute to fusion data processing of L-band and Ka-band. The BeiDou global navigation satellite system (BDS-3) has established the Ka-band inter-satellite link (ISL) to realize a two-way ranging function between satellites, which provides a new observation technology for the orbit determination of BDS-3 satellites. Therefore, this study presents a BDS satellite orbit determination model based on ground tracking station (GTS) observations and ISL ranging observations firstly to analyze the impact of the ISL ranging observations on the orbit determination of BDS-3 satellites. Subsequently, considering the data fusion processing, the variance component estimation (VCE) algorithm is applied to the parameter estimation process of the satellite orbit determination. Finally, using the measured data from China’s regional GTS observations and BDS-3 ISL ranging observations, the effects of ISL ranging observations on the orbit determination accuracy of BDS-3 satellites are analyzed. Moreover, the impact of the VCE algorithm on the fusion data processing is evaluated from the aspects of orbit determination accuracy, Ka-band hardware delay parameter stability, and ISL ranging observation residuals. The results show that for China’s regional GTSs, the addition of BDS-3 ISL ranging observations can significantly improve the orbit determination accuracy of BDS-3 satellites. The observed orbit determination accuracy of satellite radial component is improved from 48 cm to 4.1 cm. In addition, when the initial weight ratio between GTS observations and ISL ranging observations is not appropriate, the various indicators which include orbit determination accuracy, ISL hardware delay, and ISL observation residuals were observed to have improved after the adjustment of the VCE algorithm. These results validate the effectiveness of the VCE algorithm for the fusion data processing of the GTS observations and ISL ranging observations.

GREAT-UPD: An open-source software for uncalibrated phase delay estimation based on multi-GNSS and multi-frequency observations

To meet the demands of precise orbit and clock determination, high-precision positioning, and navigation applications, a software called GREAT (GNSS + Research, Application and Teaching) was designed and developed at Wuhan University. As one important module in the GREAT software, GREAT-UPD was developed for multi-GNSS and multi-frequency uncalibrated phase delay (UPD) estimation. It can provide extra-wide-lane (EWL), wide-lane (WL), and narrow-lane (NL) UPDs for GPS, GLONASS, Galileo and BDS (GREC) satellites for precise point positioning (PPP) ambiguity resolution (AR) in a multi-GNSS and multi-frequency environment. The open-source GREAT-UPD software is written in C + + 11 language following object-oriented principles and can be compiled and run on several popular operating systems, such as Windows, Linux, and Macintosh. Observations from 222 stations spanning days from DOY 091 to 120 were used to conduct multi-GNSS and multi-frequency UPD estimation and PPP AR. Results indicate that GREAT-UPD can generate stable and reliable UPD products with multi-GNSS and multi-frequency observations. After applying the UPD corrections, the multi-frequency GREC PPP AR was achieved with the averaged time to first fix of 9.0 min. The software package can be obtained at https://geodesy.noaa.gov/gps-toolbox , including the source code, user manual, batch processing scripts, example data, and some auxiliary tools.

An In-Depth Assessment of the New BDS-3 B1C and B2a Signals

An in-depth and comprehensive assessment of new observations from BDS-3 satellites is presented, with the main focus on the Carrier-to-Noise density ratio (C/N0), the quality of code and carrier phase observations for B1C and B2a signal. The signal characteristics of geosynchronous earth orbit (GEO), inclined geosynchronous satellite orbit (IGSO) and medium earth orbit (MEO) satellites of BDS-3 were grouped and compared, respectively. The evaluation results of the new B1C and B2a signals of BDS-3 were compared with the previously B1I/B2I/B3I signals and the interoperable signals of GPS, Galileo and quasi-zenith satellite system (QZSS) were compared simultaneously. As expected, the results clearly show that B1C and B2a have better signal strength and higher accuracy, including code and carrier phase observations. The C/N0 of the B2a signal is about 3 dB higher than other signals. One exception is the code observation accuracy of B3I, which value is less than 0.15 m. The carrier precision of B1C and B2a is better than that of B1I/B2I/B3I. Despite difference-in-difference (DD) observation quantity or zero-base line evaluation is adopted, while B1C is about 0.3 mm higher carrier precision than B2a. The BDS-3 MEO satellite and GPS, Galileo, and QZSS satellites have the same level of signal strength, code and phase observation accuracy at the interoperable frequency, namely 1575.42 MHz and 1176.45 MHz which are very suitable for the co-position application.

Spaceborne atomic clock performance review of BDS-3 MEO satellites

On June 23, 2020, the final satellite of the third generation Beidou satellite navigation system (BDS-3) was launched, which represents the BDS-3 is fully operational and can provide global services. Due to the fact that BDS-3 constellation mainly consists of Medium Earth Orbit (MEO) satellites, we only discussed the in-orbit atomic clock performance of MEO, and compared to that of the latest generation of GPS as well as Galileo satellites. Based on the precise satellite clock product provided by German Research Center for Geosciences (GFZ), a level of 0.2 ns fitting precision is obtained, and it is better than BDS-2. The periodic terms of satellite clock are analyzed by means of spectrum analysis. Except some mutual periodic terms, several exclusive periodic terms, approximately 8 h, 6 h and 4.8 h harmonics, are found in passive hydrogen masers (PHMs) of BDS-3, and possible reasons are given. Clock systematic characteristics, such as frequency accuracy and drift rate are examined and found that PHMs have better result than rubidium atomic clocks. Finally, frequency stability is evaluated: all BDS-3 satellite clocks can reach the order of 10-15 at the average interval of 86400 s, which is much better than BDS-2 and comparable to the latest type of GPS III as well as Galileo satellites.

Enhancing BDS-3 precise time transfer with DCB modelling

This article is the first report of BDS-3 differential code bias (DCB) correction models. Satellite differential code biases (SDCBs) were regarded as a source of error; if there was no correction, the quality of the GNSS Position & Navigation & Timing service deteriorated. As a newly built global system, the BDS-3 satellite broadcasted B1I, B3I, B1C and B2a signals. To fully exploit all the BDS-3 signals, DCB correction models related to multiple Dual-Frequency (DF) Ionosphere-Free (IF) combinations were developed for BDS-3 application. Two prevailing global navigation satellite system positioning technologies, namely, standard point positioning (SPP) and precise point positioning (PPP), were utilized to validate the efficacy of the DCB parameters provided by the Multi-GNSS Experiment (MGEX). Our numerical analyses clarified how the DCB model performed when it was applied to positioning and time transfer events under the B1C&B3I, B1I&B2a, B1C&B2a cases. With the use of DCB correction in three combinations, the positioning accuracy and frequency stability were significantly improved. The E, N and U position accuracies of the ondcb scheme compared to those of the offdcb scheme were enhanced in the range of 39.51–86.24%, 35.44–78.64% and 41.84–78.64%, respectively. In particular, we observed that the long-term stability of the time link was obviously better than the short-term stability. The stability percentages were improved by at least 3%, and some were improved by up to 66.5%. The frequency stability obtained by B1I&B3I is equivalent to that obtained by B1C&B3I and B1I&B2a. Finally, all the statistics indicate that the B1C&B2a combination recommended as the optimal option is applicable to time transfer communities. The result agrees with our expectation that the appropriate modelling of DCB is vital, which allows for more precise positioning and a stable time link.

On long-baseline relative positioning with BDS-2/BDS-3/GPS data

BeiDou global navigation satellite system (BDS-3) reached the global coverage in June 2020. To study the performance of the precise relative positioning using the BDS-3 alone and the improvement due to adding BDS-3 satellites to BDS-2 and GPS, this paper analysed the data of 033-039d provided by the MGEX in 2021. The fusion of BDS-2, BDS-3 and GPS data was conducted for static and dynamic high-precision long-baseline solution experiments. The influence of the individual BDS-2 / BDS-3 / GPS and by adding BDS-3 satellites to BDS-2 and GPS on precise relative positioning convergence speed and positioning accuracy were analyzed, respectively. The experimental results show that the current BDS-3 positioning performance (convergence speed and positioning accuracy) is similar to GPS, and the BDS-3 satellites effectively improve the positioning convergence speed upon BDS-2 and GPS. In the static positioning processing mode, with the aid of the BDS-3 satellites, the RMS (Root-Mean-Square) of the positioning errors using GPS only and the combination of BDS-2 and GPS was increased only by 20 % in the up direction, and for the BDS-2 system alone, the positioning accuracies in the E, N and U components were increased by 60%, 71% and 65%, respectively. In the dynamic positioning processing mode, after the addition of BDS-3 satellites, the positioning accuracies using GPS and GPS+BDS-2 in the E, N and U components were improved by about 15 %, 23 % and 23 %, respectively, and the BDS-2 positioning accuracies were improved by about 46 %, 38 % and 36 % in the E, N and U components, respectively.