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    20 September 2020, Volume 49 Issue 9
    Design and implementation of the ground test and verification bed for BeiDou navigation satellite system
    GUO Shuren, WANG Wei, GAO Weiguang, LU Jun, CHAI Qiang, LIU Wenxiang, CHEN Ying
    2020, 49(9):  1073-1083.  doi:10.11947/j.AGCS.2020.20200330
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    The BeiDou Navigation Satellite System (BDS) is a constantly-updated huge system with tightly-coupled space and ground segments and long construction period. Faced with the multiple challenges of highly complicated key technical scheme, high-intensity satellite deployment and launching tasks, and high difficulty in system stable operation, BDS has very high demands for the system's test and verification. This paper analyzes the test and verification tasks of BDS including system design and verification, space and ground segment integration, and simulation operation, and proposes the system architecture of the ground test and verification bed which equivalent to the full-system, full-scale and full-elements BDS. The bed is featured with synchronized work of the software and hardware platforms, equivalent operation of the simulation and real systems, and remote connection between the test bed and system segments, and solves the problems of multi-system simulation modeling, high-dynamic simulation as well as software and hardware synchronized simulation; realizes the whole process verification of the BDS design, deployment and operation, and also realizes the test and verification of the BDS new technical schemes such as inter-satellite links, BDS satellite autonomous integrity monitoring, and precise orbit determination. The bed for BDS is the only test and verification system in the field of satellite navigation that can realize all-states system simulation, and synchronized iterative evolution and operation with the real system, and can serve as important reference for the verification work of other space missions.
    New progress of PPP/PPP-RTK and positioning performance comparison of BDS/GNSS PPP
    ZHANG Xiaohong, HU Jiahuan, REN Xiaodong
    2020, 49(9):  1084-1100.  doi:10.11947/j.AGCS.2020.20200328
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    This paper begins with a brief review of the current state of development of precise point positioning (PPP) in recent years, with a focus on summarizing the latest research progress of several hotspots such as real-time rapid estimation of high-rate satellite clocks, multi-GNSS PPP ambiguity resolution, multi-frequency GNSS PPP models and ambiguity resolution, rapid initialization of PPP and PPP-RTK. Then, the evaluation of positioning performance of single/multi-GNSS PPP with latest observation of GPS, GLONASS, Galileo and BDS, especially the positioning accuracy, convergence time and time to first fix of BDS-2+3, is given. The results show that:PPP performance of BeiDou Navigation Satellite System is comparable with other GNSS. The convergence time and time to first fix of BDS-2 and BDS-3 combined PPP are about 20~30 minutes, positioning accuracy of static PPP in east, north and up directions is at millimeter to centimeter level, while that of kinematic PPP reach 5 cm in horizontal and 7 cm in elevation direction. Multi-GNSS combination can improve positioning accuracy and shorten the convergence time and time to first fix significantly, the positioning accuracy of ambiguity-fixed solutions can be improved by 14.8, 12.0 and 12.8% in the east, north and up directions compared to float solutions, respectively. The convergence time and time to first fix of multi-GNSS PPP can be reduced by 36.5 and 40.4% compared to that of GPS PPP.
    Refinement of BeiDou satellite antenna phase center correction model and its impact on precision orbit determination and positioning
    ZHANG Qin, YAN Xingyuan, HUANG Guanwen, XIE Shichao, CAO Yu
    2020, 49(9):  1101-1111.  doi:10.11947/j.AGCS.2020.20200289
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    Aiming at the lack of high accurate antenna phase correction (PCC) model for the BDS-2 IGSO and MEO satellites, in the BDS-2 and BDS-3 satellites joint precision orbit determination (POD) and precise point positioning (PPP), in this paper, an improved method for estimating PCV and z-offset parameters is used to refine the B1I/B3I ionosphere-free linear combination PCC model for the BDS-2 IGSO and MEO satellites. Compared with the results obtained using the ground-calibrated PCO model published by BeiDou official, the STD of SLR residuals obtained by the POD using the refined PCC model is reduced by 0.6~2.4 cm, and the improvement percentage is about 8.6%~33.3%. The floating PPP solution results, based on the refined BDS-2 satellite PCC model in this paper and the existing BDS-3 PCC model, have been significantly improved by 9.5 mm (37.2%) in the elevation direction.
    Atmospheric water vapor climatological characteristics over Indo-China region based on BeiDou/GNSS and relationships with precipitation
    SHI Chuang, ZHANG Weixing, CAO Yunchang, LOU Yidong, LIANG Hong, FAN Lei, C SATIRAPOD, C TRAKOLKUL
    2020, 49(9):  1112-1119.  doi:10.11947/j.AGCS.2020.20200339
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    The historical BeiDou/GNSS observations from 2006 to 2016 at stations over Indo-China region were reprocessed with consistent models and strategies. Precipitable water vapor (PWV) products were then retrieved, and were used to study the atmospheric water vapor climatological characteristics over Indo-China region, including the average PWV, PWV annual amplitudes and PWV semi-annual amplitudes. The climatological characteristics of the PWV are found to be mainly affected by the station latitude, station altitude and the monsoon system in this region. Relationship between PWV and precipitation were explored in the study region. A clear correlation is found and the correlation coefficient (R) generally decreases with the station latitude where R can reach 0.8 in Yunnan Province while decrease to about 0.2 in the southern Thailand region close to the equator. In addition, PWV and precipitation monthly anomaly also show weak positive correlation in this region with R of about 0.2~0.4.
    Galileo triple-frequency uncombined precise orbit determination: model and quality assessment
    LI Xingxing, HUANG Jiande, YUAN Yongqiang, LI Jie, LIU Chengbo, ZHU Yiting
    2020, 49(9):  1120-1130.  doi:10.11947/j.AGCS.2020.20200320
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    Multi-frequency signals have brought new opportunities and challenges to global navigation satellite system (GNSS) precise data processing. In this contribution, the uncombined (UC) observation model suitable for multi-frequency precise orbit determination (POD) was derived, and the double-differenced constraining strategy for multi-frequency UC ambiguity was given. Afterwards, Galileo POD was carried out by using the observation data of 150 multi-GNSS experiment (MGEX) stations. Different solutions using E1/E5a double-frequency, E1/E5b double-frequency and E1/E1a/E5b triple-frequency observations based on UC model and the traditional ionosphere free (IF) model respectively are applied to generate Galileo orbits.The comparison with external precise orbit products, orbit boundary discontinuities comparison and satellite laser ranging (SLR) validation were used to evaluate the orbit accuracy of different solutions. The results showed that the UC model and IF model having comparable orbit accuracy for double-frequency solutions.The differences of orbit accuracy between UC and IF model are within 1 mm, and the differences of the estimated satellite clock and SLR residuals are within 0.01 ns and 2 mm, respectively. When triple-frequency observations (E1/E5a/E5b) were used, the accuracy of float UC and float IF solutions can be improved by 1~2 mm compared to that using E1/E5b observations.
    Characterization of pesudorange bias and its effect on positioning for BDS satellites
    TANG Chengpan, SU Chengeng, HU Xiaogong, GAO Weiguang, LIU Li, LU Jun, CHEN Ying, LIU Cheng, WANG Wei, ZHOU Shanshi
    2020, 49(9):  1131-1138.  doi:10.11947/j.AGCS.2020.20200329
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    Distortions of GNSS signals result in pesudorange biases. The biases are different for different receivers depending on frontend bandwidth and employed filters of receivers. Pesudorange biases are treated as a new error sources in this paper. Firstly, two pesudorange bias calibration calculation methods namely collocated receivers-based method and differential code bias (DCB) based method are proposed to separate pesudorange biases and other error sources. Then pesudorange biases are calculated, estimated and separated from other error sources using collocated receivers-based method. The STD of BDS estimated pesudorange biases series is approximately 0.1 m. The BDS estimated pesudorange biases keep constant with time. Pesudorange biases are also irrelevant with geographical locations of receivers. At 1.5 G band, B1I pesudorange bias is the largest. The pesudorange biases of BDS new signal B1C improve a lot compared with pesudorange biases of BDS B1I and are also much better than the GPS L1C/A. At other bands, pesudorange biases of GPS L2C are slightly larger than BDS B3I, followed by GPS L5C, pesudorange biases of BDS B2a are the smallest. The influence of pesudorange biases is also analyzed with measurements. Pesudorange biases are highly-correlated with total group delay (TGD). If the user receivers are quite different with those used for TGD calculation, the positioning accuracy will get worse.
    Geometry-free single-epoch resolution of BDS-3 multi-frequency carrier ambi-guities
    ZHANG Zhetao, LI Bofeng, HE Xiufeng
    2020, 49(9):  1139-1148.  doi:10.11947/j.AGCS.2020.20200325
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    The BDS-3 navigation satellite system currently provides five-frequency observations, so theoretically it can perform multi-frequency carrier ambiguity resolution (MCAR). This paper systematically studies the basic theory and method of the MCAR, including three-frequency (TCAR), four-frequency (FCAR) and five-frequency (FiCAR) carrier ambiguity resolution. Firstly, from the perspective of linear combination, basic mathematical models including TCAR, FCAR and FiCAR methods are given. Secondly, the high-quality signals and the optimal linear combinations under different conditions of baseline lengths-are discussed. In addition, single-epoch ambiguity resolution using geometry-free model is analyzed. Finally, experiments were conducted using the real BDS-3 five-frequency data. The results showed that the MCAR can effectively fix the ambiguities in single epoch, and the increase of the frequency number can significantly improve the success rates of the ambiguity resolution.
    Inter-satellite clock offsets adjustment based on closed-loop residual detection of BDS inter-satellite link
    LIU Cheng, GAO Weiguang, PAN Junyang, TANG Chengpan, HU Xiaogong, WANG Wei, CHEN Ying, LU Jun, SU Chengeng
    2020, 49(9):  1149-1157.  doi:10.11947/j.AGCS.2020.20200319
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    It is a common and effective method in engineering science to use the "closed-loop detection" idea to detect and correct the cumulative error of system measurement. This paper indicates the favorable closed-loop conditions in the global inter-satellite link (ISL) of BeiDou-3 navigation satellite system (BDS-3), and proposes to detect and analyze the closed residuals. On this basis, a network adjustment model of the closed residuals is constructed by which the correction of inter-satellite clock offsets is realized. Calculations based on the on-orbit measured data indicate that the obvious constant or periodic non-zero closed residuals really exist in the global ISL of BDS-3. Attributed to the correction of the closed-loop residuals of the ISL, the non-closed inter-satellite clock corrections are basically eliminated, the random noise of the inter-satellite clock corrections is reduced by 30% to 50% and the accuracy of the inter-satellite clock corrections are effectively improved. It is helpful forthe service capability of BeiDou.
    Development of GNSS technology for high earth orbit spacecraft
    WANG Meng, SHAN Tao, WANG Dun
    2020, 49(9):  1158-1167.  doi:10.11947/j.AGCS.2020.20200170
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    With the increasing number of high orbit spacecraft, the use of high orbit GNSS technology to a certain extent avoids the limitations of traditional ground-based measurement and orbit determination, reduces the operation burden and maintenance cost of ground-based measurement and communication, and improves the autonomous navigation ability and orbit measurement accuracy of the satellite platform. Therefore, GNSS technology has significant research significance and application value for high earth orbit spacecraft. In this paper, the technology development of GNSS used in high orbit spacecraft is reviewed. Compared with low orbit spacecraft, the characteristics of GNSS in high orbit space are analyzed. From the three aspects of high orbit GNSS availability, on orbit technology verification and high sensitive receiver technology, the characteristics and development of high orbit GNSS autonomous navigation technology are summarized. The new tasks of high orbit post precise orbit determination and formation satellite relative navigation are analyzed. From the perspective of the future development of high orbit spacecraft, the future research prospect of high orbit GNSS technology is given.
    Retrieval of coastal typhoon storm surge using multi-GNSS-IR
    HE Xiufeng, WANG Jie, Wang Xiaolei, SONG Minfeng
    2020, 49(9):  1168-1178.  doi:10.11947/j.AGCS.2020.20200228
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    Typhoon storm surges cause great losses to coastal cities every year. In recent years, a ground-based remote sensing technology using GNSS reflected signal has been proved to be able to monitor tidal level, called global navigation satellite system interferometric reflectometry (GNSS-IR), which can supplement the data of tide gauge. However, storm surges have strong destructiveness and occur quickly, the time resolution of GPS cannot meet the requirement to monitor this marine disaster. In this paper, three coastal storm surge events were monitored using the multi-mode and multi-frequency GNSS data of HKQT site in Hong Kong, China and BHMA site in Bahamas Islands. Firstly, we analyzed the quality of multiple system data. Then, multi-mode and multi-frequency GNSS-IR was used for monitoring three storm surges caused by hurricane "Dorian" in 2019, typhoon "Mangkhut" in 2018 and typhoon "Hato" in 2017.The sliding window least squares method was used to correct the results and compared with tide gauge observations. The experimental results showed that the accuracy of "Dorian" is better than 14 cm, and the accuracy of "Hato" and "Mangkhut" are both better than 9 cm. Compared with GPS, it could improve the accuracy and the time resolution of the results, which is helpful to record the whole process of tide rising, peak and falling in storm surge. And it can play an important role in the study of marine disasters.
    GNSS-IR model of snow depth estimation combining wavelet transform with sliding window
    BIAN Shaofeng, ZHOU Wei, LIU Lilong, LI Houpu, LIU Bei
    2020, 49(9):  1179-1188.  doi:10.11947/j.AGCS.2020.20200268
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    Currently, GNSS interferometric reflectometry technology has become a high-precision method for monitoring land surface snow depth. Aiming at the problems of signal separation and random estimation biases, we developed a GNSS-IR refined model with multi-satellite fusion for snow depth estimation combining wavelet transform with sliding window. The common polynomial method was replaced by discrete wavelet transform to obtain the high-quality SNR sequences of the reflected signals which can calculate the reflected height of GPS antenna. Then, these reflected heights from SNR observations of multi-satellite were effectively selected and averaged using the sliding window under a constrained threshold. The refined model was established using GNSS observations for snow season from 2016 to 2017, and then the snow depth datasets of both PBO H2O and SNOTEL were regarded as reference to verify the performance of the refined model. The results show that there is a high agreement between snow depths derived from the refined model and in situ measurements, and the RMSE is 10 cm. Compared with the results of a single satellite, the accuracy and the stability of the refined model with multi-satellite fusion are obviously better. In terms of RMSE, the accuracy of the refined model has been improved by 50% when compared with PBO H2O dataset. In addition, taking into consideration that land surface roughness is an error factor, a relative RMSE value of snow depth estimations corrected by a new datum of the reflection height is approximately 4 cm, and the correlation coefficient between snow depth estimations and in situ measurements reaches 0.98.
    Analysis and comparisons of the BDS/Galileo quad-frequency PPP models performances
    SU Ke, JIN Shuanggen
    2020, 49(9):  1189-1201.  doi:10.11947/j.AGCS.2020.20200236
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    Chinese BeiDou navigation satellite system (BDS) and Galileo system can provide the services of quad-frequency observations. In this paper, we assess the BDS/Galileo quad-frequency precise point positioning (PPP) models, i.e., PPP model with two ionosphere-free combinations (QF1), PPP model with one ionosphere-free combination (QF2), undifferenced uncombined PPP model (QF3) and ionosphere-constrained undifferenced uncombined PPP model (QF4), by comparing the dual-frequency ionosphere-free PPP model (DF). The equivalence of the QF1, QF2 and QF3 models is theoretically demonstrated by the equivalence principle. The static and simulated kinematic PPP performances are evaluated and investigated with one-month period observations from the network stations and the kinematic PPP performances are verified with a kinematic experiment in the campus. The results show that the pseudorange noises of BDS-3 B1C and B2a signals are larger than the B1I and B3I signals and the pseudorange noises differences for the Galileo quad-frequency signals are not obvious. The performances of the QF1, QF2 and QF3 models are basically consistent for the static and simulated kinematic PPP. By adding the external ionospheric constraint, the quad-frequency PPP performances are affected. Compared to the QF1, QF2 and QF3 models, the mean convergence time of the static BDS (BDS-2+BDS-3) model are reduced by 4.4%, 4.4% and 5.4%, respectively. The mean convergence time of static Q4 model increases 16.8 minutes when compared to the QF3 model. Compared to the dual-frequency PPP, the quad-frequency kinematic PPP performances are obviously improved. The three-dimensional positioning accuracy of BDS and Galileo QF4 models are improved by 11.4% and 31.4%, respectively, when compared to the QF1 models. Furthermore, the BDS/Galileo kinematic PPP models perform better than the single-system solutions.
    An experimental validation method on GNSS signal attenuation model in soil
    HAN Mutian, YANG Yi, ZHANG Bo
    2020, 49(9):  1202-1212.  doi:10.11947/j.AGCS.2020.20200259
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    The attenuation of GNSS signals in soil is of great significance for the related research of using GNSS signals to measure soil moisture. In this paper, for the first time, the attenuation of BDS (BeiDou navigation satellite system) and GPS (global positioning system) signals in the soil was studied through experiments. In the experimental design, the GNSS antenna was placed into the soil, then the soil thickness and moisture above the antenna were continuously changed to collect the power attenuation data of the GNSS signal. Finally, these data were used to retrieve soil moisture in order to validate the GNSS signal attenuation model. Experimental results show that soil can significantly attenuate GNSS signals. The greater the soil moisture value and thickness value is, the more severe the attenuation is. In the case of clay type soil and soil moisture of 0.15~0.30 cm3/cm3, the GNSS signal power has been attenuated to be undetectable by the GNSS receiver when the soil thickness reaches 21 cm. Further retrieval of soil moisture based on the GNSS signal attenuation model was carried out, the results show that the attenuation model is more accurate when the soil thickness is larger than or equal to 10 cm and when the satellite elevation angle is larger than 50°. And under this situation, the root mean square error of soil moisture retrieval using BeiDou B1 signal and GPS L1 signal is less than 0.04 cm3/cm3 and 0.09 cm3/cm3, respectively.
    Evaluation and comparative analysis of BDS-3 signal-in-space range error
    LIU Weiping, HAO Jinming, Lü Zhiwei, XIE Jiantao, LIU Jing, JIAO Bo
    2020, 49(9):  1213-1221.  doi:10.11947/j.AGCS.2020.20200266
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    BDS-3 is a global satellite navigation system independently built by China. Its performance level and performance comparison with other satellite navigation systems have an important impact on the follow-up promotion and application. In this paper, the signal in space range error (SISRE) is used as a key performance index of the system. Taking the multi-system precise orbit and clock offset provided by GFZ as the standard, the comparison and evaluation method of satellite orbit, satellite clock offset and SISRE is given. Based on the measured data of three months from January to March 2020, the accuracy improvement of BDS-3 relative to BDS-2 is verified, and the performance comparison between BDS-3, GPS, Galileo and GLONASS is analyzed emphatically. The results show that the accuracy level of BDS-3 is significantly higher than that of BDS-2 both in satellite orbit and in satellite clock offset. The orbit accuracy of BDS-3 in the R, T and N direction is 0.07 m, 0.30 m and 0.26 m respectively, which is at the optimal level among the four global systems. The satellite clock offset accuracy is 1.83 ns, which is basically the same as that of GPS, superior to GLONASS, but slightly worse than Galileo. In terms of the signal in space range error, if only orbit error is considered, BDS-3 SISRE(orb) is averagely 0.08 m. Next, Galileo SISRE(orb) is 0.26 m, GPS SISRE(orb) is 0.57 m, and GLONASS SISRE(orb) is 0.98 m. If the orbit and clock error are considered comprehensively, the average SISRE of BDS-3 is 0.50 m, which is slightly lower than 0.38 m of Galileo, better than 0.58 m of GPS, and significantly better than 2.35 m of GLONASS.
    The integer ambiguity resolution of BDS triple-frequency between long range stations with GEO satellite constraints
    ZHU Huizhong, LEI Xiaoting, XU Aigong, LI Jun, GAO Meng
    2020, 49(9):  1222-1234.  doi:10.11947/j.AGCS.2020.20200263
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    The BDS triple-frequency carrier phase ambiguity resolution is greatly affected by the residual of atmospheric error between long range stations, and GEO satellites are also unfavorable for carrier phase ambiguity resolution relative to geostationary. Taking advantage of the relatively stable signal path of GEO satellites and the influence of atmospheric delay which do not change with the space position of satellites, the constraint of atmospheric delay errors more in line with actual conditions for GEO satellites are carried out. Using the linear relationship of carrier phases ambiguities of GEO satellites B2 and B3 to reduce the effect of single-difference ionospheric delay error between stationson the ambiguity candidates, the candidates of carrier phases ambiguity of B2 and B3 areselected. The linear relationship of triple-frequency phase ambiguity which does not include the influence of errors makes evaluation of the ambiguity candidates, and the ambiguity search space can be constrained. The ambiguity candidates of the GEO satellites are used to determine the residual change of the ionospheric delay between adjacent epochs, and the parameter estimation of GEO satellites is more constrained in accordance with actual situation.The method of long range BDS triple-frequency carrier phase ambiguity resolution with actual atmospheric delay variation constraints and the integer ambiguity constraints of GEO satellite is studied.And determining the ionospheric delay constraint value by using the candidates of ambiguity between epochs is proposed, adjusting the constraint value of the random walk between GEO satellite according to the actual situation.The experimental results show that the method in this paper can improve the efficiency of the triple-frequency carrier phase ambiguity resolution and the accuracy of positioning.
    A piecewise pre-distortion optimization method based on spaceborne digital filter
    LIU Han, YANG Zhimei, XU Qibing, CHEN Lingling, ZHANG Lixin
    2020, 49(9):  1235-1242.  doi:10.11947/j.AGCS.2020.20200331
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    The satellite navigation system can provide high-precision navigation, positioning and timing services for all kinds of users around the world. Navigation signal quality is an important factor affecting system service performance. The transmission channel of navigation load has non-ideal characteristics, which will cause the change of navigation signal amplitude frequency characteristic and phase frequency characteristic. Therefore, the channel characteristics need to be compensated by means of pre-distortion. BDS-3 satellite is equipped with a pre-distortion filter in the digital segment. However, due to the limited resources on-board and the low accuracy of broadband pre-distortion algorithm. It is difficult for the navigation signal quality to converge to the index range in the process of pre-distortion adjustment. A piecewise high-precision predistortion method based on spaceborne digital filter is designed. Based on the existing parameters of the wideband filter, the pred-istortion compensation is carried out dynamically and elaborately according to the weights and adjustment requirements, which makes the key indexes of attention converge with higher precision. This method has been verified by 20 satellites in orbit and is an important technical means for in-orbit navigation signal quality optimization.