Table of Content

20 July 2019, Volume 48 Issue 7

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Review

The BeiDou coordinate system

WEI Ziqing, WU Fumei, LIU Guangming

2019, 48(7):  805-809.  doi:10.11947/j.AGCS.2019.20190131

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The BeiDou coordinate system(BDCS) is the geodetic datum of the BeiDou Satellite Navigation System. After a brief description of the backgrounds, the definition of origin, scale, and orientation, and the defining and deriving constants of the reference ellipsoid of the BDCS are given. Then addressed is the first realization of the BDCS, namely, the ITRF2014 coordinates at epoch 2010.0 and velocities of 8 monitoring stations of the BeiDou system, which has been achieved by processing four-phases GPS data for a global GNSS network including the monitoring stations, and by the frame alignment and the linear regression fitting of station coordinate series. Discussions and suggestions are made about the period of re-realization of the system, the monitoring stations' overseas layout and continuously observing, the generation and distribution of precise ephemerides of BeiDou satellites, and the standardization of the BDCS. Finally, a summary concludes the paper.

BDS-3 RNSS technical characteristics and service performance

GUO Shuren, CAI Hongliang, MENG Yinan, GENG Changjiang, JIA Xiaolin, MAO Yue, GENG Tao, RAO Yongnan, ZHANG Huijun, XIE Xin

2019, 48(7):  810-821.  doi:10.11947/j.AGCS.2019.20190091

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A detailed description of BDS-3 including constellation design, different types of services, signal structure, coordinate and time frame, mechanics of orbit determination and time synchronous is provided. A RNSS service performance assessment parameters list is provided. It consists of parameters about time and coordinate frame, signal quality, signal in space accuracy and service performance. Tracking data is collected from stations globally distributed, and performance of BDS-3 initial service is assessed. From results, ratio bias of signal component effective power is less than 0.25 dB, S curve bias is less than 0.3 ns, position dilution of precision (PDOP) availability of current constellation is better than 85% at global level, signal-in-space ranging error is 0.48 m(RMS), signal-in-space availability reached 99.78%, signal-in-space continuity reached 99.99%, UTC offset error is 19.1 ns. All of them fulfill the design specification and requirements of BDS open service performance standard. Compared to BDS-2 satellites, current BDS-3 constellation realized global coverage and achieved a better performance in signal-in-space error, signal-in-space availability and signal-in-space continuity.

Geodesy and Navigation

PPP algorithm using BDS new SBAS differential corrections

CHEN Junping, WANG Ahao, ZHANG Yize, ZHOU Jianhua, WANG Binghao, WANG Jiexian

2019, 48(7):  822-830.  doi:10.11947/j.AGCS.2019.20180545

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Since January 2017, BDS provides users with decimeter-level SBAS point positioning capability through the broadcasting of upgraded differential corrections, including satellite orbit error corrections, satellite clock error corrections, ionosphere grid corrections and zone corrections. In practice, however, the real-time zone corrections experience discontinuities causing the re-initialization of carrier-phase based positioning. We develop a new BDS SBAS precise point positioning (PPP) algorithm based on the switching of zone corrections among neighboring zones. Applying the new algorithm, users switch to use the zone corrections of the neighboring zones in case of no corrections received of the current zone, and thus avoid positioning re-initialization. Data of 7 BDS tracking stations and real-time kinematic on-road tracks are used to evaluate the new algorithm. Results show that with the implementation of the new algorithm, the positioning accuracy of BDS kinematic dual-frequency PPP is better than 0.3 m and 0.5 m in horizontal and vertical during the period of zone switching. On-road real-time single and dual frequency kinematic PPP shows no positioning re-initialization and the positioning accuracy remains at the level of better than 0.5 m during the discontinuity of zone corrections.

Comparison and analysis of two orbit determination methods for BDS-3 satellites

YANG Yufei, YANG Yuanxi, HU Xiaogong, TANG Chengpan, ZHAO Liqian, XU Junyi

2019, 48(7):  831-839.  doi:10.11947/j.AGCS.2019.20180560

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The basic system of BeiDou-3 (or simply called BDS-3) navigation satellite system has been completed by the end of 2018, and the whole system is planned to be completed in 2020. The precise orbit is the pre-condition of high-performance global service. The observation noise and ranging accuracy of Inter-satellite links (ISL) were analyzed based on the basic constellation of 18 satellites. The joint orbit determination experiment of the basic system was conducted based on both 12 regional stations (RS) within Chinese mainland and ISLs, and the results were compared with orbits only based on regional stations. The overlapping orbit difference (OOD), orbit prediction accuracy and satellite laser ranging (SLR) validation accuracy of both methods were analyzed. The results show that the observation noise is about 2.9 cm and ranging accuracy is about 4.4 cm. The 3D position RMS (root mean square) of ODDs for RS mode is about 66.7 cm. The accuracy of 3D position is improved by 76.9% after ISL observations are added. The 3D position accuracy of 24-hour orbit prediction can also be decreased from 114 to 20.3 cm, improved by 83.2%. The SLR validation accuracy of BDS-3 satellite orbit is about 8.4 cm, which is much better than that of BDS-2.

Global ionospheric Kriging interpolation and precision analysis by considering gross error

ZHU Yongxing, TAN Shusen, DU Lan, JIA Xiaolin

2019, 48(7):  840-848.  doi:10.11947/j.AGCS.2019.20180049

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The Kriging interpolation is affected by the gross error data and its global applicability is unknown for ionospheric data. Based on the variation function, the gross error elimination statistics of the TEC is created for automatic gross error elimination. Using the global ionosphere TEC grid products, the analysis results show that:① The gross error can be eliminated effectively, and the interpolation accuracy is equivalent to the sample accuracy. ② The accuracy of Kriging global interpolation is 1.0~5.0 TECU at different latitude regions, different times.

Spatio-temporal baseline analysis of lunar-based repeat-track SAR interferometry

DONG Jinglong, JIANG Liming, JIANG Houjun, SHEN Qiang, LI Dewei, WANG Hansheng, MAO Song

2019, 48(7):  849-861.  doi:10.11947/j.AGCS.2019.20180269

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Based on the relative motion features of the Earth-Moon system, we constructed the observation geometry of lunar-based repeat-track synthetic aperture radar interferometry (LB-SAR RTI), and calculated the revisit time of lunar-based synthetic aperture radar (LB-SAR) and the corresponding perpendicular baseline by using NASA/JPL ephemeris (DE430). The results show that the revisit period of LB-SAR is about 24.8 h; the length of perpendicular baseline varies greatly with the number of revisits and exhibit a periodic variation about one sidereal month resulting from the varied Moon's N-S velocity. Furthermore, we investigated the features related to the spatio-temporal baselines under the constraint of critical baseline. We find that at a specific time, only part of revisits can be used as temporal baseline, and the revisits suitable as temporal baseline will also vary with the initial observation time. Furthermore, the number of interferometric combinations vary significantly with temporal baseline and the Moon's declination. Near 50% effective interferometric combinations have the temporal baseline near the integer multiples of sidereal month. About 80% effective interferometric combinations occur in regions with the Moon's declination greater than 10°. Therefore, the design of the observation data acquisition plan of LB-SAR RTI should comprehensively take into account the factors related to temporal baseline, spatial baseline and initial observation time.

A hight-accuracy method for tropospheric zenith delay error correction by fusing atmospheric numerical models

MAO Jian, CUI Tiejun, LI Xiaoli, CHEN Li, SUN Yanling, GAO Shuang, ZHANG Hui

2019, 48(7):  862-870.  doi:10.11947/j.AGCS.2019.20190003

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The complexity and intensity of water vapor variations are the fundamental reasons why it is difficult for tropospheric zenith delay models to get accurate estimation. To solve this problem, a new method for estimating tropospheric zenith delay based on WRF(weather research and forecasting model) atmospheric numerical model is proposed. By analyzing the numerical simulation mechanism and data structure characteristics of WRF model, a hybrid method of direct integration and correction model is used to estimate the hourly tropospheric zenith delay at any position in the world. The validation results show that the accuracy of the hourly ZTD reanalysis value calculated by this method is 13.6 mm, and the daily average value is 9.3 mm, which is about 5 times and 3.5 times higher than that of the traditional model UNB3m and the current model GPT2w, respectively. In the 30-hour forecast period, the accuracy of the forecast value can also reach 22 mm. The accuracy is higher than existing tropospheric zenith delay models whether for the ZTD reanalysis value or the forecast value.

A method of dynamic positioning with the medium and long baseline for aerial measurement scenarios

ZHANG Yuxi, ZHANG Xiaohong, LIU Quanhai, ZHU Feng

2019, 48(7):  871-878.  doi:10.11947/j.AGCS.2019.20180513

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Medium and long baseline kinematic relative positioning with a single base station will be affected by atmospheric residual errors and the ambiguities could not be fixed fast. Therefore, the performance of long-baseline relative positioning with low ambiguity-fixed rate is not as good as short-baseline situations. While considering baselines varying from short to long in airborne scenes, we propose a new method to improve the performance of kinematic relative positioning. First, high precision ionospheric delays can be easily obtained from the ambiguity-fixed resolutions in short-baseline scenes when the air just taking off. Second, the subsequent ionospheric delays are predicated epoch by epoch based on previous ionospheric delays. At last, filter with ionosphere predication constrained is implemented to fix the ambiguities quickly and obtain continuous high-precision positioning results. In this paper, the characteristics of double-differenced ionospheric residual errors for the dynamic measurement scenarios with long-baseline are analyzed. A sliding window will be applied for modeling and prediction of ionospheric delays. Next, we have discussed the implementation conditions, performance of positioning and the ambiguity resolution with this proposed method. The presented algorithm is evaluated by a set of airborne data and the results show that the new scheme can obtain the 100% fixing rate nearly and centimeter-level positioning accuracy in long baseline relative positioning with single base station as long as a few minutes short-baseline data. With this method, the costs of airborne measurement tasks will be significantly reduced.

The minimum analysis of geometric dilution of precision in celestial positioning

CHEN Zhanglei, LI Chonghui, ZHENG Yong, CHEN Bing, HE Donghan

2019, 48(7):  879-888.  doi:10.11947/j.AGCS.2019.20180479

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Celestial positioning is an important method in positioning and navigation, which is widely used in the fields of geodetic astrometry and celestial navigation. The selection of stars in this method affects the accuracy of positioning. At present, there is a lack of research on the optimal selection of stars for the celestial positioning algorithm which determinates longitude and latitude simultaneously. With the improvement of the automation level of observation instruments, the acquisition of observation data becomes more efficient. Therefore, it is necessary to study the optimal selection of stars to achieve the highest accuracy of positioning. Based on the concept of geometric dilution of precision (GDOP) in satellite navigation, this paper studies the influence of the number and distribution of stars in zenith-method on the accuracy of positioning. Finally, the conclusion is verified by the simulation experiment and the measured data. When the statistical characteristic of observation error on zenith distance is fixed, GDOP can be used to describe the effect of star distribution on the accuracy of positioning, and the error of positioning is minimal when the azimuth of stars is uniformly distributed. Considering that the residual error of atmospheric refraction correction of zenith distance of stars with different heights is different, the stars with equal zenith distance and azimuth distributed evenly should be used as far as possible in actual measurement.

Time and frequency transfer based on modified integer phase clock method

LÜ Daqian, ZENG Fangling, OUYANG Xiaofeng, YU Heli

2019, 48(7):  889-897.  doi:10.11947/j.AGCS.2019.20180248

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The integer phase clock method is one of the most widely used integer ambiguity resolution method in precise point positioning (PPP). The stability of frequency transfer based on integer phase clock method is better than that of the conventional PPP. However, the clock offsets calculated by integer phase clock method contain the systematic biases, which affect the accuracy of time transfer. In this paper, we introduce the integer phase clock method, and analyze the origin of this systematic bias. Then based on single-difference ambiguity resolution method and atomic-clock refinement, a modified integer phase clock method is proposed. And some experiments are carried out to assess the modified integer phase clock method for ambiguity resolution and time-frequency transfer. The experiment results prove that the modified algorithm can eliminate the systematic bias effectively. The accuracy of time transfer can reach 0.1~0.2 ns, and the stability of frequency transfer can achieve 1.1×10^-15/d.

Non-isotropic Gaussian combination filtering method of GRACE time-variable gravity

GUO Feixiao, SUN Zhongmiao, REN Feilong, WANG Feifei

2019, 48(7):  898-907.  doi:10.11947/j.AGCS.2019.20180365

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Affected by factors of measurement errors and so on,the surface mass variation inversion results would be dominated by serious north-to-south stripe noise with using time-variable gravity models directly. Filtering methods must be adopted. Optimal parameters of different filtering methods are determined by taking signal-to-noise ratio maximum as the criterion. On this basis, a new non-isotropic combination filtering method is proposed based on the features of time-variable gravity model spherical harmonic serrors. The new non-isotropic combination filtering method is different from traditional two-steps combination filtering methods. It just performs in one step with combining non-isotropic Gaussian filtering and RMS filtering. The basic idea of the new combination filtering method is to apply larger weights to low-order spherical harmonic coefficients while smaller weights to high-order spherical harmonic coefficients for keeping more signals and suppressing noise. The experiment results show that the new combination filtering method is simple to calculate and stripe noise can be removed effectively. The new combination filtering method improves the signal-to-noise ratio compared with previous filtering methods and traditional two-steps combination filtering methods.

The impacts of different satellite antenna parameters on BDS precise orbit determination and precise point positioning accuracy

HU Yifan, ZHANG Shuai

2019, 48(7):  908-918.  doi:10.11947/j.AGCS.2019.20180226

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The necessity that the satellite antenna parameters used in BDS PPP should be consistent with the satellite orbit and clock products is demonstrated. Based on four groups of satellite antenna parameters BDS precise orbit inner accuracy in RTN three-way, GEO satellites are all around 9.3、18.6、11.5 cm, IGSO satellites are around 1.7、4.2、2.7 cm, MEO satellites are about 2.1、5.1、4.8 cm, the difference of the orbit determination accuracy in the R direction is less than 5 mm, and the maximum difference in the TN direction is 2.4 cm. Compared with the GFZ's post precision products, the outer accuracy of RTN is significantly different, excluding the factors of the difference orbit determination strategy with GFZ in GEO, the IGSO and MEO orbit precision are similar between ESA and WHU, and the RTN is within 10 cm in all directions, which are better than IGS and EST from 1 cm to 10 cm in three components, where TN direction have the most significant difference. While keeping the satellite antenna parameters used by BDS PPP consistent with the satellite orbit and clock products, the positioning accuracy of the four satellite antenna parameters are similar, in which the last epoch coordinate bias of static PPP is within 5 cm, the RMS of kinematic PPP coordinate bias is within 10 cm in horizontal direction after convergence, and within 15 cm in vertical direction. The average convergence time of kinematic PPP based on ESA and WHU antenna parameters was about 46 min, while the average convergence time based on IGS and EST antenna parameters was about 56 min, slightly worse than the convergence time based on GFZ post-products, and its average convergence time was about 34 min.

Engineering Survey

Dynamic Bayesian ELM method for deformation monitoring data prediction

FAN Qian, FANG Xuhua, XU Chengquan, YANG Ronghua

2019, 48(7):  919-925.  doi:10.11947/j.AGCS.2019.20180504

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Bayesian extreme learning machine (BELM) has the characteristics of making full use of the prior information of data and self-adaptive estimation of model parameters. However, when the sample size increases, the computational efficiency will be reduced if BELM training is repeated every time. To solve this problem, a dynamic bayesian extreme learning machine (DBELM) method is proposed for real-time prediction of deformation monitoring data. This method takes BELM training model parameters as initial values. According to the new sample information, the initial model parameters can be updated dynamically, and the relevant calculation formula is deduced theoretically. The detailed analysis of simulation data and actual deformation data show that the prediction accuracy of DBELM method is better than that of BELM, RELM and ELM.Especially in the long term continuous forecast, its forecasting performance has obvious advantages over the other three methods.This fully demonstrates the feasibility and validity of the proposed method in the field of deformation monitoring data prediction.

Academic Research

Weighted total least square adjustment EIO model and its algorithms

DENG Xingsheng, PENG Sichun, YOU Yangsheng

2019, 48(7):  926-930.  doi:10.11947/j.AGCS.2019.20170021

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EIO (errors-in-observations) model is proposed for the weighted total least squares adjustment problem. The EIO model only corrects the independent observations. The observation cofactor matrix has the simplest structure. The flaw of EIV model is overcome. Based on the EIO model, the precise parameter estimation and cofactor matrix formulations are derived and proved by several examples, which show that the results are correct and the algorithm is efficient.

EIV models and algorithms of weighted total least squares problem^*: discuss with “Weighted total least square adjustment EIO model and its algorithms”

WANG Leyang, YU Hang, ZOU Chuanyi, LU Tieding

2019, 48(7):  931-937.  doi:10.11947/j.AGCS.2019.20190155

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A weighted total least squares (WTLS) method is a kind of parameter estimation method which takes into account the observation errors in both the observation vector and the coefficient matrix of the EIV (errors-in-variables) model. The EIV model presents different structural characteristics in terms of different application scenarios. A EIO model is proposed by the paper "Weighted total least square adjustment EIO model and its algorithms" to deal with the structural problem of the EIV model^*. In order to compare the EIO model method with the existing EIV model parameter estimation method, four kinds of methods are listed to deal with the structural characteristics of the EIV model, and eight parameter estimation formulas are summed up. Furthermore, the first-order and higher-order approximate precision estimation methods of WTLS solutions are discussed. It is emphasized that the EIV model and its parameter estimation theory can be developed from three aspects:functional model, stochastic model and the WTLS parameter estimation method. Although different methods are proposed, the problem is solved in an equivalent way.

Summary of PhD Thesis

Land subsidence monitoring and analysis of influencing factors in Su-Xi-Chang area based on multi-source SAR data

LU Yanyan

2019, 48(7):  938-938.  doi:10.11947/j.AGCS.2019.20180345

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