Remote real-time performance evaluation of standard time and frequency is critical for full-scope time traceability and quantity unification. Conventional time and frequency calibration schemes predominantly adopt on-site accompanying calibration or laboratory submission, which are hampered by bulky equipment, high operational costs and poor flexibility, rendering them incompatible with continuous real-time monitoring and evaluation scenarios. While GNSS common-view and all-in-view techniques are available for remote performance evaluation, their limited precision cannot satisfy the demands of high-precision timekeeping systems with a frequency stability at the 10−15 level. This paper constructs a remote real-time performance evaluation system based on self-developed BDS real-time PPP (RTPPP) receivers. Two validation experiments are carried out: a 43 km short-baseline link between Lintong and Hangtian campus of the National Time Service Center (NTSC) referenced to an optical-fibre time transfer link, and a 1150 km long-baseline link between Lintong and Anhui referenced to a GNSS IPPP link. Experimental results indicate that the frequency stability of the BDS RTPPP link is better than 8 × 10−15 at an averaging time of one day. For the single hydrogen-maser timekeeping system located at Hangtian, the average frequency deviation and frequency stability measured via BDS RTPPP reach 1.93 × 10−15 and 2.07 × 10−15/1 d, respectively, showing close agreement with the independently calibrated optical-fibre reference. For the Anhui timekeeping system, the one-day average frequency deviation is 8.90 × 10−15 and frequency stability reaches 7.95 × 10−15 after one day of averaging, which are in close agreement with the GNSS IPPP reference results. The overall findings verify that BDS RTPPP based on PPP-B2b service is qualified for remote real-time performance evaluation and monitoring of high-precision timekeeping systems.

The Beidou Navigation Satellite System (BDS) provides the precise point positioning (PPP) service in China and surrounding areas, with broadcasting PPP correction data through B2b signal of BDS. By receiving signals and demodulating correction message, user receivers can carry out real-time precise point positioning calculation, and obtain high-precision position information. This paper introduces the PPP model and parameter estimation. The calculation of the navigation chip is designed for PPP, with applying two independent CPU cores. The application of the chip in agricultural machinery is indicated in this paper.

More and more researches have focused on many aspects of Signal Quality Monitoring (SQM) algorithms which have advanced significantly over these years. However, traditional methods could disclose only the partial aspects of signal quality, little has been done on the comprehensive analysis of different parameters that could together reflect the whole characteristics of signal quality. In this paper, we provided a new method for synthetic power spectrum deviation evaluating, and the analysis parameters related to signal power, waveform, frequency spectrum and correlation curve were further improved based on the traditional methods. The evaluating methods of repairing capability, which is unique for BDS-3, and signal polarization mode were proposed and suggested to be added to SQM algorithms. Based on this, we presented a comprehensive analysis of SQM algorithms. Finally, based on the latest data collected by the 40-m large-aperture antenna at the Haoping Radio Observation from May to September of 2020, the initial signal quality analysis of the new-generation BDS-3 navigation signals for all the thirty in-orbit satellites were given. Results showed that compared with the traditional signal quality assessment system, the satellite signal quality assessment system presented in this paper can fully reflect the merits and disadvantages of satellite navigation signals. Research results can further complement the existing signal quality evaluation system, and provide guide reference for BDS and other GNSS satellite in signal performance testing at various stages.

为分析日界不连续误差对GNSS载波相位频率传递的影响,采用MGEX/BIPM并址站10 d的GPS/BDS观测数据进行PPP测站钟差解算,在分析测站钟差估计结果日界不连续误差统计特性的基础上,从理论分析和实验验证两方面研究日界不连续误差对两地时钟相对频率偏差估计的影响.结果表明,日界不连续误差基本服从高斯分布,导致两地时钟相对频率偏差估计结果在相邻批单元边界处出现偏移,难以准确估计总时段中相对频率偏差,严重影响两地时钟频率比对结果.

Global navigation satellite system (GNSS) precise point positioning (PPP) has been widely used for high-precision time and frequency transfer. However, the day-boundary discontinuities at the boundary epochs of adjacent days or batches are the most significant obstacle preventing PPP from continuous time transfer. The day-boundary discontinuities in station estimates and time comparisons are mainly caused by the code-pseudorange noise during the analysis of observation data in daily batches, where the absolute clock offset is determined by the average code measurements. However, some discontinuities with amplitudes even more than 0.15 ns may still appear in station clock estimates and time comparisons, although several methods had been proposed to remove such discontinuities. The residual small amplitude of the day-boundary discontinuities in some PPP station clock estimates and time comparisons through new GNSSs like Galileo seems larger, especially using precise clock products with large discontinuities. To further understand the origin of the day-boundary discontinuities, the influence of GNSS precise products on the day-boundary discontinuities in PPP station clock estimates and time comparisons is investigated in this paper. Ten whole days of Multi-GNSS Experiment (MGEX) from modified Julian date (MJD) 59028 to 59037 are used as the observation data. For a comparative analysis, the station clock estimates are compared with global positioning system (GPS) and Galileo observations through PPP and network solutions, separately. The experimental results show that the daily discontinuities in current combined GPS final and rapid clock products are less than 0.1 ns, and their influence on the origin of day-boundary discontinuities in PPP station clock estimates and time comparison are statistically negligible. However, the daily discontinuities in individual Analysis Centers (ACs) GPS products are more extensive, and their influence on the origin of the day-boundary discontinuities in GPS PPP station clock estimates cannot be ignored. The day-boundary discontinuities demonstrate random walk noise characteristics and deteriorate the station clocks' long-term frequency stability, especially at an average time of more than one day. Although Galileo clock daily discontinuities are different from those of GPS, their influence on the day-boundary discontinuities in station clock estimates is nearly similar to the GPS PPP. The influence of daily discontinuities of Galileo clocks on PPP time comparison is similar to GPS and is not particularly critical to time comparison. However, combined and weighted MGEX products should be developed or Galileo IPPP should be used for remote comparison of high-stability clocks.

随着人们对卫星导航系统定位精度需求的不断提升,近年来伪距偏差问题越来越受到广泛关注.然而截至目前,国内外专家学者对北斗系统的伪距偏差问题研究甚少.本文首先深入分析了北斗系统的伪距偏差产生根源,在此基础上对伪距偏差随接收机参数设置变化的关系进行详细阐述.然后基于昊平观测站的40m大口径天线采集的各颗北斗卫星下行信号数据,利用软件接收机遍历并分析了在不同接收机带宽、不同相关器间隔情况下,以及不同卫星仰角情况下北斗各颗卫星的B1 I和B3 I信号伪距偏差测量结果.最后根据北斗伪距偏差的特点,首次给出了我国北斗接收机参数设置建议.研究结果表明:在一定的接收机参数设置范围内,本文提出的方法可以将北斗伪距偏差降低到20cm以内.本文的研究成果不仅能够进一步促进广大专家学者对我国北斗伪距偏差的深入了解及广泛关注,同时还能促进我国BDS(BeiDou Navigation Satellite System)及其他GNSS(Global Navigation Satellite System)系统接口控制文件的进一步完善.

Time and frequency transfer through global navigation satellite system (GNSS) precise point positioning (PPP) based on carrier-phase measurements has been widely used for clock comparisons in national timing laboratories. However, the time jumps up to one nanosecond at the day boundary epochs of adjacent daily batches lead to discontinuities in the time transfer results. Therefore, it is a major obstacle to achieve continuous carrier phase time transfer. The day-boundary discontinuities have been studied for many years, and they are believed to be caused by the long-term pseudorange noise during estimation of the clock offset in the daily batches and are nearly in accordance with a Gaussian curve. Several methods of eliminating the day-boundary discontinuity were proposed during the past fifteen years, such as shift and overlapping, longer batch processing, clock handover, and ambiguity stacking. Some errors and new noise limit the use of such methods in the long-term clock stability comparison. One of the effective methods is phase ambiguity fixing resolution in zero-differenced PPP, which is based on the precise products of wide-lane satellite bias (WSB) provided by the new international GNSS Service (IGS) Analysis Center of Centre National d’Etudes Spatiales (CNES) and Collecte Localisation Satellites (CLS). However, it is not suitable for new GNSS, such as the Beidou Satellite System (BDS), GALILEO, and QZSS. For overcoming the drawbacks above, Multi-GNSS Experiment (MGEX) observation data of 10 whole days from MJD 58624 to 58633have been network processed by batch least square resolution. These observations come from several ground receivers located in different national timing laboratories. Code and carrier phase ionosphere-free measurements of GPS and BDS satellites are used, and the time transfer results from network processing are compared with PPP results provided by Bureau International des Poids et Mesures (BIPM) and used for international atomic time (TAI) computation (TAIPPP) and universal time coordination (UTC). It is shown that the time offsets of three different time links are almost continuous and the day-boundary discontinuities are sharply eliminated by network processing, although a little extent of day-boundary discontinuities still exist in the results of UTC(USNO)-UTC(PTB). The accuracy of time transfer has been significantly improved, and the frequency stability of UTC(NTSC)-UTC(PTB) can be up to 6.8 × 10−15 on average time of more than one day. Thus, it is suitable for continuous multi-GNSS time transfer, especially for long-term clock stability comparison.

The National Time Service Center (NTSC), Chinese Academy of Sciences (CAS), Xi’an, China, has developed a method for common-view time transfer using Geostationary Earth Orbit (GEO) satellite (GCV) applicable to the time and frequency transfer at distant stations. This method is independent of Global Navigation Satellite System (GNSS) time and frequency transfer, as well as two-way satellite time and frequency transfer (TWSTFT). A master clock at the time laboratory transmits pseudorandom code signals to a GEO telecommunication satellite, and the time signals are retransmitted by the satellite. Receivers at the time laboratory and user locations coincidentally receive the time signals, and the offsets between the user clocks and the master clock of the time laboratory are determined with high precision if the precise coordinates of the user locations are known. For this technology, only a parabolic antenna with receiver devices and a demodulator are required at each user station, but the coordinates and precise orbits need to be obtained in advance. The key features of GCV include 1) continuous coverage of the signal from GEO communication satellites; 2) differential observations to reduce the effects of orbit error and the imprecision of the propagation delay model; 3) using a very small aperture terminal (VSAT) to enhance high signal ratio to noise to obtain precise ranging accuracy and antimultipath ability; and 4) utilizing the C-band or Ku-band to decrease the impact of the ionosphere. Experiments based on the TWSTFT network of CAS showed that the performance of GCV was at the same level as that of TWSTFT, and the rms of the residuals of GCV was less than 1.5 ns with respect to TWSTFT.

Pseudorange bias has become a practical obstacle in the field of high-precision global navigation satellite system (GNSS) applications, which greatly restricts the further development of high-precision applications. Unfortunately, no studies have been conducted on the pseudorange biases of the BeiDou navigation satellite system (BDS). To mitigate the effects of pseudorange biases on the BDS performance to the greatest extent possible, the origin of such BDS pseudorange biases are first thoroughly illustrated, based upon which the dependency of the biases on the receiver configurations are studied in detail. Owing to the limitations regarding the parameter re-settings for hardware receivers, software receiver technology was used to achieve the ergodicity of the receiver parameters, such as the correlator spacing and front-end bandwidth, using high-fidelity signal observations collected by a 40-m-high gain dish antenna at Haoping Observatory. Based on this, the pseudorange biases of the BDS B1I and B3I signals and their dependency on different correlator spacings and front-end bandwidths were adequately provided. Finally, herein, the suggested settings of the correlator spacing and front-end bandwidth for BDS receivers are in detail proposed for the first time. As a result, the pseudorange biases of the BDS signals will be less than 20 cm, reaching even under 10 cm, under this condition. This study will provide special attention to GNSS pseudorange biases, and will significantly promote a clear definition of the appropriate receiver parameter settings in the interface control documents of BDS and other individual satellite systems.

Due to the distortions of the broadcasted satellite signals and the inconsistencies of parameter settings for different receivers, the single difference or double difference of pseudo-ranges between two receivers are different for two pair of different receivers. Bias inconsistencies will lead to adverse effects for pseudo-range-based positioning applications. Pseudo-range biases can also hinder carrier-phase ambiguity resolution. However, fewer articles deal with pseudo-range biases for BeiDou navigation satellite System (BDS). In order to mitigate the impact of biases on BDS to the greatest extent, the generation mechanisms and characteristics of pseudo-range biases are studied in detail firstly. Then based on this, experimental verification methods are designed using Haoping Radio Observatory (HRO) of Chinese Academy of Sciences to observe BDS signals. Pseudo-range biases of all visible BDS satellites are measured and evaluated with high accuracy, using the 40 meters dish antenna and modern equipment of HRO. Finally, some important parameters of BDS receivers, such as the correlator spacing and front-end bandwidth, are suggested to mitigate the ranging errors and positioning errors result from pseudo-range biases. The achievements of this paper can provide a worthy reference for GNSS signal designers, GNSS monitoring and assessment and GNSS receiver designers.

Due to the non-ideal characteristics of navigation satellite signals, pseudo-range observations of two satellites for the same signal are different in the same receiver, in addition to that, for zero-baseline receivers, pseudo-range observations between different receivers are different even for the same satellite and the same signal. Bias inconsistencies will lead to adverse effects for pseudo-range-based positioning applications and can also hinder carrier-phase ambiguity resolution. However, fewer article deals with the generation mechanisms of pseudo-range biases for BeiDou Navigation Satellite System. In order to mitigate the impact of biases on BDS to the greatest extent, the generation mechanisms and characteristics of pseudo-range biases are studied in detail in the beginning. Then based on this, experimental verification methods are designed using Haoping Radio Observatory (HRO) of Chinese Academy of Sciences to observe BDS signals. Pseudo-range biases of all visible BDS satellites are measured and evaluated with high accuracy thanks to the 40 m dish antenna and modern equipment of HRO. Finally, some important parameters for BDS receivers, such as the correlator spacing and front-end bandwidth, are provided or suggested to mitigate the ranging errors and positioning errors result from pseudo-range biases. The achievements of this paper could be a worthy reference for GNSS signal designers, GNSS monitoring and assessment and GNSS receiver designers.

It is well known that the traditional evil waveform evaluating method for Binary Phase Shift Keying (BPSK) or Quadrature Phase Shift Keying (QPSK) is the 2nd Order Step (2OS) thread model adopted by the International Civilian Aviation Organization (ICAO).Although there are some new methods to evaluate new Global Navigation Satellite Systems (GNSS) signals, they are all for the analysis of the width and amplitude of signal waveforms.No research has been done on waveform symmetry yet.It has been observed through experiments that waveform asymmetry could also result in tracking errors, range biases, and position errors in GNSS receivers.In order to better evaluate the characteristics of most new navigation signals, the mathematical models for GNSS signals are presented and the extended general thread models are provided from the traditional 2OS thread model.A novel Waveform Rising and Falling Edge Symmetry (WRaFES) model determined for evaluating the asymmetry of GNSS signal waveform is proposed and introduced in details, as well as the evaluating method for Binary Offset Carrier (BOC) correlation curves.WRaFES model characteristics are analyzed in details from the aspects of time domain waveform, correlation domain and S curve bias.Finally, taking the first modernized BeiDou Navigation Satellite System (BDS) satellite M1-S B1Cd signal as an example, the thresholds for signal deformation detection and experimental results from these proposed methods are given.Simulation results and test results show that WRaFES model is very effective not only in detecting waveform asymmetry with high accuracy, but also in analyzing the relationship between waveform asymmetry and tracking error.

Satellite navigation analog threat generate unit abnormal will lead to signal distortion, navigation signal analog threat will directly affect the ranging, positioning performance and other user experience. In this paper, the analog threat signal of BDS are researched and analyzed. Firstly, the large-diameter antenna receiving system is used to obtain the off-line satellite navigation signal data. Using the cumulative average method to process the baseband waveforms. Then, the standard chip correlation technique is used to obtain the optimal symbol waveform. Secondly, the optimal symbol is modeled based on the 2OS model to determine the initial value of the sigma (damping factor) and f(d) (damped frequency) of oscillation. The two-dimensional search is used to search sigma and f(d) respectively. The mean of the difference between ideal signal and the real signal waveform are based for the sigma and f(d) of oscillation values. Finally, the simulation and experimental data are used to verify, the estimation method of the analog distortion parameters of the navigation signal and its influence on the ranging performance are given in detail.

With the advent of new global navigation satellite systems (GNSSs) and new signals, GNSS users will rely more on them to obtain higher-accuracy positioning. Evil waveform monitoring and assessment are of great importance for GNSS to achieve its positioning, velocity, and timing service with high accuracy. However, the advent of new navigation signals introduces the necessity to extend the traditional analyzing techniques already accepted for binary phase-shift keying modulation to new techniques. First, the well-known second-order step thread model adopted by the International Civil Aviation Organization is introduced. Then the extended new general thread models are developed for the new binary offset carrier modulated signals. However, no research has been done on navigation signal waveform symmetry yet. Simulation results showed that, waveform asymmetry may also cause tracking errors, range biases, and position errors in GNSS receivers. It is thus imperative that the asymmetry be quantified to enable the design of appropriate error budgets and mitigation strategies for various application fields. A novel evil waveform analysis method, called waveform rising and falling edge symmetry (WRaFES) method, is proposed. Based on this WRaFES method, the correlation metrics are provided to detect asymmetric correlation peaks distorted by received signal asymmetry. Then the statistical properties of the proposed methods are analyzed, and a proper deformation detection threshold is calculated. Finally, both simulation results and experimentally measured results of Beidou navigation satellite system (BDS) M1-S B1Cd signal are given, which show the effectiveness and robustness of the proposed thread models.

基于卫星双向时间传递原理,国家授时中心提出了转发式卫星测定轨方法,已将GEO通信卫星的测定轨精度提高到米级水平.近年来,转发式测定轨技术不断发展完善,观测目标已由单一GEO卫星扩展到北斗IGSO卫星(I1-S).本文描述了转发式测量模型,并给出了转发式测定轨新系统对GEO卫星和北斗IGSO卫星(I1-S)的测定轨结果.经过试验验证,GEO、IGSO卫星的重叠弧段的轨道差的RMS值已分别达到2m和0.9in.长期的试验应用和分析表明,转发式测定轨技术的主要特色和优越性在于:转发测距与钟差分离,便于实现精密定轨;以精密时间测量为基础,测距精度高(2cm),并且不受气象条件制约;使用微波频段和扩频技术,易于远距离测轨;该技术所需的星上透明转发器载荷成熟且易于小型化.从未来应用来看,转发式测定轨技术适用于中高轨航天器的精密测定轨,尤其对高轨卫星测定轨有明显优势,可用于开展相关科学研究.

In the signal generation,emission,transmission and receiving processes of the global navigation satellite system (GNSS),due to some unpredictable anomalies or effects,there are various abnormalities that would affect GNSS signal quality.Consequently,those unpredictable anomalies and effects received by users would influence the position,velocity and time (PVT)performance of the GNSS.By monitoring and assessment of GNSS signal quality,it would be possible to detect signal distortions and warn the users as soon as possible, thus guaranteeing a safe and efficient use of the GNSS,especially for civil aviation and maritime affair users. However,for the monitoring and assessment of a variety of abnormal GNSS navigation signals,there are no comprehensive and systematic research results yet,and no real-time or quasi real-time automatic identification and mechanism for signal distortion analyzing at present.The idea of establishing a GNSS signal distortion mo-del is introduced.To detect GNSS signal distortions caused during the processes from signal generation to signal receiving,and to further analyze their impacts on the service performance of the GNSS,a relatively complete mathematical method is proposed through simulation and data verification.Besides,the generation mechanisms and characteristics of GNSS signal distortions are analyzed in detail,and based on that,the influences of GNSS signal distortions on signal quality assessment and user positioning are given both in qualitative and quantitative methods.The achievements in this paper could be used as a valuable technical reference for automatic identifica-tion and locating of GNSS satellite faults.In addition,it could also be a good reference for satellite signal gene-ration designers in signal designing and optimizing.

Nowadays, multipath remains as an unsolved key problem for high precision applications, such as satellite navigation and position. Because GNSS broadcasted signals are subject to reflection and diffraction, just like any other type of electromagnetic waves, multipath is caused by the reception of direct signal together with the reflected or/and diffracted signals at the same time. Multipath will result in errors in pseudorange measurement and carrier phase measurement, and thus affect the positioning accuracy. To verify the effects of multipath propagation on positioning performance, the principle of multipath is introduced in the beginning of this paper. Then based on a simulation platform and mathematical verifying model, the multipath performance and its impact on positioning performance of received GNSS signal is comprehensively demonstrated from different aspects, such as pseudorange multipath error, waveform shape, modulation error, and correlation characteristics and so on. Results indicate the degradation of positioning performance when multipath signals were present, causing position error with several meters to tens of meters. Those results and data we have obtained from this paper could be used as a valuable reference for BDS future signal design and system constructions.

As one of the high accuracy time comparison methods, Two-way Satellite Time and Frequency Transfer (TWSTFT) is one of the important methods for Bureau International des Poids et Mesures (BIPM) to organize international comparison, calculation of the International Atomic Time (TAI) and Coordinated Universal Time (UTC). Due to the interference of emitting device of TWSTFT to co-located devices of Very Long Baseline Interferometry (VLBI) or International GNSS Service (IGS) and the limitation of satellite transponder resources, the comparison is not proceed continuously. Ten consecutive days of data from C-band TWSTFT Network of National Time Service Center (NTSC) was used for analyzing the precision of non-continuous TWSTFT. The raw data of non-continuous TWSTFT was dealt with linear interpolation method. Then the result was compared with continuous TWSTFT which was seen as the true value. Finally, the influence of interval to the precision of non-continuous TWSTFT was analyzed. The comparison shows that: when the interval time is less than 2.5 days, the RMS of the difference between non-continuous and continuous TWSTFT is better than 1 ns; when the interval time is 0.5 day, the RMS is better than 0.5 ns.

The building algorithm of system time is one of the research focus of auto-navigation. This paper bases on centralized processing mode of auto-timekeeping, gives a kind of building algorithm of system time. On-board clock's weight are determined by the clock quantity which referring to the computation, change of clock velocity can reflect change of clock frequency, so it uses clock velocity instead of clock frequency for determining weight; this algorithm uses two kinds of method of predicting initial velocity: a. with the former two epochs each satellite of each day predict initial velocity, b. with the former two epochs each satellite of first day predict initial velocity. Choosing one clock as the main clock, according to defined sample interval, getting all the available clock error relative to the main clock. This paper gives the computation model of system time, testing the algorithm with IGS precise clock error(system time is IGST). The result shows: comparing to the equal weighted algorithm, the weighted algorithm has better stability. Adding prediction has a significant improvement on the system time's stability, method a: the difference between TA(weight) and TA(IGST) (International GNSS Service Time) is about 45ns, method b: the difference be-tween TA(weight) and TA(igst) (International GNSS Service Time) is about 65ns. The algorithm is in reason, it can be used for the building algorithm of system time of auto-navigation.

The multistage depressed collector (MDC) of the traveling wave tube (TWT) was mo dified by introduction of the non-axisymmetric magnetic field, generated with a piece of magnetic steel sheet closely contacting the MDC outer wall to increase the collector efficiency and decrease the back-streaming rate. The impact of the magnetic field distribution in the MDC of a Ku-band TWT on the electron trajectories was modeled and calculated with MTSS simulation code. The simulated results show that the optimized magnetic field distribution significantly affects the current distributions on different electrodes. For instance, quite a few electrons can reach the lower potential electrode. At the middle operating frequency, the magnetic field simultaneously reduces the back-streaming rate from 2.46% to 0.54%; and increases the collector efficiency by more than 3%, up to 79.07%.