Orbit determination for geostationary satellites with the combination of transfer ranging and pseudorange data

Geostationary satellites(GEOs) play a significant role in the regional satellite navigation system.Simulation experiments show that the clock corrections could be mitigated through a single strategy or double differencing strategies for a navigation constellation,but for the mode of individual GEO orbit determination,high precision orbit and clock correction could not be obtained in the orbit determination based on the pseudorange data.A new GEO combined precise orbit determination(POD) strategy is studied in this paper,which combines pseudorange data and C-band transfer ranging data.This strategy overcomes the deficiency of C-band transfer ranging caused by limited stations and tracking time available.With the combination of transfer ranging and pseudorange data,clock corrections between the GEO and the stations can be estimated simultaneously along with orbital parameters,maintaining self-consistency between the satellite ephemeris and clock correction parameters.The error covariance analysis is conducted to illuminate the contributions from the transfer ranging data and the psudoranging data.Using data collected for a Chinese GEO satellite with 3 C-band transfer ranging stations and 4 L-band pseudorange tracking stations,POD experiments indicate that a meter-level accuracy is achievable.The root-mean-square(RMS) of the post-fit C-band ranging data is about 0.203 m,and the RMS of the post-fit pseudorange is 0.408 m.Radial component errors of the POD experiments are independently evaluated with the satellite laser ranging(SLR) data from a station in Beijing,with the residual RMS of 0.076 m.The SLR evaluation also suggests that for 2-h orbital predication,the predicted radial error is about 0.404 m,and the clock correction error is about 1.38 ns.Even for the combination of one C-band transfer ranging station and 4 pseudorange stations,POD is able to achieve a reasonable accuracy with the radial error of 0.280 m and the 2-h predicted radial error of 0.888 m.Clock synchronization between the GEO and tracking stations is achieved with an estimated accuracy of about 1.55 ns,meeting the navigation service requirements.     

Orbit determination and prediction for Beidou GEO satellites at the time of the spring/autumn equinox

Geostationary(GEO) satellites form an indispensable component of the constellation of Beidou navigation system(BDS). The ephemerides, or predicted orbits of these GEO satellites(GEOs), are broadcast to positioning, navigation, and timing users. User equivalent ranging error(UERE) based on broadcast message is better than 1.5 m(root formal errors: RMS) for GEO satellites. However, monitoring of UERE indicates that the orbital prediction precision is significantly degraded when the Sun is close to the Earth's equatorial plane(or near spring or autumn Equinox). Error source analysis shows that the complicated solar radiation pressure on satellite buses and the simple box-wing model maybe the major contributor to the deterioration of orbital precision. With the aid of BDS' two-way frequency and time transfer between the GEOs and Beidou time(BDT, that is maintained at the master control station), we propose a new orbit determination strategy, namely three-step approach of the multi-satellite precise orbit determination(MPOD). Pseudo-range(carrier phase) data are transformed to geometric range(biased geometric range) data without clock offsets; and reasonable empirical acceleration parameters are estimated along with orbital elements to account for the error in solar radiation pressure modeling. Experiments with Beidou data show that using the proposed approach, the GEOs' UERE when near the autumn Equinox of 2012 can be improved to 1.3 m from 2.5 m(RMS), and the probability of user equivalent range error(UERE)2.0 m can be improved from 50% to above 85%.     

Accounting of fundamental components of the rotation parameters of the Earth in the formation of a high-accuracy orbit of navigation satellites

An analysis of perturbing factors influencing the motion of a navigation satellite (NS) is carried out, and the degree of influence of each factor on the GLONASS orbit is estimated. It is found that fundamental components of the Earth??s rotation parameters (ERP) are one substantial factor commensurable with maximum perturbations. Algorithms for the calculation of orbital perturbations caused by these parameters are given; these algorithms can be implemented in a consumer??s equipment. The daily prediction of NS coordinates is performed on the basis of real GLONASS satellite ephemerides transmitted to a consumer, using the developed prediction algorithms taking the ERP into account. The obtained accuracy of the daily prediction of GLONASS ephemerides exceeds by tens of times the accuracy of the daily prediction performed using algorithms recommended in interface control documents.     

Orbit determination for Chang’E-2 lunar probe and evaluation of lunar gravity models

The Unified S-Band (USB) ranging/Doppler system and the Very Long Baseline Interferometry (VLBI) system as the ground tracking system jointly supported the lunar orbit capture of both Chang’E-2 (CE-2) and Chang’E-1 (CE-1) missions. The tracking system is also responsible for providing precise orbits for scientific data processing. New VLBI equipment and data processing strategies have been proposed based on CE-1 experiences and implemented for CE-2. In this work the role VLBI tracking data played was reassessed through precision orbit determination (POD) experiments for CE-2. Significant improvement in terms of both VLBI delay and delay rate data accuracy was achieved with the noise level of X-band band-width synthesis delay data reaching 0.2–0.3 ns. Short-arc orbit determination experiments showed that the combination of only 15 min’s range and VLBI data was able to improve the accuracy of 3 h’s orbit using range data only by a 1–1.5 order of magnitude, confirming a similar conclusion for CE-1. Moreover, because of the accuracy improvement, VLBI data was able to contribute to CE-2’s long-arc POD especially in the along-track and orbital normal directions. Orbital accuracy was assessed through the orbital overlapping analysis (2 h arc overlapping for 18 h POD arc). Compared with about 100 m position error of CE-1’s 200 km×200 km lunar orbit, for CE-2’s 100 km×100 km lunar orbit, the position errors were better than 31 and 6 m in the radial direction, and for CE-2’s 15 km×100 km orbit, the position errors were better than 45 and 12 m in the radial direction. In addition, in trying to analyze the Delta Differential One-Way Ranging (ΔDOR) experiments data we concluded that the accuracy of ΔDOR delay was dramatically improved with the noise level better than 0.1 ns and systematic errors better calibrated, and the Short-arc POD tests with ΔDOR data showed excellent results. Although unable to support the development of an independent lunar gravity model, the tracking data of CE-2 provided evaluations of different lunar gravity models through POD. It is found that for the 100 km×100 km lunar orbit, with a degree and order expansion up to 165, JPL’s gravity model LP165P did not show noticeable improvement over Japan’s SGM series models (100×100), but for the 15 km×100 km lunar orbit, a higher degree-order model can significantly improve the orbit accuracy.

     

Satellite-station time synchronization information based real-time orbit error monitoring and correction of navigation satellite in Beidou System

Satellite-station two-way time comparison is a typical design in Beidou System(BDS)which is significantly different from other satellite navigation systems.As a type of two-way time comparison method,BDS time synchronization is hardly influenced by satellite orbit error,atmosphere delay,tracking station coordinate error and measurement model error.Meanwhile,single-way time comparison can be realized through the method of Multi-satellite Precision Orbit Determination(MPOD)with pseudo-range and carrier phase of monitor receiver.It is proved in the constellation of 3GEO/2IGSO that the radial orbit error can be reflected in the difference between two-way time comparison and single-way time comparison,and that may lead to a substitute for orbit evaluation by SLR.In this article,the relation between orbit error and difference of two-way and single-way time comparison is illustrated based on the whole constellation of BDS.Considering the all-weather and real-time operation mode of two-way time comparison,the orbit error could be quantifiably monitored in a real-time mode through comparing two-way and single-way time synchronization.In addition,the orbit error can be predicted and corrected in a short time based on its periodic characteristic.It is described in the experiments of GEO and IGSO that the prediction accuracy of space signal can be obviously improved when the prediction orbit error is sent to the users through navigation message,and then the UERE including terminal error can be reduced from 0.1 m to 0.4 m while the average accuracy can be improved more than 27%.Though it is still hard to make accuracy improvement for Precision Orbit Determination(POD)and orbit prediction because of the confined tracking net and the difficulties in dynamic model optimization,in this paper,a practical method for orbit accuracy improvement is proposed based on two-way time comparison which can result in the reflection of orbit error.     

一种通用的卫星导航信号码时延估计误差评估方法

卫星导航信号的码时延估计误差是决定系统服务性能的关键因素,迫切需要对多种不同调制与复用方式的导航信号进行全面的码时延估计误差性能评估,从而为后期的系统应用提供重要的选择依据.为此,本文提出了通用的码时延估计误差评估方法.首先,概括了导航接收机的码跟踪环路模型,根据是否匹配接收以及是否相干处理,将目前的导航接收机归纳为四种类型.其次,在假设码时延估计误差非常小的条件下,分别给出了匹配接收下相干处理和非相干处理时的估计误差以及相互之间的关系;推导了非匹配接收下非相干处理时的估计误差,并讨论了与相干处理时的关系.最后,推导了码时延估计误差的齐夫-扎凯界限,解决了估计误差不满足非常小这一假设条件时的评估.本文提出的评估方法均以导航信号的功率谱密度表示,为信号设计和接收机的研制提供了重要的理论指导,同时也给具体信号的评估带来了极大便利.仿真实验中对新一代典型导航信号的码时延估计误差做了有效评估.     

基于改进UKF的加油机位姿预测方法

针对无人机自主空中加油保持阶段加油机位姿跟踪精度不高的问题,提出了一种改进UKF（无损卡尔曼滤波）预测方法。建立了视觉导航系统模型,利用Harris算法检测角点,并用RANSAC（随机序列一致性）算法进行角点匹配。将历史预测数据引入当前时刻UKF预测值,并通过匹配角点所得姿态观测值对改进UKF预测值进行修正,从而实现加油机姿态的高精度预测。仿真结果表明,改进UKF在遭遇突发强干扰时姿态预测性能明显优于标准UKF,所预测误差小于5.8%,满足空中加油精度要求。该算法避免了强干扰引发的预测出错,有效抑制了突发干扰。     

测向误差特征辅助两步式网络的水声纯方位定位方法

围绕水声分布式纯方位定位问题,针对传统方法的远距离定位精度低、定位结果易受初值影响等缺点,提出了一种测向误差特征辅助两步式全连接层神经网络(DFE-TS-FCNN)的纯方位定位方法。使用神经网络进行定位,提高远距离定位精度并消除初值影响,输入特征是目标方位角测量值和测向误差标准差估计值。使用两步式网络结构抑制网络过拟合,分类网络确定目标区域后,再用对应的定位网络估计目标位置。蒙特卡洛仿真实验中,所提方法在近距离达到了与迭代加权最小二乘算法和迭代总体最小二乘算法相近的定位精度,在远距离定位精度大幅提高、约束均方根误差(RMSE)小于2.5 km的条件下,最远可定向距离相比传统方法从12.6 km提升至22.7 km。在实际数据中,该方法也获得了较好的定位结果。     

基于滑模变结构控制及预测跟踪的光电吊舱控制方法北大核心CSCD

光电吊舱在稳像和目标跟踪过程中存在摩擦力矩、不平衡力矩等干扰力矩,从而影响速度环响应精度;另一方面,吊舱视频跟踪器图像传输和处理造成的延迟也会造成跟踪滞后,因此必须进行延时补偿。提出基于预测跟踪的滑模变结构控制方法,采用微分预测跟踪器实现对视频跟踪器的延迟补偿,采用预测跟踪器估计出的目标运动角速率构成自适应补偿参数,以调整滑模变结构控制量,改进的滑模控制算法在补偿干扰力矩的同时抑制了抖振现象。仿真及实验结果表明:改进的控制方法能够有效补偿干扰力矩和跟踪器延迟造成的误差,相对于传统PID控制,其跟踪误差减小为原来的1/3,并且该算法已经在相关系统上得到应用。     

基于大数据分析的移动对象轨迹预测方法

对移动对象的轨迹预测将在移动目标跟踪识别中具有较好的应用价值。移动对象轨迹预测的基础是移动目标运动参量的采集和估计,移动目标的运动参量信息特征规模较大,传统的单分量时间序列分析方法难以实现准确的参量估计和轨迹预测。提出一种基于大数据多传感信息融合跟踪的移动对象轨迹预测算法。首先进行移动目标对象进行轨迹跟踪的控制对象描述和约束参量分析,对轨迹预测的大规模运动参量信息进行信息融合和自正整定性控制,通过大数据分析方法实现对移动对象运动参量的准确估计和检测,由此指导移动对象轨迹的准确预测,提高预测精度。仿真结果表明,采用该算法进行移动对象的运动参量估计和轨迹预测的精度较高,自适应性能较强,稳健性较好,相关的指标性能优于传统方法。     

Adaptive Two-Step Bearing-Only Underwater Uncooperative Target Tracking with Uncertain Underwater Disturbances

The bearing-only tracking of an underwater uncooperative target can protect maritime territories and allows for the utilization of sea resources. Considering the influences of an unknown underwater environment, this work aimed to estimate 2-D locations and velocities of an underwater target with uncertain underwater disturbances. In this paper, an adaptive two-step bearing-only underwater uncooperative target tracking filter (ATSF) for uncertain underwater disturbances is proposed. Considering the nonlinearities of the target’s kinematics and the bearing-only measurements, in addition to the uncertain noise caused by an unknown underwater environment, the proposed ATSF consists of two major components, namely, an online noise estimator and a robust extended two-step filter. First, using a modified Sage-Husa online noise estimator, the uncertain process and measurement noise are estimated at each tracking step. Then, by adopting an extended state and by using a robust negative matrix-correcting method in conjunction with a regularized Newton-Gauss iteration scheme, the current state of the underwater uncooperative target is estimated. Finally, the proposed ATSF was tested via simulations of a 2-D underwater uncooperative target tracking scenario. The Monte Carlo simulation results demonstrated the reliability and accuracy of the proposed ATSF in bearing-only underwater uncooperative tracking missions.     

基于ARM的视觉导航AGV图像处理方法研究

为了实现更加简单高效的标识线图像的检测与处理,提出了一种优化的基于ARM的视觉导航AGV标识线图像处理方法。首先对采集到的图像进行灰度化处理并使用Otsu算法对图像进行阈值分割;然后采用优化的中值滤波算法进行图像滤波并使用高效的边缘提取策略获取路径边缘特征;最后采用角度判断的方法剔除错误点并使用最小二乘法拟合成路径的中心线。实验结果表明,该方法有较高的准确率和较好的实时性,可以满足工业生产中的实际需求,适用于基于嵌入式系统开发的视觉导航AGV。     

Failure strength prediction of glass/epoxy composite laminates from acoustic emission parameters using artificial neural network

The ageing effect of glass/epoxy composite laminates exposed to seawater environment for different periods of time was investigated using acoustic emission (AE) monitoring. The mass gain ratio and flexural strength of glass fiber reinforced plastic (GFRP) composite laminates were examined after the seawater treatment. The flexural strength of the seawater treated GFRP specimens showed a decreasing trend with increasing exposure time. The degradation effects of seawater are studied based on the changes in AE signal parameters for various periods of time. The significant AE parameters like counts, energy, signal strength, absolute energy and hits were considered as training data input. The input data were taken from 40% to 70% of failure loads for developing the radial basis function neural network (RBFNN) and generalised regression neural network (GRNN) models. RBFNN model was able to predict the ultimate failure strength and could be validated with the experimental results with the percentage error well within 0.5–7.2% tolerance, whereas GRNN model was able to predict the ultimate failure strength with the percentage error well within 0.5–4.4% tolerance. The prediction accuracy of GRNN model is found to be better than RBFNN model.     

基于椭圆对数极坐标变换的尺度变化目标跟踪算法

针对传统对数极坐标变换局限于跟踪圆形或类圆形尺度变化目标这一问题,提出一种基于椭圆对数极坐标变换域下目标匹配的尺度变化目标跟踪算法。算法利用Mean Shift进行空间定位,确定目标的形心,通过椭圆对数极坐标变换域中目标和候选的最大相关匹配系数来确定目标的尺度参数。实验结果表明:该文算法在目标小形变和光照变化条件下,跟踪误差较小,尺度跟踪准确率更高,具有较好的鲁棒性。     

Applications of two-way satellite time and frequency transfer in the BeiDou navigation satellite system

A two-way satellite time and frequency transfer(TWSTFT) device equipped in the BeiDou navigation satellite system(BDS)can calculate clock error between satellite and ground master clock. TWSTFT is a real-time method with high accuracy because most system errors such as orbital error, station position error, and tropospheric and ionospheric delay error can be eliminated by calculating the two-way pseudorange difference. Another method, the multi-satellite precision orbit determination(MPOD)method, can be applied to estimate satellite clock errors. By comparison with MPOD clock estimations, this paper discusses the applications of the BDS TWSTFT clock observations in satellite clock measurement, satellite clock prediction, navigation system time monitor, and satellite clock performance assessment in orbit. The results show that with TWSTFT clock observations, the accuracy of satellite clock prediction is higher than MPOD. Five continuous weeks of comparisons with three international GNSS Service(IGS) analysis centers(ACs) show that the reference time difference between BeiDou time(BDT) and golbal positoning system(GPS) time(GPST) realized IGS ACs is in the tens of nanoseconds. Applying the TWSTFT clock error observations may obtain more accurate satellite clock performance evaluation in the 104 s interval because the accuracy of the MPOD clock estimation is not sufficiently high. By comparing the BDS and GPS satellite clock performance, we found that the BDS clock stability at the 103 s interval is approximately 10.12, which is similar to the GPS IIR.     

轮式机器人轨迹跟踪控制系统的设计

针对轮式机器人轨迹跟踪控制系统误差收敛速率低、精度和实时性差的问题,采用反演控制算法并结合李雅普诺夫稳定性分析方法对轮式机器人的轨迹跟踪系统进行了优化设计。建立了轮式机器人轨迹跟踪控制系统的运动学模型,并对该模型进行位置偏差分析;在反演控制算法中引入了分部虚拟控制量,并分析和设计了其他间接受控量,提高了算法运行的效率;采用李雅普诺夫收敛定理对系统的收敛性进行分析,根据分析的结果提出了算法更加简单的控制律。利用Matlab软件的Simulink库对设计的轨迹跟踪控制系统试验研究。结果表明,与基于李雅普诺夫直接法或者迭代学习算法设计的轮式机器人轨迹跟踪控制系统相比较,设计的控制系统具有跟踪精度高、收敛速度快、实时性好的优点。     

Design of Nonlinear Autoregressive Exogenous Model Based Intelligence Computing for Efficient State Estimation of Underwater Passive Target

In this study, an intelligent computing paradigm built on a nonlinear autoregressive exogenous (NARX) feedback neural network model with the strength of deep learning is presented for accurate state estimation of an underwater passive target. In underwater scenarios, real-time motion parameters of passive objects are usually extracted with nonlinear filtering techniques. In filtering algorithms, nonlinear passive measurements are associated with linear kinetics of the target, governing by state space methodology. To improve tracking accuracy, effective feature estimation and minimizing position error of dynamic passive objects, the strength of NARX based supervised learning is exploited. Dynamic artificial neural networks, which contain tapped delay lines, are suitable for predicting the future state of the underwater passive object. Neural networks-based intelligence computing is effectively applied for estimating the real-time actual state of a passive moving object, which follows a semi-curved path. Performance analysis of NARX based neural networks is evaluated for six different scenarios of standard deviation of white Gaussian measurement noise by following bearings only tracking phenomena. Root mean square error between estimated and real position of the passive target in rectangular coordinates is computed for evaluating the worth of the proposed NARX feedback neural network scheme. The Monte Carlo simulations are conducted and the results certify the capability of the intelligence computing over conventional nonlinear filtering algorithms such as spherical radial cubature Kalman filter and unscented Kalman filter for given state estimation model.     

Optical testing of star sensor (II): Alignment of star sensor from off-line pictures taken in orbital test

Alignment error of an image sensor relative to the optical axis of a star sensor head and alignment errors between four heads were calculated from pictures taken in an orbital experiment. When comparing the image sensor alignment parameters in the orbital test and a ground test, both values matched well. By determining the relative relationship of the four heads using four pictures taken at the same moment in orbit and uploading the parameters to the star sensor system in orbit, the estimated attitude error was improved from 0.29 to 0.17°, though the accuracy was limited by the ±0.2° determination accuracy of the satellite itself. We estimated the attitude determination accuracy from separation angles between boresights of the four heads, calculated from pattern matching between downloaded pictures and a star catalogue. The estimated accuracy, in terms of potential optical performance, was 0.60 arcmin at 3σ, which is sufficient to satisfy the specification of 1 arcmin.     

Precise orbit determination of a maneuvered GEO satellite using CAPS ranging data

Wheel-off-loadings and orbital maneuvers of the GEO satellite result in additional accelerations to the satellite itself. Complex and difficult to model, these time varying accelerations are an important error source of precise orbit determination (POD). In most POD practices, only non-maneuver orbital arcs are treated. However, for some applications such as satellite navigation RDSS services, uninterrupted orbital ephemeris is demanded, requiring the development of POD strategies to be processed both during and after an orbital maneuver. We in this paper study the POD for a maneuvered GEO satellite, using high precision and high sampling rate ranging data obtained with Chinese Area Positioning System (CAPS). The strategy of long arc POD including maneuver arcs is studied by using telemetry data to model the maneuver thrust process. Combining the thrust and other orbital perturbations, a long arc of 6 days’ CAPS ranging data is analyzed. If the telemetry data are not available or contain significant errors, attempts are made to estimate thrusting parameters using CAPS ranging data in the POD as an alternative to properly account for the maneuver. Two strategies achieve reasonably good data fitting level in the tested arc with the maximal position difference being about 20 m. Supported by the National Natural Science Foundation of China (Grant No. 10703011) and the Science & Technology Commission of Shanghai Municipality of China (Grant No. 06DZ22101)     

结合卡尔曼滤波器噪声分析的车道线检测跟踪算法

使用卡尔曼滤波对视频序列图像中的具体信息进行跟踪的研究目前是跟踪方向的一个热点.但是在处理卡尔曼滤波跟踪过程中的过程噪声和测量噪声,大部分研究普遍采用的是初始赋值.通过不断的调整数,达到较好的跟踪效果.但是这样做不但没有遵循原始数据的规律,同时调整参数是一项耗时的工作.基于这个原因,提出了一种对卡尔曼滤波的过程噪声和测量噪声进行预估计的方法并将其应用到车道线跟踪过程中.通过对一部分离线数据进行处理,可以基本估计出系统的噪声参数.最后采用车道线跟踪算法对论文中的方法进行验证,实验证明,提出的参数估计方法在车道线的跟踪过程中达到很好的效果,同时处理每帧的时间为50ms左右,满足了实时性的要求.