Fast approximate maximum likelihood period estimation from incomplete timing data

 To estimate the period of a periodic point process from noisy and incomplete observations, the classical periodogram algorithm is modified. The original periodogram algorithm yields an estimate by performing grid search of the peak of a spectrum, which is equivalent to the periodogram of the periodic point process, thus its performance is found to be sensitive to the chosen grid spacing. This paper derives a novel grid spacing formula, after finding a lower bound of the width of the spectral mainlobe. By employing this formula, the proposed new estimator can determine an appropriate grid spacing adaptively, and is able to yield approximate maximum likelihood estimate (MLE) with a computational complexity of O(n²). Experimental results prove that the proposed estimator can achieve better trade-off between statistical accuracy and complexity, as compared to existing methods. Simulations also show that the derived grid spacing formula is also applicable to other estimators that operate similarly by grid search.     

Radiation efects on MHD stagnation point flow of nano fluid towards a stretching surface with convective boundary condition

The aim of the present paper is to study the numerical solutions of the steady MHD two dimensional stagnation point flow of an incompressible nano fluid towards a stretching cylinder.The effects of radiation and convective boundary condition are also taken into account.The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis.The resulting nonlinear momentum,energy and nano particle equations are simplifed using similarity transformations.Numerical solutions have been obtained for the velocity,temperature and nanoparticle fraction profles.The influence of physical parameters on the velocity,temperature,nanoparticle fraction,rates of heat transfer and nanoparticle fraction are shown graphically.     

可数非扩张映像族公共不动点迭代算法

提出一种计算可数个非扩张映像族公共不动点的新的迭代算法，并证明了算法的强收敛性。本算法由于避免使用形一映像，因而使得算法简洁，计算工作量较小。     

数控机床回零的控制方法及常见故障分析

数控机床有挡块返回参考点方式是数控机床上广泛采用的一种回零方式,同时,有挡块返回参考点在数控机床故障中发生的概率也比较大,快速而有效的排除此类故障是保证企业顺利生产的必然要求。本文重点介绍有挡块回参考点的工作原理、动作方式、参数设置、PMC控制及常见故障分析与排除方法,以期为数控机床此类故障的排除提供可借鉴的方法。     

工业 S型热电偶1100℃以上温度点校准方法

越来越多的单位要求对工业S型热电偶1100℃以上温度点进行校准，但目前没有任何技术规范可供参考。本文就这一需求提供了三种解决方案，并对这三个方案的校准结果及不确定度进行对比分析，为计量机构根据客户的需求制定科学合理的校准方案提供参考。     

Towards PPP-RTK: Ambiguity resolution in real-time precise point positioning

Integer ambiguity resolution at a single station can be achieved by introducing predetermined uncalibrated phase delays (UPDs) into the float ambiguity estimates of precise point positioning (PPP). This integer resolution technique has the potential of leading to a PPP-RTK (real-time kinematic) model where PPP provides rapid convergence to a reliable centimeter-level positioning accuracy based on an RTK reference network. Nonetheless, implementing this model is technically subject to how rapidly we can fix wide-lane ambiguities, stabilize narrow-lane UPD estimates, and achieve the first ambiguity-fixed solution. To investigate these issues, we used 7 days of 1-Hz sampling GPS data at 91 stations across Europe. We find that at least 10 min of observations are required for most receiver types to reliably fix about 90% of wide-lane ambiguities corresponding to high elevations, and over 20 min to fix about 90% of those corresponding to low elevations. Moreover, several tens of minutes are usually required for a regional network before a narrow-lane UPD estimate stabilizes to an accuracy of far better than 0.1 cycles. Finally, for hourly data, ambiguity resolution can significantly improve the accuracy of epoch-wise position estimates from 13.7, 7.1 and 11.4 cm to 0.8, 0.9 and 2.5 cm for the East, North and Up components, respectively, but a few tens of minutes is required to achieve the first ambiguity-fixed solution. Therefore, from the timeliness aspect, our PPP-RTK model currently cannot satisfy the critical requirement of instantaneous precise positioning where ambiguity-fixed solutions have to be achieved within at most a few seconds. However, this model can still be potentially applied to some near-real-time remote sensing applications, such as the GPS meteorology.     

基于分段连续推力的晕轨道控制方法

基于小偏差理论,推导了三体动力学模型的误差线性模型,并将此假设下的控制归纳为终端固定的有限时间调节器问题。在此基础上,进一步利用该最优控制方法推导了晕轨道周期内的连续小推力控制方案,验证了控制加速度及状态量的收敛。同时针对整周期控制方式在超调后状态量收敛速度慢的问题,通过分段连续推力控制模式(Sectional Continuous Thrust Control,SCTC)来近似瞬时脉冲推力控制模式,给出了最短分段控制时间的计算方法。实验表明,SCTC模式加快了轨道状态的收敛速度。对于km级入轨偏差,通过1次控制即可使实际轨道收敛至标称轨道。     

改进的ICCP地磁匹配算法

本文在分析等值线约束迭代最近点(ICCP)匹配算法基本原理的基础上,针对ICCP算法在非刚性变换情形下,就难以取得理想匹配效果的情况,提出了改进的ICCP算法。该算法首先用ICCP匹配算法修正初始位置误差和航向误差,然后采用逐点匹配,再迭代该过程的方法,逐点修正位置误差。仿真实验表明,改进的ICCP算法能够实现准确定位,定位精度在一个网格精度以内。     

The GLONASS-M satellite yaw-attitude model

The proper modeling of the satellites’ yaw-attitude is a prerequisite for high-precision Global Navigation Satellite System (GNSS) positioning and poses a particular challenge during periods when the satellite orbital planes are partially eclipsed. Whereas a lot of effort has been put in to examine the yaw-attitude control of GPS satellites that are in eclipsing orbits, hardly anything is known about the yaw-attitude behavior of eclipsing GLONASS-M satellites. However, systematic variations of the carrier phase observation residuals in the vicinity of the orbit’s noon and midnight points of up to ±27 cm indicate significant attitude-related modeling issues. In order to explore the GLONASS-M attitude laws during eclipse seasons, we have studied the evolution of the horizontal satellite antenna offset estimates during orbit noon and orbit midnight using a technique that we refer to as “reverse kinematic precise point positioning”. In this approach, we keep all relevant global geodetic parameters fixed and estimate the satellite clock and antenna phase center positions epoch-by-epoch using 30-second observation and clock data from a global multi-GNSS ground station network. The estimated horizontal antenna phase center offsets implicitly provide the spacecraft’s yaw-attitude. The insights gained from studying the yaw angle behavior have led to the development of the very first yaw-attitude model for eclipsing GLONASS-M satellites. The derived yaw-attitude model proves to be much better than the nominal yaw-attitude model commonly being used by today’s GLONASS-capable GNSS software packages as it reduces the observation residuals of eclipsing satellites down to the normal level of non-eclipsing satellites and thereby prevents a multitude of measurements from being incorrectly identified as outliers. It facilitates continuous satellite clock estimation during eclipse and improves in particular the results of kinematic precise point positioning of ground-based receivers.     

牵连运动为转动时动点加速度求解的研究

本文通过分析和实例，论证了动参考系为转动时的大部分加速度问题可以不计算科氏加速度而采用较简单的基点法求解。