Satellite VHF transponder time synchronization

This paper describes an experiment designed to transfer accurate time between two widely separated clocks using a VHF satellite transponder. The satellite used was the NASA Applications Technology Satellite, ATS-1. The experiment used atomic oscillators to maintain accurate time at each station, and the synchronization was accomplished by measuring the round-trip delay times between the stations. The goal of the experiment was to evaluate a VHF system, because of the low-cost ground equipment involved, in contrast to microwave systems. The paper discusses the results and the various factors that contributed to the timing errors.     

Time-transfer performance in burst-mode communication systems

This paper studies the performance of the clock transfer scheme for burst-mode communication systems for which data are received during short, equally spaced intervals. Its main focus is on satellite-based time-division multiple-access (TDMA) communication systems with data regeneration and switching onboard the satellite, although the results apply to other TDMA systems as well. The system reference clock is generated onboard from an incoming, very stable ground source, based on a burst-mode demodulator that extracts the clock from a discontinuous modulated carrier due to the bursty nature of TDMA signals. If good enough, this onboard regenerated clock avoids the use of bulky and expensive clocks in the satellite payload and can act as the master clock of the TDMA system     

GPS-Based Satellite Tracking System for Precise Positioning

NASA is developing a Global Positioning System (GPS) based measurement system to provide precise determination of Earth satellite orbits, geodetic baselines, ionospheric electron content, and clock offsets between worldwide tracking sites. The system will employ variations on the differential GPS observing technique and will use a network of nine fixed ground terminals. Satellite applications will require either a GPS flight receiver or an on-board GPS beacon. Operation of the system for all but satellite tracking will begin by 1988. The first major satellite application will be a demonstration of decimeter accuracy in determining the altitude of TOPEX in the early 1990's. By then the system is expected to yield long-baseline accuracies of a few centimeters and instantaneous time synchronization to 1 ns.     

高重复率激光时间比对激光发射时序精确控制

我国空间站即将首次开展高重复率（~kHz）星地激光时间比对,搭载的星载探测器拟采用固定门控开启模式,对地面激光发射时序的控制提出了高实时、高重复率和高精度等要求。基于卫星激光测距（SLR）的距离门控原理,提出高重复率激光时间比对地面激光点火信号精确产生方法,以使上行激光脉冲能在门控信号之后短时间内到达星载探测器,极大减少噪声干扰。该方法可在单片可编程门阵列FPGA中实现,具有重复率大于10 kHz、控制精度5 ns以及软件交互简单等优势,结合方法计算精度和半导体泵浦激光器的纳秒级触发抖动,预计地面激光发射时刻精度最终控制在10 ns以内,满足空间站激光时间比对激光发射时序的控制需求,并可为其他激光时间比对工程的实施提供技术支持。     

5G网络时钟同步研究

本文介绍了网络时钟同步方案,针对5G网络中,4G与5G基站时钟同步不对齐,会导致5G用户下载速率下降、4/5G无法正常互操作等问题,分场景深入分析问题现象和原因,提出通过信号分路方式为5G提供基站同步信号、新建卫星接收天线引入、选用1588v2时钟方案等解决方案,并展望后续推进移动基站北斗/GPS 授时系统双模改造。为5G网络时钟同步建设提供了建议。     

星地时钟同步方案设计及性能分析

在宽带卫星通信系统中,星载通信设备与地面通信设备间的时钟同步性能将会直接影响到整个卫星通信网络的性能。针对基于星上处理和星上交换的宽带卫星通信系统的技术特点,设计了3种星地时钟同步方案,并从链路传输滑动损伤、接收缓存容量、时钟信号的抖动和漂移等几个方面对星地时钟同步方案的性能进行了比较和分析,总结了上述几种同步方式的优缺点,最后给出了建议的星地时钟同步方式。     

Satellite time synchronization of a NASA network

A satellite time synchronization technique has been used for synchronization-of remotely separated clocks during the past several years. The NASA Network has been successfully synchronized to an accuracy of tens of microseconds via the NASA Geodetic Earth Orbiting Satellite GEOS-11. The results indicate that a polor orbit satellite having an on-board clock can effectively be used to synchronize clocks on a global basis.     

遮挡环境下原子钟和气压测高仪辅助北斗定位方法研究

在城市峡谷等遮挡环境下,接收机无法连续进行定位解算;并且高程定位精度不能满足用户在立交桥或者盘山公路等环境下的定位需求。在接收机内使用原子钟,可以利用原子钟的高稳定性,对钟差进行高精度的预测。并通过与气压测高仪共同辅助北斗系统定位,可以有效提高接收机的定位精度和连续性;该文首先理论分析了原子钟和气压测高仪辅助定位算法;然后,提出一种气压测高仪初始化校正方法,并通过对钟差噪声类型的分析确定了钟差预测方法;最后,模拟遮挡环境,进行原子钟和气压测高仪辅助北斗卫星导航系统定位试验,并分析了定位结果。结果表明:仅跟踪两颗可见卫星,便可以进行定位解算,并且垂直方向上的定位误差从8.2 m (RMSE)下降到了5.2 m,定位结果的波动从4.6 m下降到了0.8 m。     

Internet time synchronization: the network time protocol

The network time protocol (NTP), which is designed to distribute time information in a large, diverse system, is described. It uses a symmetric architecture in which a distributed subnet of time servers operating in a self-organizing, hierarchical configuration synchronizes local clocks within the subnet and to national time standards via wire, radio, or calibrated atomic clock. The servers can also redistribute time information within a network via local routing algorithms and time daemons. The NTP synchronization system, which has been in regular operation in the Internet for the last several years, is described, along with performance data which show that timekeeping accuracy throughout most portions of the Internet can be ordinarily maintained to within a few milliseconds, even in cases of failure or disruption of clocks, time servers, or networks     

摄动对北斗授时机的影响及修正方法探讨

北斗卫星导航试验系统是我国第一代自行研制建立的卫星导航定位系统。利用该系统实现单向授时的原理是计算地面中心站与用户机间卫星信号传输延迟,经修正实现本地时钟和系统时钟同步,得到本地的精确时间。为了实现同步,消除＂摄动＂的影响是至关重要的。这里提出利用＂最小二乘＂对卫星速度进行多项式拟合,消除卫星运动对授时精度影响的方法。通过与直接法（没有进行最小二乘拟合）比较,发现前者能将＂摄动＂对授时精度的影响降低在10ns以内。介绍并分析了这种方法,并得出了分析结论。     

BDS在轨卫星钟性能评估与对比

从卫星钟基本特性、频率稳定度、钟差预报精度等方面对 BDS 三种类型卫星钟 (RAFS-1、RAFS-2、PHM)进行综合对比分析，结果表明：PHM 卫星钟性能指标显著优于 RAFS； RAFS-2 卫星钟性能指标显著优于 RAFS-1，频率稳定度指标和钟差预报精度提升了 1 倍左右，且 RAFS-2 卫星钟增加了相位连接功能，保证了在轨卫星钟在调整过程中不会影响时间频率服务。     

一种微型化制造的双腔结构芯片原子钟87Rb蒸汽腔

碱金属蒸汽腔是芯片原子钟（CSACs）中重要的核心部件之一,其微型化制造具有重要的实用价值,同时也非常具有挑战性。采用MEMS 技术批量化制作了具有双腔结构的芯片原子钟87Rb 蒸汽腔阵列。在阳极键合过程中,通过原位化学反应产生纯净的87Rb 元素蒸汽,缓冲气体（N2）采用反充的方法充入到87Rb 蒸汽腔内以保证缓冲气体的压强可以精确的控制。所设计的双腔结构可以防止原位化学反应中产生的杂质阻挡光路,从而能够提高探测到的光信号的强度。通过原子钟桌面系统测试,得到了87Rb 元素D1 线的光学吸收谱和用于芯片原子钟锁频的误差信号,在90℃时,87Rb 元素D1 线纠偏信号的线宽（波峰与波谷间距）可达到0.53 kHz。测试结果表明,双腔结构的87Rb 蒸汽腔满足芯片原子钟或其他芯片级原子器件的设计要求。     

芯片级原子钟专用射频模块设计

为进一步实现原子钟的低功耗、微型化,设计了一种用于85Rb原子钟的专用射频模块芯片。该芯片采用了交叉耦合差分结构,利用串联的平面集成螺旋电感达到3 GHz的输出频率,同时采用了累积型MOS变容管,实现控制电压对于输出频率的单调调节。最终对设计芯片进行了仿真测试,并完成了流片与封装,基本达到了设计指标。     

伪卫星时钟同步方法的研究

提出了一种全新的利用光网络实现伪卫星时钟同步的方法,并设计出了一套基于光网络的高可靠性、高精度的时钟同步系统,实现了不同时钟之间的精密时间同步,同步精度优于1 ns.     

基于MATLAB的飞船测控系统仿真

为了更好地描述地面监测站对天空中卫星的监测情况,首先建立了椭圆形模型来描述地面监测站的覆盖区域,后来根据飞船轨道的调整逐渐近似为圆形轨道模型.通过对模型的求解,计算出了实现全程监控所需要的测站数量.通过编写MATLAB程序,实现了测控站点在飞船运动时的覆盖区域的动态仿真,验证了测控站点与飞船轨道在同一平面内的情况下的分布是合理的.     

Standard time and frequency generation

The basic properties of atomic primary frequency standards are reviewed. A continuously running frequency source combined with counting, storage, and display devices results in a clock. Time scales are obtained by setting clocks with respect to a convenient origin. The accuracy of a primary frequency standard is a combined theoretical and experimental assessment of the uncertainties of all known possible sources of bias from the idealized definition. Recommended standard measures for the stability are the spectral density of fractional frequency fluctuations (frequency domain) and the two-sample no-dead time Allan variance (time domain). The operation of atomic frequency standards is based on various methods of particle interrogation to observe the transitions, particle confinement to obtain sufficient interaction time, and particle preparation to obtain the desired energy level populations. Passive resonator and active oscillator (maser) modes of operation are discussed; the former has some fundamental advantages. A review of the state of the art and current practice shows that cesium beam resonator standards have the best documented accuracy capability (5 × 10^-13) and, in their commercial versions, are presently the best available clocks. There are three concepts related to time: time interval, date, and synchronization. In order to assign dates to events, time scales have to be established. The steadily growing need for a very precise and uniform time scale has resulted in a new internationally coordinated time scale, IAT. Frequency metrology will have increasing impact on both length and voltage metrology in the near future.     

卫星导航系统星载钟时域频率稳定度探讨

介绍了四大卫星导航系统采用的时间系统,分析了频率标准的时域频率稳定度研究内容、研究方法,推导了其实用计算方法和公式,主要对近十年以来研究GNSS星载原子钟频率稳定度的测算数据进行了梳理和归纳总结,对比分析了各个导航系统星载原子钟的频率稳定度,综合数据表明大部分星载钟天稳均可达到10^-14量级,其中GPS星载铷钟和Galileo星载PHM钟稳定性最好。     

Effect of broadcast and precise ephemerides on estimates of the frequency stability of GPS Navstar clocks

Frequency stability analysis of on-orbit Navstar clocks is performed by the Naval Research Laboratory using both the broadcast and the precise post-processed ephemerides. The phase offset between the Navstar clock and the reference clock is computed from pseudorange measurements obtained by dual-frequency GPS receivers at the five GPS monitor sites and at the U.S. Naval Observatory precise-time site. The broadcast ephemerides are generated at the GPS master control station by a Kalman filter using data collected from the five GPS monitor stations. The precise post-processed ephemerides are generated by the Defense Mapping Agency (DMA) using data collected from the GPS monitor sites and from five additional DMA monitor sites. In this paper the frequency stability is estimated for two Navstar caesium clocks–a Block I caesium clock (Navstar 9) and a Block II caesium clock (Navstar 23)–using both the broadcast and the precise ephemerides. A significant improvement in the estimate of the frequency stability of the Block II clocks has been achieved using the precise ephemeris.     

Generating a fault-tolerant global clock using high-speed controlsignals for the MetaNet architecture

Describes a new technique, based on exchanging control signals between neighboring nodes, for constructing a stable and fault-tolerant global clock in a distributed system with an arbitrary topology. It is shown that it is possible to construct a global clock reference with a time step that is much smaller than the propagation delay over the network's links. The synchronization algorithm ensures that the global clock “tick” has a stable periodicity, and therefore, it is possible to tolerate failures of links and clocks that operate faster and/or slower than nominally specified, as well as hard failures. The approach taken is to generate a global clock from the ensemble of the local transmission clocks and not to directly synchronize these high-speed clocks. The steady-state algorithm, which generates the global clock, is executed in hardware by the network interface of each node. At the network interface, it is possible to measure accurately the propagation delay between neighboring nodes with a small error or uncertainty and thereby to achieve global synchronization that is proportional to these error measurements. It is shown that the local clock drift (or rate uncertainty) has only a secondary effect on the maximum global clock rate. The synchronization algorithm can tolerate any physical failure. It will continue to operate correctly on any connected segment of the network, i.e., it can tolerate any number of link and node failures, as long as the network remains connected     

Airborne collision avoidance and other applications of time/frequency

Time/frequency technology provides a reliable aircraft collision avoidance system (CAS) that can operate in either synchronous or asynchronous modes. Precision time-ordered techniques of CAS provide both range and range-rate measurements in a one-way sense to all aircraft as well as ground stations within range of transmitted microwave signals. The cooperative system utilizes exact frequency references coupled with precise synchronization: control of frequency to 1 part in 10⁸and time to less than 1 µs. In addition to performing specific functions of protecting aircraft, the time/frequency CAS provides a means for wide dissemination of submicrosecond timing. Flying clocks, which are an integral part of the airborne CAS, have been providing transcontinental and intercontinental transfer of time since 1964. CAS ground stations can serve as depositories of time and frequency derived from flying clocks, satellites, Loran-C and Omega navigation systems, or television transfer referenced to national and international time/frequency standards. Airborne relay of CAS time provides a ready means for local transfer of time to users that are not within line of sight to a ground station. Thus time and frequency can be disseminated for applications other than collision avoidance. In the process of developing the technology and originating CAS, new systems for communication, position determination, navigation, and vehicle surveillance evolved. Such systems can be structured to operate at high data rates, provide improved accuracy and use less RF spectrum than existing techniques.