卫星双向定时精度分析

介绍了Compass卫星双向定时的基本原理,在讨论双向定时计算模型的基础上,给出了考虑Sagnac效应项的详细计算模型,并计算了Sagnac效应项在几个代表性地区的影响,结果表明:对于距离中心站较远地区的用户,这种影响可达几十个纳秒。最后,对Compass卫星双向定时理论精度进行了分析,并采用几个地区的试验结果进行了验证。     

长波定时接收机系统时延的测量改进

为进一步提高用户定时校频精度,我们通过使用移相技术并利用高精度数字存储示波器和高精度计数器,对长波接收系统时间延迟的测试方法进行了改进,获得了比原先更精确的测量结果。     

导航型GPS接收机定时精度的分析

“能提供廉价并满足准确度达到或优于１μｓ的产业需求的世界范围内的时号传播”仍然是ＣＣＩＲ第７研究组的研究课题．为此，我们测量了４种型号的１３台导航星ＧＰＳ接收机输出的１ｐｐｓ与陕西天文台主钟ＵＴＣ（ＣＳＡＯＭＣ）的时间偏差．经电缆延迟和主钟相对子ＵＴＣ（ＣＳＡＯ）的钟差改正，测量结果表明，单次（采样时间约３０ｓ）定时测量的精度约为０．２μｓ，准确度为１～３μｓ；不同型号的ＧＰＳ接收机的硬件延时、测量偏倚误差和接收机噪声特性各不相同．只有测定这些接收机的时钟偏差并加以改正，才能达到准确度优于１μｓ的定时精度或时间同步精度。     

基于GPS射频信号模拟的定时方法

针对采用GPS定时的设备提出了基于GPS射频信号模拟的定时方法,在不更改原有设备的情况下,可利用外部时间基准进行同步。首先建立卫星的轨道模型,模拟出卫星的导航电文;然后根据设备的位置模拟产生观测数据;再将导航电文调制到卫星的伪随机码上,根据观测数据计算出伪距,对调制的信号进行延迟;最后,通过正交射频调制得到GPS模拟信号。已建立的实验模型能实现以上模拟过程。测试表明,该方法可达到10ns级的定时精度。     

基于UTC（NTSC）的GPS定时接收机时延测量

GPS提供了一种精密定时的方法,但接收机绝对时延的测量是一个难题,现有的测量方法成本高且难以操作,因而提出了一种GPS定时接收机时延测量的简单方法。这种方法根据国家授时中心保持的时间尺度UTC（NTSC）与GPS定时接收机输出的秒脉冲信号（1PPS）的比对结果,利用UTC（NTSC）的国际比对数据以及UTC与GPST的时差数据,可以测量GPS定时接收机时延,该方法简单易行。最后,对这种方法的测量误差进行了分析。     

BPL长波授时系统定时控制方法

中国科学院国家授时中心BPL长波授时系统现代化技术改造的核心是用一部固态发射机替代两部电子管发射机.结合固态发射机工作原理和罗兰-C脉冲信号定时控制特点,提出了BPL长波授时发播信号时刻控制方法,并对发射机输出信号控制精度进行了初步估算.结果表明,该控制精度符合项目任务书中对定时控制精度在±100ns之内的要求.     

定时系统及时间统一和同步(摘要)

定时系统是航天通信测控网、靶场测控系统的一个重要组成部分,向各测控设备提供标准频率信号和标准时问信息,使靶场内的各测控设备在统一的时间尺度上工作。定时系统国内通常称为时间统一勤务系统(简称时统),主要由标准频率源(如原子频标、石英频标)、时间编码产生器、信号分配设备、定时校频、监测控制设备组成。定     

BPL时号定时校准原理和方法

为保证BPL长波授时时号(以国家授时中心(NTSC)保持的UTC(NTSC)为基准)的准确度,必须对该时号进行定时校准(确定发射时号与发播工作钟同步时定时校准信号的相位)。阐述了定时校准的原理和方法。与传统罗兰-C系统校准方法不同,该方法选择发射天线电流取样信号基准过零点而非定时控制单元基本定时信号为定时校准点,消除了因锁相控制精度不足引起的误差,提高了时号精度。该方法可以作为罗兰-C授时系统的通用校准方法。     

双向定时近周日误差成分初步分析

该文介绍了利用地球同步卫星定位系统进行双向定时的基本原理,采用北京和乌鲁木齐两站的试验数据进行了计算分析,发现双向定时结果明显存在周期性变化,并且两地结果随地方时的变化趋势基本一致。谱分析表明:两站的主要周期谱基本一致,都比较明显地存在近周日和近半日变化。这种周期性变化可能主要由电离层延迟随地方时的变化引起。     

Two decades of pulsar timing of Vela

Pulsar timing at the Mt Pleasant observatory has focused on Vela, which can be tracked for 18 hours of the day. These nearly continuous timing records extend over 24 years allowing a greater insight into details of timing noise, micro glitches and other more exotic effects. In particular we report the glitch parameters of the 2004 event, along with the reconfirmation that the spin up for the Vela pulsar occurs instantaneously to the accuracy of the data. This places a lower limit of about 30 seconds for the acceleration of the pulsar to the new rotational frequency. We also confirm of the low braking index for Vela, and the continued fall in the DM for this pulsar.     

Software timing calibration of the ARGO-YBJ detector

The ARGO-YBJ experiment is mainly devoted to search for astronomical gamma sources. The arrival direction of air showers is reconstructed thanks to the times measured by the pixels of the detector. Therefore, the timing calibration of the detector pixels is crucial in order to get the best angular resolution and pointing accuracy. Because of the large number of pixels a hardware timing calibration is practically impossible. Therefore an off-line software calibration has been adopted. Here, the details of the procedure and the results are presented.     

The visible and near infrared module of EChO

The Visible and Near Infrared (VNIR) is one of the modules of EChO, the Exoplanets Characterization Observatory proposed to ESA for an M-class mission. EChO is aimed to observe planets while transiting by their suns. Then the instrument had to be designed to assure a high efficiency over the whole spectral range. In fact, it has to be able to observe stars with an apparent magnitude M_v?=?9–12 and to see contrasts of the order of 10^?4–10^?5 necessary to reveal the characteristics of the atmospheres of the exoplanets under investigation. VNIR is a spectrometer in a cross-dispersed configuration, covering the 0.4–2.5 μm spectral range with a resolving power of about 330 and a field of view of 2 arcsec. It is functionally split into two channels respectively working in the 0.4–1.0 μm and 1.0–2.5 μm spectral ranges. Such a solution is imposed by the fact the light at short wavelengths has to be shared with the EChO Fine Guiding System (FGS) devoted to the pointing of the stars under observation. The spectrometer makes use of a HgCdTe detector of 512 by 512 pixels, 18 μm pitch and working at a temperature of 45 K as the entire VNIR optical bench. The instrument has been interfaced to the telescope optics by two optical fibers, one per channel, to assure an easier coupling and an easier colocation of the instrument inside the EChO optical bench.     

On the timing of an interstellar communication

By considering prominent events that are observable from both Earth and nearby stellar systems it is possible to establish common clocks that may be useful in estimating arrival times for signals of intelligent extraterrestrial origin. The geometry and statistics of a timing strategy are developed together with quantitative estimates of its effectiveness and limits on its application. Effectiveness is measured by comparing the timing strategy with one randomized in time. Limitations arise from inaccuracies inherent in the determination of stellar parallaxes and result in standard deviations of the order of weeks to months for time estimates. The problem can be alleviated by choosing clocks close to Sender in angular distance. Signal opportunities for several nearby Sun-like stars are calculated using the bright Nova Cygni 1975 as a clock.     

Effect of weak gravitational microlensing on pulsar timing

The effect of weak gravitational microlensing on the pulsar timing is considered. The residuals of the times of arrival due to this effect for the pulsar B1937+21 will be ～10 ns if the time span of observations is about 20 years. It has also been shown that the residuals due to this effect will be several microseconds in the same 20 years in future SKA (Square Kilometer Array) observations of pulsars located behind the bulge.     

Arecibo timing and single-pulse observations of 17 pulsars

We have analysed the soft X-ray emission in a wide area of the Sculptor supercluster by using overlapping ROSAT Position Sensitive Proportional Counter pointings. After subtraction of the point sources, we have found evidence for extended, diffuse soft X-ray emission. We have investigated the nature of such extended emission through the cross-correlation with the density of galaxies as inferred from the Münster Redshift Survey. In particular, we have analysed the correlation as a function of the temperature of the X-ray emitting gas. We have found a significant correlation of the galaxy distribution only with the softest X-ray emission (0.1 – 0.3 keV) and only for gas temperatures   kT < 0.5 keV  . We have excluded the fact that this soft X-ray diffuse emission, and its correlation with the galaxy distribution, is significantly contributed by unresolved active galactic nuclei, groups of galaxies or individual galaxies. The most likely explanation is that the soft, diffuse X-ray emission is tracing warm–hot intergalactic medium, with temperatures below 0.5 keV, associated with the large-scale structures in the Sculptor supercluster.     

Sequence and timing of conditions on early Mars

The geological record of early Mars displays a variety of features that indicate fundamental differences from more recent conditions. These include evidence for: (1) widespread aqueous alteration and phyllosilicate formation, (2) the existence of an active magnetic dynamo, (3) the erosion of extensive valley networks, some thousands of kilometers long, (4) a much more significant role of impact cratering, forming structures up to the scale of large basins, and (5) the construction of much of the Tharsis volcanic province. Mars also is likely to have had a much thicker atmosphere during this early period. We discuss and review the temporal relationships among these processes and conditions. Key observations from this analysis suggest the following: (1) the last large impact basins, Argyre, Isidis, and Hellas, all pre-date the end of valley network formation, potentially by several hundred million years, (2) the magnetic dynamo is likely to be ancient (pre-Hellas), since the center of Hellas and other young basins lack magnetic remanence, and (3) the period of phyllosilicate formation is not readily connected to the period of valley network formation. Concepts for the possible formation and evolution of life on Mars should address this time sequence of conditions.