文章来源:未知 文章作者:enread 发布时间:2016-12-14 08:37 字体: [ ]  进入论坛
Precision time signals sent through the Global Positioning System (GPS) synchronize1 cellphone calls, time-stamp financial transactions, and support safe travel by aircraft, ship, train and car. What if GPS goes down? The National Institute of Standards and Technology (NIST) and the U.S. Naval2 Observatory3 (USNO), which operate U.S. civilian4 and military time standards, respectively, have worked with two companies--Monroe, Louisiana-based CenturyLink, and Aliso Viejo, California-based Microsemi--to identify a practical backup possibility: Commercial fiber-optic telecommunications networks.
In GPS systems, transmissions can be disrupted unintentionally by radio interference or the weather in space, for instance. Various types of intentional5 interference are possible also. Federal agencies have long recognized the need to back up GPS, a collection of several dozen satellites that has provided users with time and position information since the 1970s.
To explore the possibility of using commercial telecom networks as a backup for time services, an ongoing6 experiment connects the NIST time scales in Boulder7, Colorado, with the USNO alternate time scale at Schriever Air Force Base in Colorado Springs by means of CenturyLink's fiber-optic cables. The two federal time scales, 150 kilometers apart, are ensembles8 of clocks that generate versions of the international standard for time, Coordinated9 Universal Time (known as UTC), in real time.
In this experiment, time signals were sent at regular intervals10 in both directions between the two locations. Researchers measured the differences between the remote (transmitted) and local time. 
The results, just presented at a conference, showed UTC could be transferred with a stability of under 100 nanoseconds (ns, or billionths of a second)--thus meeting the project's original goal for this metric--as long as the connection remained unbroken. Stability refers to how well the remote and local clocks remain synchronized11. Because the signals were forwarded by various pieces of equipment along each path, they experienced significant unequal delays in the two different directions. This reduced overall performance, resulting in an accuracy that did not meet the stated goal of 1 microsecond (millionths of a second).* With the GPS available to calibrate12 (and thus correct for) the unequal delays, time transfer could be accomplished13 maintaining that calibration within 100 ns if GPS were to "disappear," the study suggests.


1 synchronize Lqlwy     
v.使同步 [=synchronise]
  • The sound on a film must synchronize with the action. 影片中的声音必须与动作配合一致。
  • You must synchronize your Inbox before selecting additional folders. 在选择其他文件夹前,您必须同步您的收件箱。
2 naval h1lyU     
  • He took part in a great naval battle.他参加了一次大海战。
  • The harbour is an important naval base.该港是一个重要的海军基地。
3 observatory hRgzP     
  • Guy's house was close to the observatory.盖伊的房子离天文台很近。
  • Officials from Greenwich Observatory have the clock checked twice a day.格林威治天文台的职员们每天对大钟检查两次。
4 civilian uqbzl     
  • There is no reliable information about civilian casualties.关于平民的伤亡还没有确凿的信息。
  • He resigned his commission to take up a civilian job.他辞去军职而从事平民工作。
5 intentional 65Axb     
  • Let me assure you that it was not intentional.我向你保证那不是故意的。
  • His insult was intentional.他的侮辱是有意的。
6 ongoing 6RvzT     
  • The problem is ongoing.这个问题尚未解决。
  • The issues raised in the report relate directly to Age Concern's ongoing work in this area.报告中提出的问题与“关心老人”组织在这方面正在做的工作有直接的关系。
7 boulder BNbzS     
  • We all heaved together and removed the boulder.大家一齐用劲,把大石头搬开了。
  • He stepped clear of the boulder.他从大石头后面走了出来。
8 ensembles 0e9198bc7343b463793ceb2e25beb9dd     
整体( ensemble的名词复数 ); 合奏; 乐团; 全套服装(尤指女装)
  • I love to play in all types of ensembles. 我喜欢参与吹奏各种各样的合奏曲。
  • The 5th Brigade is now taking 895 Land Warrior ensembles to Afghanistan. 第五旅现在携带895套陆地勇士装备去阿富汗。
9 coordinated 72452d15f78aec5878c1559a1fbb5383     
  • The sound has to be coordinated with the picture. 声音必须和画面协调一致。
  • The numerous existing statutes are complicated and poorly coordinated. 目前繁多的法令既十分复杂又缺乏快调。 来自英汉非文学 - 环境法 - 环境法
10 intervals f46c9d8b430e8c86dea610ec56b7cbef     
n.[军事]间隔( interval的名词复数 );间隔时间;[数学]区间;(戏剧、电影或音乐会的)幕间休息
  • The forecast said there would be sunny intervals and showers. 预报间晴,有阵雨。
  • Meetings take place at fortnightly intervals. 每两周开一次会。
11 synchronized f6dbc93312ac2dd66d3989fc9050167f     
  • Do not use the synchronized keyword in Managed Objects. 不要在管理对象上使用synchronized关键字。 来自互联网
  • The timing of the gun was precisely synchronized with the turning of the plane's propeller. 风门的调速与飞机螺旋桨的转动精确同步。 来自辞典例句
12 calibrate vTvyu     
  • Pesticide levels in food are simply too difficult to calibrate.食品中杀虫剂的含量很难精确测定。
13 accomplished UzwztZ     
  • Thanks to your help,we accomplished the task ahead of schedule.亏得你们帮忙,我们才提前完成了任务。
  • Removal of excess heat is accomplished by means of a radiator.通过散热器完成多余热量的排出。
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TAG标签: time GPS networks