【连线杂志20140214】GPS 25周年纪念–庆祝25年来没迷路的日子

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【中文标题】GPS 25周年纪念–庆祝25年来没迷路的日子

【原文标题】Celebrating 25 Years of NotGetting Lost Thanks to GPS

【登载媒体】《连线》杂志 Wired Magazine

【原文作者】Adam Mann

【原文链接】http://www.wired.com/wiredscience/2014/02/happy-anniversary-gps/

【译者】nanotube

【翻译方式】人工

【声明】本翻译供Anti-CNN使用，未经AC或译者许可，不得转载。

【译文】

    假如有人要问需要空间技术的理由是什么，答案就是它让像我这样的人不至于不停地迷路。如今，我的智能手机比我认路认得还好，这得归功于天上的一只卫星舰队，不停地告诉它现在在哪。

    虽然不算一个特别浪漫的纪念日，25年前的今天（2月14日）美国全球定位系统（GPS）的第一颗卫星从卡纳维拉尔角发射，从此开启了现代世界的一大奇迹。从那时起的25年间，GPS已经不可分割地嵌入到我们生活中的一切，从绘制地图，智能手机应用，地理标记和寻宝，救灾，到数以百计的其他用途，与此同时也提起了人们对隐私的争议。

    GPS依赖于至少24颗卫星，他们运行在2万公里高空中的六个不同的轨道上，画出看起来像一个原子模型的轨迹。 当他们的太阳能电池板张开时，这些1吨重的卫星大约跟一个长颈鹿一般大小。每时每刻，每颗卫星都在发送出一束信号，包括自己的代号和它的时间和所在位置。

模型显示在轨道上最原始的24颗GPS卫星和一个随地球旋转的地点在任何时刻看到的卫星数量。萨尔瓦多小唐 /维基百科

    你的带GPS功能的手机或车载导航收到这些信号，并拿收到信号的时间跟信号发出的时间作比较。时间差乘以光速可以很快的算出它到该卫星的距离。如果你掌握了到两个或三个卫星的距离，就可以用三角法画出在地球上的位置。当所有的GPS卫星在工作时，用户在天上总是能看到至少4颗，使他们能够推算出像高度、速度和方向等数据。

    为了保证三角法测量的准确性，GPS需要极其精确的计时，这就是为什么每颗卫星携带着原子钟。这些卫星也是直接应用爱因斯坦理论的重要科技之一。爱因斯坦教导我们，在（太空中）引力场外运行的钟表因为时空扭曲比在（地球上）引力场里面的钟表更快。此外还有一个相反的效应，考虑到GPS卫星以时速1万4千公里（0.001％的光速）快速运行，这意味着它们感觉到轻微的时间扩张使它们的时钟比一个静止在地面上的钟运行相对缓慢。这两种效应结合在一起，使得GPS卫星上面的时钟每一天比地球上的时间变快约38微秒。而GPS需要20至30纳秒（1微秒为1000纳秒）的精度，因此这两个效应在任意时刻计算离每个卫星有多远的时候都是计算过程的组成部分。

    GPS的想法最初来自于美苏太空竞赛的最早期。 1957年，前苏联刚刚发射了第一颗人造地球卫星斯普特尼克1号，当飞越头顶的时候它发出的一个特殊的无线电声音可以直接在收音机里听到。当大多数美国人被此吓坏了的时候，在应用物理实验室（APL，NASA在加州帕萨迪纳市的一个国有实验室）的两位科学家认识到他们可以利用这些信号来准确找到卫星的位置。当斯普特尼克卫星走近时，其无线电信号会得到压缩一点，缩短其波长，当它走远时，波长会延长。这被称为多普勒效应，就像救护车向你开来的时候，很容易听到它的警笛的音调越来越高一样。

    APL的科学家们使用UNIVAC，美国的第一台商用计算机，成功计算出了斯普特尼克卫星的轨道。 一年后，政府要求他们研究一个相反的问题：基于发射信号的卫星，定位在地球上收信的人的位置。这个问题很快就由国防部高级研究计划局部门（后来被命名为DARPA，曾经负责发明互联网的机构）接管。该机构最早从1964年开始为了启动TRANSIT（过境）计划，第一个卫星导航计划的一部分，发射过卫星。美国海军是TRANSIT计划的卫星的主要用户，用它们给导弹核潜艇提供位置信息。

    研究，发射，并维持一个完整的GPS系统的卫星耗价惊人（最终花费大约为80亿今天的美元）。如果它不是为了冷战需要，确保美国需要从世界任何地方和所有的地方发射核导弹，GPS可能永远不会实现。偏执的美军希望确保他们能够在它的核武库被前苏联的核攻击摧毁的情况下仍然能做出反应。靠战略轰炸机和陆基洲际弹道导弹是不够的。这就需要核潜艇的潜射弹道导弹来提供从海上的反击。（前苏联当然同样有类似的分散部署。）

    但潜艇需要在发射导弹之前准确地知道自己的位置。海军依赖了TRANSIT卫星计划。与此同时在整个20世纪60年代，美国空军平行地开发了一个叫做MOSAIC（马赛克）的类似方案，利用陆军发射的西科尔计划的卫星使他们的轰炸机可以判断某个部队在地球上的位置。

    到了1973年，美国军方的几大分支意识到他们可以综合各自的做法，于是提出了一个比这三个旧计划更强的想法。在这一年的九月，军方高层在五角大楼会面后决定了最终命名为“定时和测距导航系统”的计划，简称为导航星-GPS，后缩短到仅仅GPS。从1978年到1985年，军方发射了11颗卫星（其中1颗失败），用来为新的GPS系统做研究和测试。

    1983年，大韩航空007航班偏航到前苏联领空被击落。里根总统承诺，GPS系统建成以后将允许客机民用。当代第一颗GPS卫星在1989年2月14日发射升空。空军曾计划在1986年使用航天飞机来完成这次发射，但因为挑战者号的灾难被推迟了，并最终用德尔塔II火箭来完成。 整个完整的GPS舰队建成于1994年，目前至少有32颗卫星在轨运行，其中有一些用来提供冗余。与此同时，俄罗斯研制并发射的GLONASS系统，建于类似GPS的原理上，是目前唯一能替代GPS在全球工作的系统。【注：北斗系统的服务区不覆盖全球】

    在其开始，美国军方担心GPS技术将被敌人利用，于是故意降低民用信号的精度，让它只能精确到100米范围内。2000年，克林顿总统下令把这个限制关闭 ，如今民用设备通常能精确到5至10米。欧盟和中国正在建设自己的全球导航系统，被称为伽利略和北斗，这将有助于在未来十年进一步替代GPS。看起来人们在未来将永远不必担心迷路了。

作者亚当.曼

亚当是《连线》杂志科学撰稿人。他住在加州奥克兰附近的一个湖，享受空间，物理，和其他跟科学有关的东西。

【原文】

Celebrating 25 Years of Not Getting Lost Thanks toGPS

BY ADAM MANN02.14.1410:00 AM

Navstar-2F

Artist rendering of a GPS satellite above theEarth. Image: U.S. Air Force

If there was ever a justification needed for spacetechnology, it’s that it keeps people like me from constantly being lost. Thesedays, my smart phone is much better than me at getting around thanks to a fleetof satellites that tells it where it is at all times.

Though not a particularly romantic anniversary,today marks 25 years since the first satellite in the U.S. Global PositioningSystem launched from Cape Canaveral, beginning the set up for one of thewonders of the modern world. In the two and a half decades since then, GPS hasbecome inextricably embedded into just about everything we own, finding use incartography, smart phone apps, geotagging and geocaching, disaster relief, andhundreds of other applications, while simultaneously raising privacy concerns.

GPS relies on at least 24 satellites flying 20,000kilometers overhead in one of six different orbital paths, tracing out whatlooks like a toy model of an atom. With their solar panels extended, each ofthese 1-ton satellites is about the same size as a giraffe. At any givenmoment, each satellite beams out a signal identifying itself and giving itstime and location.

ConstellationGPS

A model showing the 24 original GPS satellites inorbit and a point on the Earth rotating. Animation: El pak/Wikimedia

Your GPS-enabled phone or car captures that signaland compares the time it was received to the time it was transmitted. A quickcalculation involving the speed of light allows the device to figure out thedistance to that satellite. If you have your distance to two or threesatellites, you can triangulate your position on the Earth. When all the GPSsatellites are working, a user always has at least four in view, allowing themto determine things like altitude, speed, and direction.

In order to properly triangulate, GPS requiresextremely accurate timekeeping, which is why each satellite carries an atomicclock. The satellites are also some of the most important technology usinglessons learned from Einstein, who taught us that clocks outside agravitational well will run faster than those inside of it because of thewarping of space-time. An opposite effect comes from the fact that GPSsatellites move at 14,000 kilometers per hour (0.001 percent the speed oflight), meaning that they experience a slight time dilation making their clocksrun slow relative to one at rest on the ground. The two effects taken togethermean that the clock on a GPS satellite runs about 38 microseconds faster eachday than ones here on Earth. GPS requires accuracy of 20 to 30 nanoseconds (onemicrosecond is 1,000 nanoseconds), so both effects are part of the calculationdetermining how far away each satellite is at any given time.

The idea behind GPS comes from the very beginningsof the Space Race. In 1957, the Soviet’s newly launched Sputnik satelliteemitted a characteristic radio beep that could be tuned in to as the objectpassed overhead. While the rest of the U.S. was freaking out, two scientists atthe Applied Physics Laboratory realized they could use those transmissions topinpoint where the satellite was. As Sputnik approached, its radio signalswould get compressed a little, shortening their wavelength, and as it receded,the wavelengths would lengthen. This is known as the Doppler effect and caneasily be heard as an ambulance speeds toward you, the pitch of its sirengetting higher.

The APL scientists used UNIVAC, one of the firstcommercial computers in the U.S., to figure out Sputnik’s orbit. A year later,they were asked to do the opposite problem: Find out where someone was on Earthbased on the location of an overhead satellite. This was soon taken up by theDepartment of Defense’s Advanced Research Projects Agency (later named DARPA,the agency responsible for developing the internet), which launched satellitesstarting in 1964 as part of the TRANSIT program, the first satellite navigationprogram. The U.S. Navy was the main user of the TRANSIT satellites, using themto provide location information for their missile submarines.

Developing, launching, and maintaining thesatellites necessary for a full GPS system was horrendously expensive(eventually costing roughly $8 billion in today’s dollars). If it hadn’t beenfor the Cold War and the fact that the U.S. needed to launch nuclear missilesfrom anywhere and everywhere, GPS might never have happened. The paranoid U.S.military wanted to make sure they would be able to respond to a Soviet nuclearattack even if some of its nuclear arsenal was destroyed. It wasn’t enough tohave aircraft bombers and land-based intercontinental ballistic missilelaunchers. Submarine-launched ballistic missiles were needed to provide acounterattack from the sea. (The Soviets, of course, had similarly spread-outcountermeasures.)

But submarines needed to accurately know theirposition before launching a missile in order to hit their target. The Navy hadTRANSIT for this. Working in parallel throughout the 1960s, the Air Forcedeveloped a similar concept called MOSAIC for their bombers and the Armylaunched satellites under the SECOR program that could determine the locationof a unit somewhere on the globe.

By 1973, the branches of the U.S. military realizedthey could combine their ideas and come up with something superior to allthree. In September of that year, the top brass met at the Pentagon and came upwith what would eventually become known as the Navigation System Using Timingand Ranging program, called Navstar-GPS, which was later shortened to just GPS.Between 1978 and 1985, the military launched 11 satellites (10 of which worked)to test the new GPS system.

An unlaunched GPS unit, which looks like probablythe most satellitey satellite ever. Image: Scott Ehardt

An unlaunched GPS unit, which looks like probablythe most satellitey satellite ever. Image: Scott Ehardt

After Korean Air Lines flight 007 was shot down in1983 for wandering into prohibited U.S.S.R. airspace, President Reagan promisedthat GPS would be opened up for civilian use on passenger aircraft once it wascompleted. The first GPS satellite in the modern fleet launched on Feb. 14,1989. The Air Force had planned to use the space shuttle for this launch in1986 but was delayed by the Challenger disaster and eventually used a Delta IIrocket. The full GPS fleet was completed in 1994 and now at least 32 satellitesare in orbit to provide redundancy. During the same time, the Russiansdeveloped and launched GLONASS, which works on principles similar to GPS, andis currently the only alternative location-finding system in the world.

At its beginning, the U.S. military feared that GPStechnology would be used by enemies, and purposely degraded civilianinformation so that it could only provide accurate location information towithin 100 meters. In 2000, President Clinton had this feature turned off andnow civilian devices are usually accurate to within 5 to 10 meters. TheEuropean Union and China are currently building their own global navigationsystems, known as Galileo and Beidou, respectively, that will serve as furtheralternatives to GPS in the coming decade. It seems likely that folks in thefuture will never have to worry about being lost again.

Adam Mann

Adam is a Wired Science staff writer. He lives inOakland, Ca near a lake and enjoys space, physics, and other sciency things.