附录A 译文

GPS及其在精密单点定位中的应用

1　概述

GPS技术已成为测绘中建立控制大地控制网的有效手段。要实现这一技术，至少确切知道一点在WGS84坐标系中的绝对坐标。若某点在WGS84坐标系中的坐标未知，且要用它做控制点，则可利用单点定位技术，为了建立基准站，可以在该点位上进行连续3天数个小时的观测。这种方法可获得精确的二维平面坐标，但用这种方法所获得的高程精度却很低。然而，随着GPS技术的发展，其单点定位精度不断提高，因此，此项研究具有重要的意义。

在测绘过程中，控制网的建立是一项重要的工作。测绘成果的精度直接取决于控制网的布设的好坏。控制网是由一组高精度且已知三维坐标（X，Y，Z）的点组成的，其质量由精度决定，而其它点位精度由控制点决定。所以，为了实现高精确度的定位，应采用可靠的而准确的测量方法。尽管现在的一些技术能够建立控制网，提供满足用户要求的三维坐标，但最好的定位技术是在满足上述条件外，还能使费用最少。GPS精密单点定位就是这样的一种技术。

2 GPS简介

自从五七年第一颗人造卫星上天，六十年代的人造卫星导航定位技术，七十年代美国军方开始发展GPS（Global Positioning System）卫星导航定位系统，直至1995年4月27日美国国防部宣部“GPS系统已具备全部运作能力”。GPS计划的实现历时23年，耗资200多亿美元，前后共发射35颗卫星，目前仍在轨道上正常工作的有二25颗卫星，其中1颗为实验卫星，24颗为工作卫星。它具有海、陆、空全方位实时三维导航与定位能力，是美国第二代卫星导航系统，其特点是全天候、高精度、应用广，是迄今最好的导航定位系统。它广泛的应用价值，引起了各国科学家的关注和研究，前苏联和西欧各国的科学家在积极开发利用GPS信号资源的同时，还致力于研究开发各自的卫星导航定位系统，如前苏联建成的GLONASS卫星导航系统，我国也在致力于发展自已的卫星导航定位系统。同时，它的出现也导致了测绘行业一场深刻的技术变革。

2.1 GPS的组成

由三大部分组成，即空间部分、地面监控部分、用户设备部分。

2.2 空间部分

GPS系统的空间部分是指GPS工作卫星星座，其由24颗卫星组成，其中21颗工作卫星，3颗备用卫星，均匀分布在6个轨道上。卫星轨道平面与地球赤道面倾角为55°，各个轨道平面的升交点赤经相差60°，轨道平均高度为20200km.卫星运行周期为11小时58分（恒星时），同一轨道上的各卫星的升交角距为90°，GPS卫星的上述时空配置，基本保证了地球上任何地点，在任何时刻均至少可以同时观测到4颗卫星，以满足地面用户实时全天候精密导航和定位。GPS卫星的主体呈圆柱形，直径约为1.5m，重约774kg，两侧各安装两块双叶太阳能电池板，能自动对日定向，以保证卫星正常工作用电。每颗卫星带有四台高精度原子钟，其中2台为铷钟，2台为铯钟。GPS卫星上设有微处理机，可以进行必要的数据处理工作，它主要的3个基本功能：根据地面监控指令接收和储存由地面监控站发来的导航信息，调整卫星姿态、启动备用卫星；向GPS用户播送导航电文，提供导航和定位信息；通过高精度卫星钟向用户提供精密的时间标准。

2.3 地面监控部分

由5个地面站组成。1个主控站，其位于美国本土科罗拉多斯平土（Colorado Spings）的联合空间执行中心CSOC，3个注入站，其分别设在印度洋的迭哥加西、南大西洋的阿松森岛和南太平洋的卡瓦加兰。5个监控站，其中4个与主控站、注入站重叠，另外一个设在夏威夷。主控站的主要任务为：根据各监控站提供的观测资料推算编制各颗卫星的星历、卫星钟差、和大气层修正参数并把这些数据传送到注入站；提供GPS系统的时间标准；调整偏离轨道的卫星，使之沿预定的轨道运行；启用备用卫星以取代失效的工作卫星。注入站的主要任务为：在主控站的控制下，把主控站传来的各种数据和指令等正确并适时地注入到相应卫星的存储系统。监测站的主要任务为：给主控站编算导航电文提供观测数据，每个监控站均用GPS信号接收机，对每颗可见卫星每6秒钟进行一次伪距测量和积分多普勒观测，并采集气象要素等数据。

2.4 用户接收设备部分

由GPS接收机硬件和相应的数据处理软件以及微处理机及其终端设备组成。其主要功能是接收GPS卫星发射的信号，获得必要的导航和定位信息及观测量，并经简单数据处理实现实时导航和定位，用后处理软件包对观测数据进行精加工，以获取精密定位结果。

3 GPS用于导航定位的特点

GPS导航定位系统之所以在许多领域得到广泛应用，出现了与GPS系统相关的产业，这都得益于其本身所具有的诸多优点，概括起来主要有以下几个方面。

·定位精度高

通过很多应用实践已经证明，GPS相对定位精度在50km以内可达10^-6，100km～500km可达10^-7，1000km以上可达10^-9，在300-1500m工程精密定位中，1小时以上观测的解算，其平面位置误差小于1mm。基线边长越长越能突显是定位精度高的优势。

·观测时间短

由于GPS系统的不断完善，软件不断更新，目前20km以内相对静态定位，仅需15~20分钟，快速静态相对定位测量时，当每个流动站与基准站相距在15km以内时，流动站只需观测1~2分钟，动态相对定位测量时，流动站出发时观测1~2分钟，然后可随时定位，每站观测仅需几秒钟。

·测站间无须通视

GPS测量不要求站点间相互通视，只需测站上空开阔即可。

·可提供三维坐标

经典大地测量将平面与高程采用不同方法施测，而GPS可同时精确测定测站点的三维坐标，目前GPS水准可达到四等水准测量的精度。

·操作简便

随着GPS机不断改进，自动化程度越来越高，体积也越来越小，重量越来越轻，有的已达“傻瓜化”的程度。

·全天候作业

使用GPS测量，不受时间限制，24小时都可以工作，也不受起雾、刮风、下雨下雪等气候的影响。

·功能多、应用广

GPS系统不仅可用于测量，还可用于测速、测时。测速精度可达0.1m/s，测时精度可达几十毫秒。随着人们对GPS系统的不断开发，其应用领域正在不断地扩大。

4 GPS定位的基本原理及方法

GPS的工作原理实际上就是利用测距后方交会原理确定点位与导航，将无线电信号发射台从地面点搬到卫星上，组成一个卫星导航定位系统，应用无线电测距交会的原理，由三个以上地面已知点（控制点）交会出卫星的位置，反之利用三个以上卫星的已知空间位置又可以交会出地面未知点（接收机）的位置。GPS卫星发射测距信号和导航电文，用户用GPS接收机在某一时刻i同时接收到三颗GPS卫星信号，测量出测站P到三颗GPS卫星的距离S1、S2、S3，并计算出该时刻GPS卫星的空间坐标为（Xj、Yj、Zj），j=1,2,3。用距离交会法解算出测站P的位置（Xp、Yp、Zp），其观测方程式为：

因此，利用GPS卫星导航时，必须同时跟踪三颗以上GPS卫星。

根据GPS导航定位的方式不同，可分为GPS绝对定位和相对定位，根据接收机天线是否处于运动状态，又可以分为静态定位和动态定位，GPS卫星导航实质上就是广义的GPS动态定位。GPS绝对定位也叫单点定位，即利用GPS卫星和用户接收机之间的距离观测值直接确定用户接收机天线在WGS－84坐标系中相对于坐标原点（地球质心）的绝对位置。GPS相对定位也叫差分GPS定位，即至少用两台GPS接收机，同步观测相同的GPS卫星，确定两台接收机天线之间的相对位置，它是目前GPS定位中精度最高的一种方法广泛用于大地测量、精密工程测量、地球动力学的研究和精密导航。现在又出现了一种叫载波相位动态实时差分RTK（Real-time kinematic）技术，实质也就是相对定位，只不过它能快速完成整周模糊度的搜索求解。其基本过程是基准站（已知点）通过数据链将其采集的观测数据和测站信息一起传送给流动站，流动站利用同步采集到的GPS观测数据，在系统内组成差分观测值进行实时处理，同时给出厘米级定位结果。

5 GPS的应用

美国政府最初开发GPS导航定位系统主要为用于军队导航、收集情报等军事目的，后来对民用领域开放，将GPS系统分为军用码和民用码，但两者的接收精度相差很大。由于GPS具有全天候、高精度、自动化、高效益的特点，特别是随着GPS系统的不断改进，软、硬件的不断完善以及美国终止降低民用GPS接收机精度的作法，应用领域得到不断地开拓，目前已遍及国民经济各个部门，并开始进入人们的日常生活。人们可以开着装有先进GPS导航系统的汽车，随时为你指示行进方向，你可以带着手表式的接收机为你在原始森林里旅游探险指明方向。

由于GPS系统具有精度高、速度快、费用省、操作简便，现今建立大地及工程控制网基本上是采取GPS定位技术，取代了常规手段。国家A级和B级GPS大地控制网分别于1996年和1997年建成并交付使用，A级网，30个点组成，其水平方向的重复精度达2×10^-8，，垂直方向不低于7×10^-8。B级网由800个点组成，其精度也分别好于4×10^-7和8×10^-7。国家A、B级网以其特有的高精度把我国传统大地网进行了全面改善和加强，从而克服了传统大地网的精度不均匀，系统误差较大等传统测量手段不可避免缺点，这一高精度三维空间大地坐标系的建成将为我国21世纪前10年的经济和社会持续发展提供基础测绘保障。据报道在三峡二期工程施工中采用GPS定位技术建立施工控制网，取得很好的效果，可以满足其相应的精度要求。

6 GPS单点定位

6.1 单点定位发展概述

GPS从投入使用以来，其相对定位的定位方式发展的很快，从最先的码相对定位到现在的RTK，使GPS的定位精度不断升高。而绝对定位即单点定位发展的相对缓慢，传统的GPS 单点定位是利用测码伪距观测值以及由广播星历所提供的卫星轨道参数和卫星钟改正数进行的。其优点是数据采集和数据处理较为方便、自由、简单，用户在任一时刻只需用一台GPS 接收机就能获得WGS284 坐标系中的三维坐标。但由于伪距观测值的精度一般为数分米至数米；用广播星历所求得的卫星位置的误差可达数米至数十米，卫星钟改正数的误差为±20 ns 左右，只能用于导航及资源调查、勘探等一些低精度的领域中。随着海洋战略的实施，海洋科研、海洋开发、海洋工程等海上活动日益增加，对定位精度的要求也呈现出多样化，如精密的海洋划界、精密海洋工程测量等，要求能够达到十几或几十厘米的定位精度，而采用伪距差分定位只能提供米级的定位精度，如果使用RTK功能，作用距离又不能达到；对于这部分定位需求，现有的定位手段无法满足要求，需要寻求新的定位方式或技术。 

 精密单点定位(Precise Point Positioning，PPP)技术由美国喷气推进实验室( JPL ) 的Zumberge 于1997年提出。20世纪90年代末，由于全球GPS跟踪站的数量急剧上升，全球GPS数据处理工作量不断增加，计算时间呈指数上升。为了解决这个问题，作为国际GPS服务组( IGS)的一个数据分析中心， JPL提出了这一方法，用于非核心GPS站的数据处理。该技术的思路非常简单，在GPS定位中，主要的误差来源于三类,即轨道误差、卫星钟差和电离层延时。如果采用双频接收机，可以利用LC相位组合，消除电离层延时的影响。如果选择地心地固系表示卫星轨道，计算的参考框架同为地心地固系，可以消去观测方程中的地球自转参数。于是，只要给定卫星的轨道和精密钟差，采用精密的观测模型，就能像伪距一样，单站计算出接收机的精确位置、钟差、模糊度以及对流层延时参数。据PPP技术的要求，定位中需要系统提供卫星的精密轨道和钟差。目前，国际GPS服务组织( IGS)的几个数据分析中心具备这个能力提供卫星的精密轨道和钟差，但是，这些都是后处理结果。根据IGS的产品报告，IGS提供的卫星轨道精度能够达到2～3cm，卫星钟差的精度优于0. 02ns，这种精度的卫星钟差和轨道，能够满足任何精度的定位要求。近10年，由于IGS的努力，GPS卫星预报轨道的精度已经达到十几厘米，预报轨道的时间也由24h预报缩短到3h预报，卫星轨道的精度已经能够满足一般定位的要求。由于IGS现在不能提供实时和外推的精密卫星钟差，制约了实时PPP技术的应用，精密的卫星钟差仍然是PPP技术实时应用瓶颈，目前IGS只有后处理卫星钟差,，JPL和GFZ已经有能力提供快速卫星钟差。

6.2 精密单点定位的概念

精密单点定位与差分GPS定位不同，精密单点定位是利用国际GPS服务机构IGS提供的或自己计算的GPS精密星历和精密钟差文件，以无电离层影响的载波相位和伪距组合观测值为观测资料，对测站的位置、接收机钟差、对流层天顶延迟以及组合后的相位模糊度等参数进行估计。用户通过一台含双频双码GPS接收机就可以实现在数千平方公里乃至全球范围内的高精度定位。它的特点在于各站的解算相互独立，计算量远远小于一般的相对定位。PPP与双差定位的主要区别在于，双差定位时部分参数和误差项通过站间和星间求差得以消除，而PPP必须采用精细的模型加以改正和用辅助参数进行估计，比如卫星天线相位中心偏差改正、固体潮改正、海洋负荷改正等。

6.3 精密单点定位在我国的研究现状即其应用

6.3.1 研究现状

目前，国内外都对精密单点定位作了大量研究，武汉大学经过数年对精密单点定位理论与方法的深入研究，在国内率先成功研制了高精度的PPP数据处理软件TriP。利用PPP进行GPS数据处理，需在数据采集两周后进行，即需要在IGS网站上下载精密星历数据后，才能进行数据处理。

6.3.2 PPP在地籍测量中应用

同RTK技术相比，精密单点定位技术的实时应用主要借助于移动通信技术和互联网技术，数据链不受空间的限制，也不必事先建立一个固定的基准站，而这恰恰是地籍测绘所遇到的困难，特别是在土地利用变更调查和监测工作中。所以，在任意地区，可以利用精密单点定位技术建立地籍控制网，进行高精度的界址点测量和地籍图测绘，方便地进行统一坐标系的土地利用变更调查和监测。同时，精密单点定位技术真正实现了测量个性化，在测区不需要高等级的测量控制点，不需要架设基准站，单人单机即可完成地籍测绘任务。由于精密单点定位技术提供的成果是ITRF坐标，它是国际通用的坐标系统，与我国的国家80坐标系和北京54坐标系之间的坐标转换极为方便。

6.3.3 PPP在航道测量中的应用

采用一定的数据处理方法，目前动态精密单点定位的精度可以达到分米级。在内河航道或近海航道测量中，测量的水平和高程的几何精度厘米级到分米级之间。精密单点定位作为一种新的定位技术，由于其自身具有比GPS其他测量方式的优越性，随着精密单点定位技术的推广，以及越来越多的商业机构和科研队伍的介入。在精化误差改正模型、减小定位的系统性误差影响以及应用其他数据处理方法后，将使精密单点定位的精度进一步的提高， 则足以完全满足现代航道测最的精度要求。由于精密单点定位测量方式简单易行，又不依赖于已知控制点，没有距离限制，能够实时动态的确定现在分米级和将来厘米级的定位精度等一系列的特点，完全可以满足航道洲量精度要求，符合航道测量的特点。因此，动态的精密单点定位技术在航道测量中是大有可为的。

6.3.4 PPP在像控测量中的应用

GPS非差相位精密单点定位在像控测量中较静态定位或RTK还存在以下突出的优势：

1) 外业作业更简单。现存在的几种作业方式都需要两台或两台以上的接收机，静态测量还需要时间上的同步，RTK需要在基准站准备好了才能作业，精密单点定位只要一台接收机就可以作业，并在时间上没有要求，工作人员到点即可以施测。

2) 点位精度均匀稳定，整体精度连续性强。静态测量或RTK测量在测区较大的情况下，通常要进行分区施测，这将导致成果转化的时候不一致，存在某些系统误差。GPS非差相位精密单点定位得到的是高精度的ITRF框架坐标，它与作业面积的大小没有关系。因此，成果只有一套转换参数，精度是均匀的。

3) 与其它差分定位模型相比，非差相位精密单点定位模型具有可用观测值多；能直接得到测站坐标；不同测站的观测值不相关；测站与测站之间无距离限制等优点。

6.3.5 PPP在航空摄影测量中的应用

精密单点定位可以实现亚分米级的飞机动态定位，能在不需要地面基准站的条件下达到短基线双差固定解相当的精度水平。因此，基于精密单点定位的技术可以实现无地面控制点的航空测量。当然，精密单点定位并不能取代传统的高精度相对定位。两者各有优势。如果所有模型处理正确，相对定位和精密单点定位的结果应该是一致的。精密单点定位中使用IGS数据处理中心发布的精密星历和卫星钟差产品，在充分考虑了所有的不能忽略的误差模型改正，且软件算法正确的前提下，精密单点定位的精度和可靠性在很大程度上取决于IGS产品的可靠性和精度。因此，IGS产品的质量分析是今后需要进一步研究的问题。由于精密单点定位中的非差组合模糊度不再具有整数特性．因此，精密动态单点定位的质量控制也需要进一步研究。此外，静态条件由于接收机静止不动，可以每数小时估计一个天顶对流层参数，并采用随机游走的方法来模拟对流层延迟的随机变化。但对于飞机等高动态载体，数小时已经飞行数百公里，大气的状态参数会发生很大的变化。因此，在高动态长距离的GPS精密单点定位中，对流层参数的估计方法还需要进一步研究。

附录B 外文文献

GPS and Its Application in Precise Point Positioning

1　Outline

GPS technique has been well established as a means for establishment of control points in surveying and mapping. In carrying out this, the absolute coordinates of at least one site have to be known accurately in WGS84 coordinate system. As WGS84 coordinates for a particular site was not known neither it could be derived, in order to establish a reference station, Single Point Positioning technique of GPS has been applied for several hours of observation spanning in three consecutive days and night. The observed value shows precise planimetric position but grossly imprecise in height. However, with the development of GPS technology, the single-point positioning accuracy continuously increased, therefore, this study still has important significance.

Control establishment is an important exercise in mapping process. The mapping accuracy is directly based on the accuracy of control network. The control network is formed by a group of points whose position (x,y,z) are known to a high degree of accuracy. Accuracy defines the quality of a control network. The positions of other features of interest are determined with respect to these control points for mapping. To achieve such a high degree of accuracy a reliable and accurate method of surveying is required. Though several techniques are used to properly establish and provide accurate horizontal and vertical control networks, meeting the requirements of the user community, the best technique will be one that provides the control networks with required mapping accuracy at the least cost in time and money. GPS point positioning is such a  technology.

2 Introduction of GPS

Since 57 year first orbiting vehicle heaven, the 60's orbiting vehicle guidance localization technologies, the 70's US military starts to develop GPS (Global Positioning System) the satellite navigation positioning system, until on April 27, 1995 American Department of Defense information department "the GPS system has had completely operates ability". The GPS plan implementation lasts for 23 years, spends more than 20 billion US dollars, around altogether emanates 35 satellites, at present still the normal work had 225 satellites on the track, 1 for tested the satellite, 24 for worked the satellite. It has the sea, the land, the empty omni-directional real-time three dimensional guidance and the localization ability, is the American. second-generation satellite navigational system, its characteristic is all-weather, the high accuracy, applies broadly, was up to now the best guidance positioning system. Its broad application value, has aroused the various countries' scientist's interest and the research, former Soviet Union and the Western Europe various countries scientist while positively is developing uses the GPS signal resource, but also devotes to the research develops respective satellite navigation positioning system, if former Soviet Union completes the GLONASS satellite navigational system, our country also is devoting to the development from already satellite navigation positioning system. Atthe same time, its appearance has also caused a mapping profession profound technology transformation.

2.1 Composition of GPS

By three major part is composed, namely the spatial part, the ground monitoring part, the subscriber's equipment are partial.

2.1.1 space part

The GPS system spatial part refers to the GPS work satellite constellation, it is composed by 24 satellites, 21 work satellite, 3 spare satellites, evenly distribute on 6 tracks. The satellite orbit plane and Earth equatorial plane inclination angle for 55°, each track plane the right ascension difference 60°, the track average height is 20200km. The satellite transit cycle is 11 hours 58 minute, on the identical track various satellites rise to hand over for 90°,the GPS satellite above space and time configuration, had basically guaranteed on the Earth any place, at least may simultaneously observe in any time to 4 satellites, satisfies the ground user real-time all-weather precise guidance and the localization. The GPS satellite main body assumes the cylinder, the diameter approximately is 1.5m, heavy approximately 774kg, two sides respectively install two double leaves solar cell board, automatedly to the date direction detection, guarantee the satellite normal work uses electricity. Each satellite has four high accuracy atomic clocks, are the rubidium clocks, for cesium clock. On the GPS satellite is equipped with the microprocessor, may carry on the essential data processing work, its main 3 basic functions: According to the ground monitoring instruction receive and the storage the guidance information which sends in by the ground monitoring station, the adjustment satellite posture, the start spare satellite; Broadcasts the guidance telegram text to the GPS user, provides the guidance and the localization information; Provides the precise time standard through the high accuracy satellite clock to the user.

2.1.2 Ground monitoring part

Is composed by 5 ground stations. 1 master station, it is located the US territory Colorado earth (Colorado Spings) the union space executes core CSOC west, 3 it separately is located in Indian Ocean the elder brother to add, the Southern University Western world's Arab League loose woods island and south the Pacific Ocean Garland. 5 monitoring stations, 4 with the master station, overlap, moreover is located in Hawaii. The master station primary mission is: The observed data provides which according to various monitoring stations calculated writes each satellite the star experience, satellite Zhong Chai, and the atmosphere revision parameter and these data transfers; Provides the GPS system the time standard; The adjustment deviation track satellite, causes it along the orbital motion which prearranges; Begins using work satellite which the spare satellite substitutes fails. The primary mission is: Under the master station check, each kind of data and the instruction which transmits the master station and so on is correct and timely pours into to the corresponding satellite memory system. The inspection station primary mission is: Provides the observation data to the master station guidance telegram text, each monitoring station uses the GPS signal receiver, carries on a time of pseudo distance survey and integral Doppler to each obviously satellite every 6 seconds observes, and data and so on gathering meteorological element.

2.1.3 User equipment part

Are composed by the GPS receiver hardware and corresponding data processing software as well as the microprocessor and its the terminal device. Its main function is receives the GPS satellite emanation the signal, obtains the essential guidance and the localization information and the view survey, and after the simple data processing implementation real-time guidance and the localization, carries on the precision work with the post-processing software package to the observation data, extracts the precise localization result.

3 characteristics of GPS in geomatic and navigation

The GPS guidance positioning system therefore obtains the broad application in many domains, appeared with the GPS system correlation industry, this all benefits many merits which has to its itself, summarizes mainly has following several aspects.

·High positioning accuracy

Through were very many using the practice already proved that, the GPS relative pointing accuracy might reach in 50km 10-6, 100km ~ 500km might reach 10-7, the 1000km above might reach 10-9, in the 300-1500m project precise localization, the solution which above 1 hour observed calculated, its planimetric position error was smaller than 1mm. The baseline length of side is longer more can suddenly reveal is the pointing accuracy high superiority.

·Observation hour short

As a result of the GPS system unceasing consummation, software unceasingly updates, at present in 20km relative static localization, only needs the 15~20 minute, when fast static relative localization survey, when each mobile stands is distanced with the datum station when 15km, mobile stands only must observe the 1~2 minute, when dynamic relative localization survey, the mobile station embarks when observes the 1~2 minute, then may locate as necessary, each station observation only needs several seconds.

    ·Observation stations need not intervisibility

The GPS survey does not require between the site the mutual intervisibility, only needs the survey station sky to be open then.

  　·Providing three dimensional coordinates

The classical geodetic survey uses the plane and the elevation the different method to execute measured, but GPS may simultaneously the accurate determination three dimensional coordinates, at present the GPS standard be possible to achieve four and so on the levelling precision.

    ·Operation easily

Unceasingly improves along with GPS computer, automaticity more and more high, volume also more and more small, the weight is more and more light, some has reached "fool" the degree.

    ·All-weather work

Uses the GPS survey, the time limit, 24 hours all may not work, also the haze, does not blow the wind, rains snows and so on the climate influence.

    ·Application broad

The GPS system not only may use in to survey, but also may use in to measure fast, measures when. Measured the fast precision may reach 0.1m/s, measured when the precision may amount to several dozens milliseconds. Along with the people to the GPS system unceasing development, its application domain unceasingly is expanding.

4 GPS basic principle and localization method

    The GPS principle of work is in fact uses behind the range finder the junction to meet the principle true fixed point position and the guidance, moves to the radio signal transmitting station from the place flour dim sum on the satellite, composes a satellite navigation positioning system, applies the radio range finding to hand over the eeting the principle, by three above grounds known points (control oint) the junction can leave the satellite the position, otherwise ses three above satellites the known space positions may hand over an leave the ground unknown spot (receiver) the position. The GPS atellite emanation range finder signal and the guidance telegram ext, the user with the GPS receiver in some time i simultaneous eception to three GPS satellite signal, surveys survey station P to hree GPS satellite distance S1, S2, S3, and calculates this time GPS atellite the space coordinates for (Xj, Yj, Zj), j=1,2,3. With is way from the solution to figure out survey station P the osition (Xp, Yp, Zp), its observation equation is:

Therefore, uses time the GPS satellite navigation, must simultaneously rack three above GPS satellite ifferent according to the GPS guidance localization mode, may ivide into the GPS absolute fix and the relative localization, ccording to receiver antenna whether is at the state of motion, also ay divide into the static localization and the dynamic localization, n the GPS satellite navigation essence is the generalized GPS dynamic ocalization. The GPS absolute fix also is called the simple point ocalization, namely uses between the GPS satellite and the user receiver range spotting value direct determination user receiver antenna is opposite in the WGS - 84 coordinate systems to the origin antenna is opposite in the WGS - 84 coordinate systems to the origin of coordinates (the Earth nature heart) the absolute location. The GPS relative localization also is called the difference GPS localization, namely at least uses two GPS receiver, the synchronized observation same GPS satellite, determined between two receiver antennas the relative position, it is in the present GPS localization the precision highest one method widely uses in the geodetic survey, the precise project survey, the Earth dynamics research and the precise guidance Now also appeared one kind to call carrier phase dynamic real-time Now also appeared one kind to call carrier phase dynamic real-time difference - RTK (Real-time kinematic) the technology, the essence also is the relative localization, only it could fast complete.

fuzziness search solution. Its unit process is the datum stands (known point) transmits together through the data chain its gathering observation data and the survey station information for the mobile station, mobile stands the GPS observation data which gathers using synchronization, composes the difference observation value in the system to carry on the real-time processing, simultaneously produces the centimeter level localization result.

5 GPS application

The American government develops the GPS guidance positioning system mainly for to use in at first military target and so on army guidance, collection information, afterwards to civil domain open, divides into t GPS system the military code and the civil code, but two receives teprecisions difference is very big. Because GPS has, the high accuracy, automated, the high benefit characteristic all-weather, along with the GPS system unceasing improvement, is specially soft, the hardware unceasing consummation as well as US expired reduces the civil GPS receiver precision the method, the application domain obtains unceasingly develops, at present has spread national economy each department, and starts to enter people's daily life. The people may open the clothing to have the advanced GPS guidance system the automobile, for you instructed as necessary marches forward the direction, you may bring to begin the manner receiver to travel the exploration for you in the virgin forest to indicate the direction.

Because the GPS system has the precision high, the speed quick, the expense province, the operation simple, nowadays establishes the earth and the project check network basically adopts the GPS localization technology, has substituted for the conventional method. The national A level and B the level GPS earth check network separately and in 1997 completed and the delivery use in 1996, the A level network, 30 spots compositions, its horizontal direction redundant precision reached 2×10^-7, the vertical direction was not lower than 7×10^-8. The B level network is composed by 800 spots, its precision also distinguishes good to 4×10 - 7 and 8×10 - 7. National A, the B level network carried on by its unique high accuracy the our country tradition big network comprehensively improved and strengthens, thus overcame the traditional big networ precision non-uniformity, the system error has been  bigger and so on the traditional survey method inevitable flaw, this high accuracy three-dimensional space earth coordinate system will complete continues for the our country 21st century previous 10 years economy and the society to develop provides the foundation mapping safeguard. It is reported uses the GPS localization technology establishment construction check network in the Three Gorges second phased projects construction, obtains the very good effect, may satisfy its corresponding precision requirement.

6 Precise point positioning using GPS

6.1 Outline of point positioning development

With the using of GPS, the methods of its relative positioning develop very quickly. From the first code relative positioning to current RTK technique, the positioning accuracy of GPS is rising fast. However, absolute positioning that point positioning development relatively slow, traditional GPS single-point measurement carry out by using of pseudorange observations code and broadcast ephemeris which provided satellites and satellite orbital parameters bell for correction. The advantage is that data acquisition and data processing is convenient, free, and simple, users can abtain WGS284 coordinates three-dimensional coordinates at any time only with one GPS receiver. However, the accuracy of pseudo-range observations is from a few decimeters to a few meters; The error of the satellite position is up to several to tens of meters with broadcast ephemeris. The error of Satellite bell correction is ± 20 ns around. So it could only be used in navigation and resource investigation, exploration and some other low-precision fields. With the implementation of the strategy for the marine, marine scientific research, marine development, marine engineering, and other marine activities increasing, the requirements of positioning accuracy have also become diversified. For example, the precision of maritime delimitation, marine engineering precision measurement, require to meet a dozen or dozens of centimeters positioning accuracy. But the use of pseudorange differential positioning can only provide the level of meter positioning accuracy. If use RTK the distance can not be reached; For this part of demand, present positioning means can not meet the requirements, so new ways of positioning or technology need to be seen.

Precise positioning(Precise Positioning Point. PPP)technology was provided by Zumberge who worked in the United States Jet Propulsion Laboratory (JPL) in 1997. 1990s, the number of global GPS tracking stations have risen sharply . Global GPS data processing ever-increasing workload, computation time increases exponentially. To solve this problem, as part of the International GPS Service (IGS) of a data analysis center. Dwayne proposed that the method for the non-core GPS data processing station. The technology is very simple ideas, GPS, the main error from the three types, namely track error Satellite bell poor and ionospheric delay. If we adopt the dual-frequency receivers, we can use the LC phase composition, eliminate ionospheric delay effects. If selected carefully and expressed solid satellite orbit calculation of the same frame of reference as to carefully solid line, Observing equation can eliminate the parameters of the Earth's rotation. Therefore, as long as the given orbital satellites and sophisticated bell poor, using sophisticated observation model, we will be able to Pseudorange like, Single Station Receiver calculated the precise location, bell poor, and the ambiguity of tropospheric delay parameters. According to the PPP technical requirements, positioning satellite system needs of the sophisticated orbit and the clock error. Currently, International GPS Service (IGS) of a few data analysis center with the ability to provide the sophisticated satellite orbit and the bell worse, but these are the results after treatment. According to the IGS product, IGS provided by the satellite orbit accuracy can be achieved 2 ~ 3cm, Satellite bell poor accuracy of better than 0. 02ns, the precision of the satellite orbit and the poor bell, be able to meet any precision positioning requirements. For nearly 10 years, due to the efforts of IGS, GPS satellite orbit prediction accuracy has reached 10 centimeters. Forecast track by the time 24 h forecast shortened to 3 h forecast the accuracy of satellite orbit has been able to meet the requirements of the general positioning. As IGS can not provide real-time and extrapolation of the sophisticated satellite bell poor, the real-time constraints PPP technology, Precise satellite bell PPP is still poor technical bottlenecks in real-time application, at current IGS only having locus processing satellite bell worse, JPL and GFZ have the ability to provide rapid satellite clock error.

6.2 Concept of Precise Positioning Point

Precise positioning is different from differential GPS, Precise point positioning using GPS precise ephemeris and precision Clock Error documents provided by IGS of International GPS services or calculated by self, The observational data is consist of   carrier phase and pseudorange observations which without the impact of the ionosphere .finally it estimates the parameters of station location, receiver clock error, tropospheric delay and composition phase ambiguity . User with a dual-frequency GPS receiver-code can achieve high-precision Positioning even thousands of square kilometers within the scope of the global. The feature is that each station of solution is independent of each other, far smaller than the calculated amount of the general relative positioning. PPP is different from the in that the error and some parameters of double poor positioning eliminated by subtraction between the station and satellite, and the PPP must adopt sophisticated models to correct or using auxiliary parameter to estimate, for example, phase error correction of satellite antenna center, tidal correction, marine load correction and so on.

6.3 Research status and application of PPP

6.3.1 Research status

At present, both at home and abroad for precision positioning single point has conducted extensive research, Wuhan University after several years of single-point precision positioning theory and method of in-depth study, first successfully developed high-precision PPP data processing software TriP in China. Using PPP for GPS data processing, the data acquisition needs two weeks. After downloading the IGS website precise ephemeris data, then the data processing can be carried out.

6.3.2 Application of PPP in cadastral survey

Compared with the RTK technology, the single-point precision positioning technology mainly depend on mobile telecommunications technology and Internet technology to realize the real-time application. So the data has no space restriction which is the main difficulty in cadastral survey to establish a fixed base station, Particularly in the land-use change monitoring and investigation work. Therefore, in any region, we can use single-point precision positioning technology to build cadastral control network for high-precision measurement and boundary points cadastral mapping. That makes it easy to unify coordinate system of land-use change investigation and monitoring. Meanwhile, single-point precision positioning technology to achieve a truly personalized measurement in determining the district does not need a high level of measurement and control, no need to set up base stations, Single Single be completed cadastral mapping mission. As the single point precision positioning technology to provide the results of ITRF coordinates. It is common international system of coordinates, so it is extremely convenient to transform the coordinates system to China 80 coordinates system or Beijing 54 coordinates system.

6.3.3 Application of PPP in fairway survey

Use of certain data processing methods, dynamic positioning precision single-point accuracy can achieve decimeter level. In coastal or inland waterway channel measurement, measurement of the level and height of the geometric accuracy to decimeter-centimeter-between. Precise positioning as a new positioning technology, because of its own other than the GPS measurement of its superiority, With single-point precision positioning technology promotion and the growing number of commercial and scientific research institutions to get involved. In terms of error correction model, positioning reduce the systemic error and the impact of alternative data-processing methods, Precision single point will the precision of positioning further raised, enough to satisfy the most modern waterway test the accuracy requirements. As the single point positioning precision measurement method is simple and does not rely on the known control points, no distance limitations Real-time dynamic can now determine the future decimeter and centimeter-level positioning precision of a series of features, Channel Island completely satisfy the volume precision and conforming to the characteristics of the IHO. Therefore, the dynamic precision single-point positioning technology in the measurement path is bright.

6.3.4 Application of PPP in photo control survey

GPS precision single-point positioning in photo control survey has highlighted advantages compared with the static or RTK positioning.

1) The operation of outwork is more simple. Several of the existing operations require two or two more receivers, static measurements also need time on the synchronization, RTK base stations in the ready to work, precision positioning single point as long as a receiver can work, and in no time, it's time that staff can administer the tests.

2) The accuracy of point is uniform, stability and the continuity of overall accuracy is consolidated. Static measurements or RTK measurement of the surveyed area larger circumstances, usually zoning tested. This will lead to the transformation of inconsistency, there is some systematic errors. GPS non-precision single-phase poor positioning of the high-precision ITRF coordinates, size of the area has no compact on accuracy. Therefore, its result only having one transformation parameter, and accuracy of the result is uniform.

3) Compared with other positioning model, the non-poor positioning precision single-point model has many advantages, such as, a lot available observations; obtaining station coordinates directly; observations having no relationship with each other, no distance restriction between stations and so on.

6.3.5 Application of PPP in aerial survey

Precise positioning can achieve decimeter-level dynamic positioning accuracy in the aircraft and reach double difference fixed solutions considerable level of accuracy without building base stations in the ground for a short baseline. Therefore, based on the single-point precision positioning technology can realize aerial survey without ground control points. Of course, the precise point positioning is not a substitute for traditional high-precision relative positioning. Both of them have their respective advantages. If all models was handled correctly, the result of the relative positioning and precision single-point positioning should be the same. Precise point positioning using precise ephemeris, satellite bell poor and products published from IGS data processing center, after considering fully of the error correction model, and software algorithms correct, Precise positioning accuracy and reliability depend on the reliability and accuracy of the IGS products to a large extent. Therefore, quality analysis of the IGS product needs further study in the future. As the non-poor composition of ambiguity is no longer having integral characteristics, Therefore, quality control of the single-point precision dynamic positioning also needs further study. In addition, static conditions the receiver is static, so it can realize to estimate a zenith troposphere parameters every few hours, and use the random walk method to simulate random changes of the tropospheric delay. But for highly dynamic aircraft carrier, it can fly hundreds of kilometers after few hours. The atmospheric state parameters will change very much. Therefore, estimation method of the tropospheric parameter requires further study in dynamic long-distance high-precision GPS point positioning.