A Unit 15 The Global Positioning System
Preamble序
The Global Positioning System (GPS) is revolutionizing surveying technology. GPS正在革新着测绘技术Like its predecessor, the TRANSIT Doppler system, 像它的前任仪器，中天仪多普勒系统 GPS shifts the scene of surveying operations from ground-to-ground measurements to ground-to-sky ,GPS在测量操作上正在改变着从地对地观测到地对空观测的测量环境with obvious implications: visibility of marks is no longer a criterion for their location :operations are possible in nearly all kinds of weather and can be performed during day or night ;and the skills required to utilize the technology are different both in filed operations and data processing.有一些明显的结论标志的可见性不在是定位的一个标准在几乎任何天气、不分昼夜操作都是可以进行的使用这项技术所要求的技能在野外测量和数据处理中都是不同的 But GPS is not merely a replacement for TRANSIT .the simultaneous visibility of multiple satellites allows effective cancellation of the major sources of error in satellite observations, with the result that with GPS ,relative positioning accuracies of one part per million ( ppm) or better over distances from one kilometer to thousands of kilometers are possible .This means that GPS Can compete with terrestrial techniques over short distances ,and can achieve more accurate results in less time than TRANSIT observations over longer distances.但GPS不只是代替中天仪。多个卫星同时可视可以有效消除卫星观测中主要误差源，因此，用GPS其相对定位精度从一公里到数百公里范围内有可能达到百万分之一。这就意味着GPS在断距离上可以与地面测量技术相当，在长距离上比中天仪用时更少，并能得到更高的精度。
GPS was designed primarily as a navigation system GPS最主要是用作导航系统, to satisfy both military and civilian needs for read-time 满足军用和民用实时定位的需要positioning is accomplished through the use of coded information ,essentially clever timing signals ,transmitted by the satellites .这种定位方式是通过使用编码信息完成的，这些编码本质上是通过卫星传输性能较好的时间信号Each GPS satellite transmits a unique single on two L-band frequencies: L1 at 1575.42 MHz and L2 at 1227.60MHz (equivalent to wavelengths of approximately 19and 24 cm , respectively). 每个GPS卫星以2个L-波段频率传输1个唯一的信号L-1波段的频率1575.2Mhz，L-2波段的频率1227.60Mhz （他们各自向对应的波长 大约为19—24cm）The satellite singles consist of the l-band carrier waves modulated with a “Standard” or S code (formerly called the C/A code ), a “precise” or P code and a Navigation Message containing ,amongst other things ,the S code which is intended mainly for civilian use ,yields a range measurement precision of about 10 m ,The navigation service provided by this code is referred to as the Standard Positioning Service(SPS). The P code is therefore referred to as the Standard Positioning Service (PPS).Although both codes can be used for surveying, a more accurate method is to measure the carrier signal. For this reason. For this reason, we will not discuss the detailed characteristics of the codes in this monograph. For details we refer the reader to Janiczek (1980).卫星信号由调制的L-波段载波组成，它是用一个“标准的”或称S的点码（以前称为C/A码，一个“精确的”或称P的点码以一个导航信息调制的，卫星的坐标作为事件—“广播星历”—的函数。S码主要是为民用，它可得到大约10m的距离测量精度，由这个码提高的导航服务为标准定为服务(GPS）,因此P码就是精确定为服务（PP是）。虽然两个码都用于测量，但更精确的方法是测量载波信号的相位。因此，在本文中我们将不详细讨论两个码的特点
There are currently eight usable satellites in orbit.当前可用的在轨卫星有8个 There are the experimental,”block1”satellites, which be progressively replaced as the”block2”, operational satellites are placed into orbit beginning in 1986 block1卫星是实验卫星，他们逐渐被black2卫星取代.这些操作卫星将在1986年底送入预定轨道 By 1989 the system should be complete, with 18 satellites in six orbit planes---at about 20200 km altitude , allowing for simultaneous visibility of at least four satellites at any time of day almost anywhere in the world. 截至1989年，大约在20200km 的6个轨道平面上的18颗卫星的系统将会完成这将会保证在世界上任何地点任何时间至少有4可卫星同时可视The present constellation is configured to provide the most favorable geometry for testing the system over North America.当前覆盖北美地区用于测试这个系统的配备的星座提供了最好的几何信息
As it happens , the observation geometry is equally favorable in Australia , and it is possible now to obtain surveying accuracies equally to those obtainable when the system is fully configured , but only for about six hours per day.凑巧的是，这些观测的几何信息在澳大利亚同样也是可用的，现在获取的测量数据的准确性与现在这个系统完全配备以后所能达到的精度是相当的，这种可能性是存在的 At the time of writing(November 1985),the period of maximum mutual visibility of the satellites in eastern Australia is between 6 pm and mid-night local time.1985年11月在写作的这个时间在澳大利亚东部卫星相互可视的最大的时间段是从下午6点到当地时间的午夜 This period regresses by 4 minutes per day (or 2hours per month),returning to the same times a year from now 这个时间段将会以每天4分钟（每月2小时）向后延迟，从现在开始1年之后回归到同样的时刻 this period of useful visibility will increase as additional satellites are launched from late 1985. 这个可以利用的可见性的时间段从1985年底随着另外一些卫星的发射将会增加
As with TRANSIT, much higher accuracies are obtained in relative positioning from observations made simultaneously at two observing stations.正如中天仪一样在2个观测站同时进行相对定位观测将会获得更加高的精度 Consequently, unless otherwise indicated, all discussion concerning data acquisition and processing will assume a two-receiver configuration因此，除了额外的的一些指示，所有关于数据获取和数据处理的讨论将会假定由配备的2个接受机. The position differences so determined constitute the baseline vector or simply the baseline between the points occupied by two receivers.由此测定的坐标差组成了基线矢量或者仅仅是2个接受机所在的测点之间的基线All satellite positioning systems provide ground coordinates of a receiver (or the baseline vrctor between a pair o receives)in an earth-centered coordinate system .The orientation of the system is determined by the tabulated coordinates or ephemerides of the GPS satellites. In order to relate coordinates determined by GPS surveying to the local geodetic datum a transformation relationship needs to be established多有卫星定为系统都提供接收机的地心坐标系（或两个接收机之间的基线矢量。系统的定向由坐标表或GPS卫星的星历确定。为了将GPS测量确定的坐标与地方大地基准联系起来，需要建立起转换关系
The following factors influence the final positioning accuracy obtainable with GPS:下列因素影响由GPS获得的最终的定位精度
1:The precision of the measurement and receiver-satellite geometry测量精度和接收机-卫星的几何图形结构
2:The measurement processing technique adopted;所采用的数据处理方法
3:The accuracy with which atmospheric and ionospheric effects can be modeled大气层和电离层模型的影响精度’
4The accuracy of the satellite ephemerides.卫星星历的精度
Each of these factors is discussed briefly in the next three sections紧接着的3个部分将会简单的讨论每个因素.
GPS measurement types :GPS measurements can be made using either the carrier signal or the codes. GPS测量的类型：GPS测量可以采用载波信号或者编码的2种形式Code measurements are called pseudo-ranges and can be based on either the P code or the S code. ;码测量可以称做伪距观测并且是基于P码或者是S码的
Knowledge of the properties of each of these types of measurements已下是有关每种测量类型的性能的指示
Pseudo-ranges are the simplest to visualize geometrically as they are essentially a measurement of distance contaminated by clock errors.由于伪距本质上是受中差影响的距离测量，它在几何可视性上是最简单的 Throughout this monograph , we use the terms clock, frequency standard and oscillator to denote the same thing, namely ,a device for precisely measuring a time interval在这篇论文中我们使用时钟，标准频率和震荡器这些术语描述的是同一个事物，即，用于精确测量时间间隔的设备. When four satellites are observed simultaneously, it is possible to determine the three-dimensional position of the ground receiver and the receiver clock offset at a single epoch .This is simply resection by distance ,in surveying
terminology ,with the technique the precision is a function of the geometry of the receiver in relation to the four visible satellites .The best geometry would be when the satellites are in each of the four quadrants and each at an elevation angle of 40-70 above the horizon. However , pseudo-range measurements are not nearly as precise as phase measurements of the carrier wave itself.当同时观测四个卫星时，就可以在单个历元确定地面接收机的三维位置以及接受的钟差。用测量术语来说，这就是简单的后方交会，GPS定位的精度受接收机与四个可视卫星的几何图形的影响。最佳的图形是：四个卫星分布在四个象限，并且每个高度角位于水平线以上的40度到70度之间。单伪距测量的精度远不如载波相位测量的精度 In order to achieve position accuracies of 10 m from P code measurements or 100 m from S code measurements , it was only necessary to design. a code structure which allowed meter level measurement precision .为了从P码测量中获取10m或从S码中获取100m的定位精度（用于导航是足够的），设计一个含有米级测量精度的码结构是必须的Moreover ,the more precise P code will likely be encrypted, and may therefore not be available for non-military use, when the system becomes operational in 1989.此外在1989年这个系统运营的时候，有可能更加精确的P码进行加密，从而使它对非军方是不可用的 An additional impediment to accurate pseudo ranging arises from multipath effects, that in the tendency of some fraction of the satellites signal to reach the receiver antenna via reflection off the ground or other surfaces .另一个妨碍伪距测量精度的因素是多路径效应， 它是卫星信号的某些部分经过地面或其他事物表面的反射进入接收机的天线 The size and signature of multipath effects depend on antenna design and height of the antenna above ground but probably cannot be reduced below a few decimeters with practical configurations.多路径效应的数量和特征取决于天线的设计类型和天线距地面的高度，但是即使拥有实际的一些配置也不可能降低到几分米以下
Carrier phase measurements are more precise than the pseudo-ranges and not as vulnerable to multipath effects. 载波相位测量比伪距测量更加精确并且不收多路径效应的影响The wavelength of the stronger of the two L-band carrier signals,L1,is 19 cm, so even rough interpolation of phase gives centimeter level precision .2个较强的L-波段载波信号的波长，L-1 19cm即使相位有粗略的一些改变也会达到厘米级的精度From the technique of Electronic Distance Measurement, we know that phase measurements are ambiguous ,and unless one can determine the absolute range difference at the initial epoch, phase measurements give only the changes in range over the observing period从电子测距技术我们知道相位测量是不确定的，除非能在初始历元测定完全的边长差，相位测量仅仅能过给出观测期间的边长的改变. However, the absolute range difference can often be easily determined. 但是，完全的边长差经常能够很容易的测定
Carrier phase can be determined from the code-modulated signal either by using the code or other techniques .通过使用码或别的技术载波相位可以通过调制的码信号测得The L1 signal ,which has both P code and S code modulation ,can thus be tracked with S or P code receivers or with codeless receivers。拥有P码和S码的调制解调器L-1信号能够通过S或者P码的接收机或者无码的接收机进行追踪The L2 signal, useful for removing ionospheric effects for very precise applications , has no S code modulation ,so that receivers for these applications must either have P code capability or operate without code.对于精密测量，L-2信号对于消除电离层的影响是非常有用的（相对定位精度小于百万分之二）L-2信号没有S码的调制解调器，因此对于这些测量接受机必须有S码的性能或者是没有码也能运行
It is also possible to track the phase of the 10.23 MHz P code transition signal
or P code sub-carrier without knowledge of the codes. 追踪10.23MHz的P码转换信号或者是没有码信息的子载波也是可能的The long wavelength of this signal compared with the L-band carrier allows relatively easy resolution of the integer-cycle ambiguity , producing in effect a pseudo-range measurement.相对于L-波段的载波这种信号的长波长（大约30M）对整周模糊度的解决是相对容易的实际上是产生了一个伪距测量 However, the long wavelength makes the measurements more susceptible to multipath effects, roughly to the same degree as pseudo-range measurements.然而长波长使得测量更容易受到多路径效应的影响定位精度跟伪距测量是同等级的
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