全球定位系统原理与应用（第二版）详解

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《理解GPS原理与应用——第二版》是一本专为探讨全球定位系统（GPS）及其相关技术的深入教材而编写的。作者Elliott D. Kaplan和Christopher J. Hegarty作为编辑，共同探讨了GPS的各个方面，从基础理论到实际应用，旨在帮助读者全面了解全球卫星导航系统的运作。本书共分为12章，涵盖了广泛的GPS知识： 1. **引言**：介绍了GPS技术的背景和目的，为后续章节提供了一个概述性的框架，解释了GPS在全球定位和导航中的核心作用。 2. **卫星导航基础**：阐述了卫星导航的基本原理，包括星座设计、信号传播模型以及定位原理，为读者打下坚实的理论基础。 3. **GPS系统组成**：详细讲解了GPS系统的主要组成部分，如空间段（卫星）、控制段（地面监控站）和用户段（接收机），解释了它们在系统中的功能和相互作用。 4. **GPS卫星信号特性**：深入研究了GPS信号的结构、编码和频率分集策略，这对于信号捕获和数据解码至关重要。 5. **信号获取、跟踪和解调**：介绍了接收机如何捕捉卫星信号、跟踪其位置并解码数据，展示了整个接收流程的关键步骤。 6. **干扰、多径效应和闪烁**：讨论了GPS信号面临的挑战，如与其他无线通信系统的干扰、多径传播带来的误差以及大气现象导致的信号不稳定（如电离层折射）。 7. **独立GPS性能**：分析了GPS系统在无外部辅助下的定位精度，包括误差来源和提高精度的方法。 8. **差分GPS**：讲解了通过与已知基准站的信号比较来提高定位精度的差分GPS技术，这是提高定位精度的关键技术之一。 9. **GPS与其他传感器和网络集成**：探讨了GPS与其他技术（如惯性导航、雷达等）的融合，以及如何利用移动通信网络进行增强定位。 10. **GALILEO系统**：介绍了欧洲的全球导航卫星系统（GNSS）——GALILEO，对比分析了其与GPS的异同和互补之处。 11. **其他卫星导航系统**：涵盖了其他全球定位系统，如GLONASS、BeiDou和QZSS，展示了全球卫星导航市场的多样性。 12. **GNSS市场与应用**：总结了全球定位系统的广泛应用领域，包括航空、航海、车辆导航、农业、测绘和物联网，以及市场发展趋势和挑战。通过《理解GPS原理与应用——第二版》，读者不仅可以掌握GPS技术的精髓，还能了解到其在不断发展的现代科技环境中的最新进展和未来趋势。

Acknowledgments

Much appreciation is extended to the following individuals for their contributions

to this effort. Our apologies are extended to anyone whom we may have inadver

tently missed. We thank Don Benson, Susan Borgeson, Bakry El-Arini, John

Emilian, Ranwa Haddad, Peggy Hodge, LaTonya Lofton-Collins, Dennis D.

McCarthy, Keith McDonald, Jules McNeff, Tom Morrissey, Sam Parisi, Ed Pow

ers, B. Rama Rao, Kan Sandhoo, Jay Simon, Doug Taggart, Avram Tetewsky,

Michael Tran, John Ursino, A. J. Van Dierendonck, David Wolfe, and Artech

House’s anonymous peer reviewer.

Elliott D. Kaplan

Christopher J. Hegarty

Editors

Bedford, Massachusetts

November 2005

xvii

CHAPTER 1

Introduction

Elliott D. Kaplan

The MITRE Corporation

1.1 Introduction

Navigation is defined as the science of getting a craft or person from one place to

another. Each of us conducts some form of navigation in our daily lives. Driving to

work or walking to a store requires that we employ fundamental navigation skills.

For most of us, these skills require utilizing our eyes, common sense, and land

marks. However, in some cases where a more accurate knowledge of our position,

intended course, or transit time to a desired destination is required, navigation aids

other than landmarks are used. These may be in the form of a simple clock to deter

mine the velocity over a known distance or the odometer in our car to keep track of

the distance traveled. Some other navigation aids transmit electronic signals and

therefore are more complex. These are referred to as radionavigation aids.

Signals from one or more radionavigation aids enable a person (herein referred

to as the user) to compute their position. (Some radionavigation aids provide the

capability for velocity determination and time dissemination as well.) It is impor-

tant to note that it is the user’s radionavigation receiver that processes these signals

and computes the position fix. The receiver performs the necessary computations

(e.g., range, bearing, and estimated time of arrival) for the user to navigate to a

desired location. In some applications, the receiver may only partially process the

received signals, with the navigation computations performed at another location.

Various types of radionavigation aids exist, and for the purposes of this text

they are categorized as either ground-based or space-based. For the most part, the

accuracy of ground-based radionavigation aids is proportional to their operating

frequency. Highly accurate systems generally transmit at relatively short wave

lengths, and the user must remain within line of sight (LOS), whereas systems

broadcasting at lower frequencies (longer wavelengths) are not limited to LOS but

are less accurate. Early spaced-based systems (namely, the U.S. Navy Navigation

Satellite System—referred to as Transit—and the Russian Tsikada system)

pro

vided a two-dimensional high-accuracy positioning service. However, the fre

quency of obtaining a position fix is dependent on the user’s latitude. Theoretically,

1. Transit was decommissioned on December 31, 1996, by the U.S. government. At the time of this writing,

Tsikada was still operational.

a Transit user at the equator could obtain a position fix on the average of once every

110 minutes, whereas at 80° latitude the fix rate would improve to an average of

once every 30 minutes [1]. Limitations applicable to both systems are that each posi

tion fix requires approximately 10 to 15 minutes of receiver processing and an esti

mate of the user’s position. These attributes were suitable for shipboard navigation

because of the low velocities, but not for aircraft and high-dynamic users [2]. It was

these shortcomings that led to the development of the U.S. Global Positioning

System (GPS).

1.2 Condensed GPS Program History

In the early 1960s, several U.S. government organizations, including the Depart

ment of Defense (DOD), the National Aeronautics and Space Administration

(NASA), and the Department of Transportation (DOT), were interested in develop

ing satellite systems for three-dimensional position determination. The optimum

system was viewed as having the following attributes: global coverage, continu

ous/all weather operation, ability to serve high-dynamic platforms, and high accu

racy. When Transit became operational in 1964, it was widely accepted for use on

low-dynamic platforms. However, due to its inherent limitations (cited in the pre-

ceding paragraphs), the Navy sought to enhance Transit or develop another satellite

navigation system with the desired capabilities mentioned earlier. Several variants of

the original Transit system were proposed by its developers at the Johns Hopkins

University Applied Physics Laboratory. Concurrently, the Naval Research Labora-

tory (NRL) was conducting experiments with highly stable space-based clocks to

achieve precise time transfer. This program was denoted as Timation. Modifications

were made to Timation satellites to provide a ranging capability for two-dimen-

sional position determination. Timation employed a sidetone modulation for

satellite-to-user ranging [3–5].

At the same time as the Transit enhancements were being considered and the

Timation efforts were underway, the Air Force conceptualized a satellite positioning

system denoted as System 621B. It was envisioned that System 621B satellites would

be in elliptical orbits at inclination angles of 0°, 30°, and 60°. Numerous variations

of the number of satellites (15–20) and their orbital configurations were examined.

The use of pseudorandom noise (PRN) modulation for ranging with digital signals

was proposed. System 621B was to provide three-dimensional coverage and contin

uous worldwide service. The concept and operational techniques were verified at the

Yuma Proving Grounds using an inverted range in which pseudosatellites or

pseudolites (i.e., ground-based satellites) transmitted satellite signals for aircraft

positioning [3–6]. Furthermore, the Army at Ft. Monmouth, New Jersey, was inves

tigating many candidate techniques, including ranging, angle determination, and the

use of Doppler measurements. From the results of the Army investigations, it was

recommended that ranging using PRN modulation be implemented [5].

In 1969, the Office of the Secretary of Defense (OSD) established the Defense

Navigation Satellite System (DNSS) program to consolidate the independent devel

opment efforts of each military service to form a single joint-use system. The OSD

also established the Navigation Satellite Executive Steering Group, which was

2 Introduction

charged with determining the viability of the DNSS and planning its development.

From this effort, the system concept for NAVSTAR GPS was formed. The

NAVSTAR GPS program was developed by the GPS Joint Program Office (JPO) in

El Segundo, California [5]. At the time of this writing, the GPS JPO continued to

oversee the development and production of new satellites, ground control equip

ment, and the majority of U.S. military user receivers. Also, the system is now most

commonly referred to as simply GPS.

1.3 GPS Overview

Presently, GPS is fully operational and meets the criteria established in the 1960s for

an optimum positioning system. The system provides accurate, continuous, world

wide, three-dimensional position and velocity information to users with the appro

priate receiving equipment. GPS also disseminates a form of Coordinated Universal

Time (UTC). The satellite constellation nominally consists of 24 satellites arranged

in 6 orbital planes with 4 satellites per plane. A worldwide ground control/monitor

ing network monitors the health and status of the satellites. This network also

uploads navigation and other data to the satellites. GPS can provide service to an

unlimited number of users since the user receivers operate passively (i.e., receive

only). The system utilizes the concept of one-way time of arrival (TOA) ranging.

Satellite transmissions are referenced to highly accurate atomic frequency standards

onboard the satellites, which are in synchronism with a GPS time base. The satellites

broadcast ranging codes and navigation data on two frequencies using a technique

called code division multiple access (CDMA); that is, there are only two frequencies

in use by the system, called L1 (1,575.42 MHz) and L2 (1,227.6 MHz). Each satel-

lite transmits on these frequencies, but with different ranging codes than those

employed by other satellites. These codes were selected because they have low

cross-correlation properties with respect to one another. Each satellite generates a

short code referred to as the coarse/acquisition or C/A code and a long code denoted

as the precision or P(Y) code. (Additional signals are forthcoming. Satellite signal

characteristics are discussed in Chapter 4.) The navigation data provides the means

for the receiver to determine the location of the satellite at the time of signal trans

mission, whereas the ranging code enables the user’s receiver to determine the tran

sit (i.e., propagation) time of the signal and thereby determine the satellite-to-user

range. This technique requires that the user receiver also contain a clock. Utilizing

this technique to measure the receiver’s three-dimensional location requires that

TOA ranging measurements be made to four satellites. If the receiver clock were

synchronized with the satellite clocks, only three range measurements would be

required. However, a crystal clock is usually employed in navigation receivers to

minimize the cost, complexity, and size of the receiver. Thus, four measurements

are required to determine user latitude, longitude, height, and receiver clock offset

from internal system time. If either system time or height is accurately known, less

than four satellites are required. Chapter 2 provides elaboration on TOA ranging as

well as user position, velocity, and time (PVT) determination.

GPS is a dual-use system. That is, it provides separate services for civil and mili

tary users. These are called the Standard Positioning Service (SPS) and the Precise

1.3 GPS Overview 3

Positioning Service (PPS). The SPS is designated for the civil community, whereas

the PPS is intended for U.S. authorized military and select government agency users.

Access to the GPS PPS is controlled through cryptography. Initial operating capabil

ity (IOC) for GPS was attained in December 1993, when a combination of 24 proto

type and production satellites was available and position determination/timing

services complied with the associated specified predictable accuracies. GPS reached

full operational capability (FOC) in early 1995, when the entire 24 production satel

lite constellation was in place and extensive testing of the ground control segment

and its interactions with the constellation was completed. Descriptions of the SPS

and PPS services are presented in the following sections.

1.3.1 PPS

The PPS is specified to provide a predictable accuracy of at least 22m (2 drms, 95%)

in the horizontal plane and 27.7m (95%) in the vertical plane. The distance root

mean square (drms) is a common measure used in navigation. Twice the drms value,

or 2 drms, is the radius of a circle that contains at least 95% of all possible fixes that

can be obtained with a system (in this case, the PPS) at any one place. The PPS pro

vides a UTC time transfer accuracy within 200 ns (95%) referenced to the time kept

at the U.S. Naval Observatory (USNO) and is denoted as UTC (USNO) [7, 8].

Velocity measurement accuracy is specified as 0.2 m/s (95%) [4]. PPS measured per-

formance is addressed in Section 7.7.

As stated earlier, the PPS is primarily intended for military and select govern-

ment agency users. Civilian use is permitted, but only with special U.S. DOD

approval. Access to the aforementioned PPS position accuracies is controlled

through two cryptographic features denoted as antispoofing (AS) and selective

availability (SA). AS is a mechanism intended to defeat deception jamming through

encryption of the military signals. Deception jamming is a technique in which an

adversary would replicate one or more of the satellite ranging codes, navigation data

signal(s), and carrier frequency Doppler effects with the intent of deceiving a victim

receiver. SA had intentionally degraded SPS user accuracy by dithering the satellite’s

clock, thereby corrupting TOA measurement accuracy. Furthermore, SA could have

introduced errors into the broadcast navigation data parameters [9]. SA was discon

tinued on May 1, 2000, and per current U.S. government policy is to remain off.

When it was activated, PPS users removed SA effects through cryptography [4].

1.3.2 SPS

The SPS is available to all users worldwide free of direct charges. There are no

restrictions on SPS usage. This service is specified to provide accuracies of better

than 13m (95%) in the horizontal plane and 22m (95%) in the vertical plane (global

average; signal-in-space errors only). UTC (USNO) time dissemination accuracy is

specified to be better than 40 ns (95%) [10]. SPS measured performance is typically

much better than specification (see Section 7.7).

At the time of this writing, the SPS was the predominant satellite navigation ser

vice in use by millions throughout the world.

4 Introduction

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