GPS深度解析：原理、应用与误差分析

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"Understanding GPS: Principle and Applications" 是一本深入解析全球定位系统（GPS）的书籍，由Elliott D. Kaplan和Christopher J. Hegarty编辑。第二版涵盖了GPS的基本原理、接收机的工作机制、定位误差分析、差分GPS技术以及GPS与其他传感器的组合导航算法等内容。 GPS（Global Positioning System）是一种基于卫星导航的全球定位系统，用于确定地球上任何地点的精确位置。其基本原理主要涉及以下几个方面： 1. **卫星信号**: GPS系统由一组分布在地球轨道上的卫星组成，这些卫星不断发射包含时间戳和位置信息的无线电信号。每个信号到达接收机时，都会记录下接收到信号的时间。 2. **三边测量法**: 接收机通过测量至少四颗卫星信号的到达时间，利用三角定位原理计算出自己的三维位置（经度、纬度和海拔）。这个过程涉及到信号传播速度（光速）和卫星与接收机之间的距离关系。 3. **伪随机码测距**: GPS信号中包含了伪随机码（PRN），接收机通过匹配这个码来精确测量信号的传播时间，从而计算距离。 4. **接收机工作原理**: GPS接收机接收到卫星信号后，解码并处理这些信息，然后通过算法计算出接收机的精确位置。这个过程可能需要解决多路径效应（信号反射造成的干扰）和信号衰减等问题。 5. **定位误差分析**: 定位精度受到多种因素的影响，包括信号干扰、大气折射、卫星钟偏和接收机钟偏等。为了提高定位精度，需要对这些误差进行修正。 6. **差分GPS (DGPS)**: DGPS通过使用一个已知精确位置的参考站来校正接收机的定位误差，显著提高了定位精度。参考站会广播修正信息，接收机结合这些信息可以减少误差。 7. **组合导航**: GPS常与其他传感器（如惯性导航系统、地磁传感器、雷达等）结合，形成多传感器导航系统，以实现更稳定、更准确的定位效果，尤其是在GPS信号受到遮挡或干扰的情况下。该书是Artech House Mobile Communications Series的一部分，对于理解和应用GPS技术的专业人士来说是一份宝贵的资源。书中包含的技术标准和实践应用，对于通信、交通、军事、测绘等多个领域的专业人士都具有重要的参考价值。

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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