This book is structured to first familiarize the reader with the fundamentals of PVT determination using GPS. Once this groundwork has been established, a description of the GPS system architecture is presented. Next, the discussion focuses on satellite signal characteristics and their generation. Received signal acquisition and tracking, as well as range and velocity measurement processes, are then examined. Signal acquisition and tracking is also analyzed in the presence of interference, multipath, and ionospheric scintillation. GPS performance (accuracy, availability, integrity, and continuity) is then assessed. A discussion of GPS differential techniques follows.
Sensor-aiding techniques, including Intelligent Transport Systems (ITS) automotive applications and network-assisted GPS, are presented. These topics are followed by a comprehensive treatment of GALILEO. Details of GLONASS, BeiDou, and the Japanese Quasi-Zenith Satellite System (QZSS) are then provided. Finally, informa- tion on GNSS applications and their corresponding market projections is presented.
Highlights of each chapter are summarized next.
Chapter 2 provides the fundamentals of user PVT determination. Beginning with the concept of TOA ranging, the chapter develops the principles for obtaining three-dimensional user position and velocity as well as UTC (USNO) from GPS.
Included in this chapter are primers on GPS reference coordinate systems, Earth models, satellite orbits, and constellation design.
In Chapter 3, the GPS system architecture is presented. This includes descrip- tions of the space, control (i.e., worldwide ground control/monitoring network), and user (equipment) segments. Particulars of the constellation are described. The U.S. government nominal constellation is provided for those readers who need to conduct analyses using a validated reference constellation. Satellite types and corre- sponding attributes are provided, including the Block IIR, Block IIR-M, and Block IIF. One will note the increase in the number of transmitted civil and military navi- gation signals as the various satellite blocks progress. Of considerable interest are interactions between the control segment (CS) and the satellites. This section pro- vides a thorough understanding of the measurement processing and building of the navigation data message. The navigation data message provides the user receiver with satellite ephemerides, satellite clock corrections, and other information that enable the receiver to compute PVT. An overview of user receiving equipment is presented, as well as related selection criteria relevant to both civil and military users.
Chapter 4 describes the GPS satellite signals and their generation. This chapter examines the properties of the GPS satellite signals, including frequency assign- ment, modulation format, navigation data, and the generation of PRN codes. This discussion is accompanied by a description of received signal power levels, as well as their associated autocorrelation characteristics. Cross-correlation characteristics are also described. The chapter is organized as follows. First, background informa- tion on modulations that are useful for satellite radionavigation, multiplexing tech- niques, and general signal characteristics, including autocorrelation functions and power spectra, is provided. Section 4.3 describes thelegacy GPS signals, defined here as those signals broadcast by the GPS satellites up through the Block IIR space vehicles (SVs). Next, an overview of the GPS navigation data modulated upon the legacy GPS signals is presented. The new civil and military signals that will be broadcast by the Block IIR-M and later satellites are discussed in Section 4.5.
Finally, Section 4.6 summarizes the chapter.
Receiver signal acquisition and tracking techniques are presented in Chapter 5.
Extensive details of the numerous criteria that must be addressed when designing or analyzing these processes are offered. Signal acquisition and tracking strategies for various applications are examined, including those required for high-dynamic stress and indoor environments. The processes of obtaining pseudorange, delta range, and integrated Doppler measurements are described. These observables are used in the formulation of the navigation solution.
Chapter 6 discusses the effects of various channel impairments on GPS perfor- mance. The chapter begins with a discussion of intentional (i.e., jamming) and nonintentional interference. Degradations to the various receiver functions are quantified, and mitigation strategies are presented. A tutorial on link budget com- putations, needed for interference analyses and useful for other GPS systems engi- neering purposes, is included as an appendix to the chapter. Section 6.2 addresses
multipath and shadowing. Multipath and shadowing can be significant and some- times dominant contributors to PVT error. These sources of error, their effects, and mitigation techniques are discussed. The chapter concludes with a discussion on ion- ospheric scintillation. Irregularities in the ionospheric layer of the Earth’s atmo- sphere can at times lead to rapid fading in received GPS signal power levels. This phenomenon, referred to as ionospheric scintillation, can lead to a GPS receiver being unable to track one or more visible satellites for short periods of time.
GPS performance in terms of accuracy, availability, integrity, and continuity is examined in Chapter 7. It is shown how the computed user position error results from range measurement errors and user/satellite relative geometry. The chapter provides a detailed explanation of each measurement error source and its contribu- tion to overall error budgets. Error budgets for both the PPS and SPS are developed and presented.
Section 7.3 discusses a variety of important concepts regarding PVT estimation, beginning with an expanded description of the role of geometry in GPS PVT accu- racy determination and a number of accuracy metrics that are commonly used. This section also describes a number of advanced PVT estimation techniques, including the use of the weighted-least-squares (WLS) algorithm, the inclusion of additional estimated parameters (beyond the userx,y,zposition coordinates and clock offset), and Kalman filtering.
Sections 7.4 through 7.6 discuss, respectively, the three other important perfor- mance metrics of availability, integrity, and continuity. Detailed examination of GPS availability is conducted using the nominal GPS constellation. This includes assessing availability as a function of mask angle and number of failed satellites. In addition to providing position, velocity, and timing information, GPS needs to pro- vide timely warnings to users when the system should not be used. This capability is known as integrity. Sources of integrity anomalies are presented, followed by a dis- cussion of integrity enhancement techniques including receiver consistency checks, such as receiver autonomous integrity monitoring (RAIM) and fault detection and exclusion (FDE), as well as SBAS and GBAS.
Section 7.7 discusses measured performance. The purpose of this section is to discuss assessments of GPS accuracy, which include but are not limited to direct measurements of PVT errors. This is a particularly complex topic due to the global nature of GPS, the wide variety of receivers, and how they are employed, as well as the complex environment in which the receivers must operate. The section con- cludes with a description of the range of typical performance users can expect from a cross-section of today’s receivers, given current GPS constellation performance.
DGPS is discussed in Chapter 8. This chapter describes the underlying concepts of DGPS and details a number of operational and planned DGPS systems. A discus- sion of the spatial and time correlation characteristics of GPS errors (i.e., how GPS errors vary from location to location and how they change over time) is presented first. These characteristics are extremely important to understanding DGPS, since they directly influence the performance achievable from any type of DGPS system.
Next, the underlying algorithms and performance of code- and carrier-based DGPS systems are described in detail. The Radio Technical Commission for Maritime Ser- vices (RTCM) Study Committee 104’s message formats have been adopted through- out the world as a standard for many maritime and commercial DGPS applications.
A discussion of RTCM message formats for both code- and carrier-based applications is presented.
Chapter 8 also contains an in depth treatment of SBAS. The discussion first starts by reviewing the SBAS requirements as put forth by the International Civil Aviation Organization (ICAO). Next, SBAS architecture and functionality are described. This is followed by descriptions of the SBAS signal structure and user receiver algorithms. Present and proposed SBAS geostationary satellite locations and coverage areas are covered.
GBAS, in particular, the U.S. FAA’s Local Area Augmentation System (LAAS), requirements and system details are then presented. The chapter closes with treat- ment and discussion of the data and products obtained from the U.S. National Geo- detic Survey’s Continuously Operating Reference Station (CORS) network and the International GPS Service.
In some applications, GPS is not robust enough to provide continuous user PVT. Receiver operation will most likely be degraded in an urban canyon where sat- ellite signals are blocked by tall buildings or when intentional or nonintentional interference is encountered. Hence, other sensors are required to augment the user’s receiver. This subject area is discussed in Chapter 9. The integration of GPS and inertial sensor technology is first treated. This is usually accomplished with a Kalman filter. A description of Kalman filtering is presented, followed by various descriptions of GPS/inertial navigation system (INS) integrated architectures includ- ing ultratight (i.e., deep integration). An elementary example is provided to illus- trate the processing of GPS and INS measurements in a tightly coupled configuration. Inertial aiding of carrier and code tracking loops is then described in detail. Integration of adaptive antennas is covered next. Nulling, beam steering, and space-time adaptive processing (STAP) techniques are discussed.
Next, Section 9.2 covers ITS automotive applications. This section examines integrated positioning systems found in vehicle systems, automotive electronics, and mobile consumer electronics. Various integrated architectures for land vehicles are presented. A detailed review of low-cost sensors and methods used to augment GPS solutions are presented and example systems are discussed. Map matching is a key component of a vehicle navigation system. A thorough explanation is given regarding the confidence measures, including road shape correlation used in map-matching techniques that aid in determining a vehicle’s true position. A thor- ough treatment of sensor integration principles is provided. Tradeoffs between posi- tion domain and measurement domain integration are addressed. The key aspects of Kalman filter designs for three integrated systems—an INS with GPS, three gyros, and two accelerometers; a system with GPS, a single gyro, and an odometer; and a system with GPS and differential odometers using an antilock brake system (ABS)—are detailed.
Chapter 9 concludes with an extensive elaboration of assisted-GPS network assistance methods (i.e., enhancing GPS performance using cellular network assis- tance). In applications in which the GPS receiver is part of an emergency response system, waiting 30 seconds for data demodulation can seem like an eternity. As such, methods to eliminate the need to demodulate the satellite navigation data mes- sage directly and to decrease the acquisition time of the signals in weak signal envi- ronments has been the basis for all assisted GPS work. The FCC requirements for
E-911 are presented. Extensive treatment of network assistance techniques, perfor- mance, and emerging standards is presented. This includes environment character- ization in terms of median signal attenuation for rural, suburban, and urban areas.
Chapter 10 is dedicated to GALILEO. An overview of the system services is pre- sented, followed by a detailed technical description of the transmitted satellite sig- nals. Interoperability factors are considered next. The GALILEO system architecture is put forth with discussions on constellation configuration, satellite design, and launch vehicle description. Extensive treatment of the downlink satellite signal structure, ground segment architecture, interfaces, and processing is pro- vided. This processing discussion covers clock and ephemeris predictions as well as integrity determination. The key design drivers for integrity determination and dis- semination are highlighted. In addition to providing the navigation service, GALILEO will also contribute to the international search and rescue (SAR) architec- ture and its associated provided services. It is planned to provide a SAR payload on each GALILEO satellite, which will be backward compatible with the present COSPAS/SARSAT system. (The COSPAS/SARSAT system is the international satellite system for search and rescue [24].)
Chapter 11 contains descriptions of the Russian GLONASS, Chinese BeiDou, and Japanese QZSS satellite systems. An overview of the Russian GLONASS system is first presented, accompanied with significant historical facts. The constellation and associated orbital plane characteristics are then discussed. This is followed by a description of the ground control/monitoring network and current and planned spacecraft designs. The GLONASS coordinate system, Earth model, and time refer- ence are also presented. GLONASS satellite signal characteristics are discussed. Sys- tem performance in terms of accuracy and availability is covered. Elaboration is provided on intended GLONASS developments that will improve all system segments. Differential services are also presented.
The BeiDou program is discussed in Section 11.2. The history of the program is briefly described. Constellation and orbit attributes are provided. These are fol- lowed by spacecraft and RDSS service descriptions. User equipment classes and types are put forth. These include general user terminals such as an emergency reporting terminal that makes emergency reports to police and a general communi- cations user terminal used for two-way text message correspondence. All classes of user terminals provide a real-time RDSS navigation service. The system architecture is described, followed by an overview of the five different types of BeiDou services.
System coverage is put forth next. Future developments including BeiDou SBAS and BeiDou-2 are discussed.
At the time of this writing, the Japanese QZSS program was under development.
When completed, QZSS will provide GPS augmentation and mobile satellite com- munications to Japan and its neighboring regions. The constellation, orbits, and sat- ellite types have not been selected. The program goal is to address the shortfalls in GPS visibility in urban canyons and mountainous terrain, which, the Japanese assess, is a problem in 80% of the country. Concepts of spacecraft design and pro- posed orbital plane design are described. This is followed by an overview of the QZSS geodetic and time reference systems. Anticipated system coverage and accuracy performance complete the chapter.
Chapter 12 is dedicated to GNSS markets and applications. As mentioned ear- lier, GPS has been widely accepted in all sectors of transportation, and it is expected that GALILEO will be as well. While predicted values (euros/dollars) of the market for GNSS products and services vary with the prognosticator, it is certain that this market will be large. As other satellite systems come to fruition, this market will surely grow. This chapter starts with reviews of numerous market projections and continues with the process by which a company would target a specific market seg- ment. Differences between the civil and military markets are discussed. It is of prime importance to understand these differences when targeting a specific segment of the military market. The influence of U.S. government and EU policy on the GNSS mar- ket is examined. Civil, government, and military applications are presented. The chapter closes with a discussion on financial projections for the GNSS industry.
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Fundamentals of Satellite Navigation
Elliott D. Kaplan and Joseph L. Leva The MITRE Corporation
Dennis Milbert NOAA (retired) Mike S. Pavloff Raytheon Company