Springer Handbook of Global Navigation Satellite Systems

This chapter is a primer on global navigation satellite systems (GNSSglobal navigation satellite system (GNSS)global navigation satellite system (GNSS)s). It assumes no prior knowledge of the systems or how they work. All of the key concepts of satellite-based positioning, navigation, and timing (PNTpositioning, navigation and timing (PNT)) are introduced with pointers to subsequent chapters for further details. The chapter begins with a history of PNT using satellites and then introduces the concept of positioning using measured ranges between a receiver and satellites. The basic observation equations are then described along with the associated error budgets. Subsequently, the various GNSSs now in operation and in development are briefly overviewed. The chapter concludes with a discussion of the relevance and importance of GNSS for science and society at large.

Geodesy is the science of the measurement and mapping of the Earth’s surface, and in this context it is also the science that defines and realizes coordinates and associated coordinate systems. Geodesy thus is the foundation for all applications of global navigation satellite system (GNSSglobal navigation satellite system (GNSS)). This chapter presents the reference systems needed to describe coordinates of points on the Earth’s surface or in near space and to relate coordinate systems among each other, as well as to some absolute system, visually, a celestial system. The topic is primarily one of geometry, but the geodynamics of the Earth as a rotating body in the solar system plays a fundamental role in defining and transforming coordinate systems. Therefore, also the fourth coordinate, time, is critical not only as the independent variable in the dynamical theories, but also as a parameter in modern geodetic measurement systems. Instead of expounding the theory of geodynamics and celestial mechanics, it is sufficient for the purpose of this chapter to describe the corresponding phenomena, textually, analytically and illustratively, in order to give a sense of the scope of the tasks involved in providing accurate coordinate reference systems not just to geodesists, but to all geoscientists.

This chapter discusses fundamentals of orbital dynamics and provides a description of key perturbations affecting global navigation satellite system (GNSSglobal navigation satellite system (GNSS)) satellites along with their impact on the orbits. Models for perturbing accelerations including Earth gravity, third body perturbations, surface forces, and relativistic corrections are described with emphasis on empirical and semiempirical solar radiation pressure models. Long-term evolution of GNSS orbits and orbit keeping maneuvers are discussed. The concepts of broadcast orbit models such as almanac models, analytical ephemeris models and numerical ephemeris models used by current GNSS systems are presented along with cook book algorithms and a summary of their performance. Complementary to the discussion of GNSS satellite orbits, the chapter introduces the basic concepts of GNSS satellite attitude, which are, for example, required to describe the antenna location relative to the center-of-mass.

Satellite navigation relies on signals radiated by orbiting satellites and received by mobile satellite navigation receivers. This chapter addresses the fundamentals of such navigation signals and introduces the most important underlying concepts. It provides an introduction to radio frequency signals including the basics of electromagnetic waves, their carrier frequency, polarization, as well as group and phase velocity. The application of waves for carrying signals, their power and spectrum are addressed. It is shown how information-carrying signals can be modulated onto the wave using various modulation schemes such as binary phase shift keying, binary offset carrier, and alternating binary offset carrier. Setting out from international agreed allocations, the frequency bands used in GNSSglobal navigation satellite system (GNSS)global navigation satellite system (GNSS) are described. The concept of pseudo-random codes which is typically used for GNSS signals is introduced as well as their receiver side processing following a correlation principle.

This chapter provides an overview of clock technology and typical clocks (Cs, Rb, H-Maser) in use today for onboard and ground systems and identifies future trends such as fountain clocks, etc. Concepts such as clock drift, trend, random variations and the statistical methods for their characterization (Allan deviation (ADEVAllan deviation (ADEV)), etc.) are introduced and performance characteristics of global navigation satellite system (GNSSglobal navigation satellite system (GNSS)) onboard clocks are presented. The handling and impact of special and general relativity on timing measurements are discussed. Finally, the generation of a GNSS time from an ensemble of ground clocks is described.

Global navigation satellite system (GNSSglobal navigation satellite system (GNSS)) satellites emit signals that propagate as electromagnetic waves through space to the receivers which are located on or near the Earth’s surface or on other satellites. Thereby, electromagnetic waves travel through the ionosphere and the neutral atmosphere (troposphere) which causes signals to be delayed, damped, and refracted as the refractivity index of the propagation media is not equal to one. In this chapter, the nature and effects of GNSS signal propagation in both the troposphere and the ionosphere, aref examined. After a brief review of the fundamentals of electromagnetic waves their propagation in refractive media, the effects of the neutral atmosphere are discussed. In addition, empirical correction models as well as the state-of-the-art atmosphere delay estimation approaches are presented. Effects related to signal propagation through the ionosphere are dealt in a dedicated section by describing the error contribution of the first up to third-order terms in the refractive index and ray path bending. After discussing diffraction and scattering phenomena due to ionospheric irregularities, mitigation techniques for different types of applications are presented.

This chapter presents an overview of the US Global Positioning System (GPSGlobal Positioning System (GPS)Global Positioning System (GPS)), which became the first operational global navigation satellite system (GNSSglobal navigation satellite system (GNSS)) core constellation when it was declared fully operational in 1995. First, the space segment is described, including key characteristics of the different satellite types. Then, an overview of the control segment is given, including its operations and evolution of capabilities. This is followed by an overview of the GPS signals, current and future, as well as a description of the navigation data content. Then, the time and coordinate systems used by GPS are described. The chapter is concluded with a brief description of services and performance.

The Global’naya Navigatsionnaya Sputnikova Sistema (GLONASSGlobal’naya Navigatsionnaya Sputnikova Sistema (Russian Global Navigation Satellite System) (GLONASS)) is a global navigation satellite system developed by the Russian Federation. Similar to its US counterpart, the NAVSTAR global positioning system (GPSGlobal Positioning System (GPS)), GLONASS provides dual-frequency L-band navigation signals for civil and military navigation. Initiated in the 1980s, the system first achieved its full operational capability in 1995. Following a temporary degradation, the nominal constellation of 24 satellites was ultimately reestablished in 2011 and the system has been in continued service since then. This chapter describes the architecture and operations of GLONASS and discusses its current performance. In addition, the planned evolution of the space and ground segment are outlined.

The European global navigation satellite system Galileo is designed as a self-standing satellite-based positioning system for worldwide service. It is independent from other systems with respect to satellite constellation, ground segment, and operation. Galileo is prepared to be compatible and interoperable with other radio navigation satellite systems, with global positioning system (GPSGlobal Positioning System (GPS)Galileo) as the main example. It uses the same physical principles as GPS, GLONASS, and others, that is radio signal-based ranging measurements from high-precision clocks as sources in orbit. The features of the first generation of Galileo comprise technological advances such as passive maser clock technology in orbit, plus modern system and signal concepts aligned to the planned and ongoing modernization of other systems. To the user, Galileo provides navigation signals on three frequencies E1, E6, and E5. The signals in E1 and E5 are coordinated with GPS L1 and L5, and both systems use equivalent modulation principles. This is expected to result in a benefit with respect to positioning accuracy, and in increased robustness of a positioning service derived from the combined use of multiple independent radio navigation systems. This chapter describes architecture and operations of Galileo.

This chapter introduces the BeiDou (COMPASS) Navigation Satellite System from its early stage as a demonstration system to its evolution to a global system. First, the development strategy and basic principle of BeiDou Demonstration System are reviewed. Its basic performance is given in details. Second, the basic information of BeiDou (regional) system including constellation, frequency, coordinate reference system, and time datum is described. Its initial performance is evaluated by using single-point positioning, code and carrier phase differential positioning. Some application examples are introduced. Third, the BeiDou (Global) Navigation Satellite System (BDSBeiDouNavigation Satellite System (BDS)) is described. Position dilution of precision is analyzed and BeiDou’s contribution is summarized. At last, Chinese Area Positioning System is briefly introduced.

Other than global positioning system (GPSGlobal Positioning System (GPS)), Russian global navigation satellite system (GLONASSGlobal’naya Navigatsionnaya Sputnikova Sistema (Russian Global Navigation Satellite System) (GLONASS)), BeiDou, and Galileo, the regional navigation satellite systems (RNSSregional navigation satellite system (RNSS)) aim to provide a regional service using a constellation of satellites in geostationary Earth orbits (GEOgeostationary Earth orbit (GEO)) and inclined geosynchronous orbits (IGSOinclined geo-synchronous orbit (IGSO)). Two regional systems implemented in Asia will be introduced in this chapter.The first one is the Japanese Quasi-Zenith Satellite System (QZSSQuasi-Zenith Satellite System (QZSS)), which was originally planned as an augmentation system to enhance GPS capability and performance in the area surrounding Japan. The other is the Indian Regional Navigation Satellite System (IRNSSIndian Regional Navigation Satellite System (IRNSS)Navigation with Indian Constellation (NavIC), also known as NavIC for Navigation with Indian Constellation), which can provide an independent positioning, navigation, and timing (PNTpositioning, navigation and timing (PNT)) service over India and surrounding areas.In this chapter, the concept of regional navigation satellite systems is first described. The combination of satellites in GEO and IGSO is a common idea to realize such a regional service platform with a low number of satellites. The orbital characteristics and geometry of the proposed RNSS constellations are explained before each RNSS is introduced in detail. Secondly, the detailed characteristics of both systems are described in the following sections. The system architecture, service provision including navigation signal properties and service performance to be provided, as well as the deployment plan or schedule are mentioned for each system. Additionally, initial demonstration results are presented.

Satellite-based augmentation systems (SBASsatellite-based augmentation system (SBAS)s) are designed to enhance the performance of standard global navigation satellite system (GNSSglobalnavigation satellite system (GNSS)) positioning. SBASs improve the positioning accuracy by providing corrections for the largest error sources. More importantly, SBASs provide assured confidence bounds on these corrections that allows users to place integrity limits on their position errors. Several systems have been implemented around the world and several more are in development. They have been put into place by civil aviation authorities for the express purpose of enhancing air navigation services. However, SBAS services have been widely adopted by other user communities, as the signals are free of charge and easily integrated into GNSS receivers.This chapter describes the basic architecture, functions, and application of SBAS. Because the key motivation behind SBAS is integrity, it is essential first to understand the error sources that affect GNSS and how they may vary with time or location. It is then explained how the corrections and confidence intervals are determined and applied by the user. The different SBASs that have been developed around the world are described and how they are developed to the same international standards such that each is interoperable with the others. The performances and services of each system are described. Finally, the evolution of SBAS from its current single-frequency single-constellation form into systems that support multiple-frequencies and multiple-constellations is described.The goal of this chapter is to explain the motivation for developing SBASs and provide the reader with a working knowledge of how they function and how they may be used to enhance GNSS positioning accuracy and integrity.

This chapter discusses the basic architecture of global navigation satellite system (GNSSglobal navigation satellite system (GNSS)) receivers. It starts with a breakdown of the receiver function into individual building blocks along the processing chain (front-end, down conversion, mixers, numerically controlled oscillators, correlators, tracking loops, data demodulation, navigation, user interface), and describes the respective functions. A dedicated section describes selected hardware solutions (example chipsets for front-end and baseband processing, offering different levels of integration and capabilities). Finally, receiver designs performing the signal processing in pure software as well as receivers based on configurable hardware are discussed.

In this chapter digital signal processing of a global navigation satellite system (GNSSglobal navigation satellite system (GNSS)) receiver is presented. It provides a high-level block diagram as well as detailed descriptions for all the internal functions of a modern digital GNSS receiver, focusing on signal acquisition and tracking, time synchronization, navigation data bit demodulation and decoding, and measurement generation. Also, several issues on the processing of upcoming GNSS signals, which may have new features like a binary offset carrier (BOCbinary offset carrier (BOC)) modulation, data/pilot channels, primary/secondary codes, and so on are addressed. Furthermore, advanced topics in designing modern digital GNSS receivers such as tracking of the global positioning system (GPSGlobal Positioning System (GPS)) P(Y)-code, various combined processing schemes, Kalman filter-based signal tracking loops, and a vector-tracking approach are also presented.

Multipathmultipath is the phenomenon whereby the signal from a satellite arrives at the receiver via multiple paths due to reflection and diffraction. These nondirect-path signals distort the received signal and cause errors in code and phase measurements. Differential techniques do not eliminate multipath and thus multipath is an important error source in high precision applications. The physical surroundings around the receiver’s antenna dictate the multipath environment and thus cause significant differences for land, marine, airborne, and spaceborne users.This chapter describes the multipath environment and presents models describing the impact of multipath on code and phase measurements. The influence of the type and rate of the broadcast code as well as the receiver architecture will be highlighted. Mitigation techniques based on receiver design will also be described along with the impact of receiver dynamics. Finally, a technique to measure multipath is described and its usage in evaluating static environments is discussed.The goal of this chapter is to provide the reader with the tools to assess the impact of multipath on both the code and phase and to understand the performance improvements and limitations associated with various multipath mitigation techniques.

Global navigation satellite system (GNSSglobal navigation satellite system (GNSS)interference) signals are so weak near the Earth’s surface that they can be easily squelched by natural or man-made interference. Moreover, the most popular GNSS signals – those offered with unrestricted access – are unencrypted and unauthenticated, which means they can be counterfeited, or spoofed. Strict international laws protect the radio frequency bands allocated to GNSS, but mother nature does not respect these laws, and man-made interference – whether accidental or intentional – is a growing concern.This chapter examines sources of GNSS signal interference and the interference effects on GNSS signal tracking. It offers a systematic treatment of natural, unintentional, and intentional interference, with emphasis on intentional jamming and spoofing. Theoretical performance bounds are developed for the simplest cases of narrowband and wideband interferences. The chapter finishes with a review of the state of the art in antenna-oriented and signal-processing-oriented interference detection and mitigation techniques.

The basic purpose of a global navigation satellite system (GNSSglobal navigation satellite system (GNSS)) user antenna is the reception of navigation signals from all visible GNSS satellites. Transmit antennas onboard the GNSS satellites, on the other hand, are quite different and employ large antenna arrays to create high-gain global beams illuminating the entire surface of the Earth.This chapter presents different design options for GNSS antennas operating in the L-band of the radio frequency spectrum. It starts with a brief discussion of key requirements for the GNSS receiving antenna, where several design parameters are introduced and explained. Thereafter, antennas of different design technologies suitable to GNSS are explored and discussed in detail. Following the introduction of major antenna candidates, different variants for specialized requirements, such as the small form factor or multipath mitigation are presented. Complementary to receiving antennas, the design of antenna arrays for signal transmission on the GNSS satellites is presented next, along with a discussion on specific antennas employed on the Global Positioning System (GPSGlobal Positioning System (GPS)), Galileo, Global’naya Navigatsionnaya Sputnikova Sistema (GLONASSGlobal’naya Navigatsionnaya Sputnikova Sistema (Russian Global Navigation Satellite System) (GLONASS)) and BeiDou satellites. Finally, a comprehensive discussion on antenna measurements and the performance evaluation is provided.

This chapter presents a review of a range of global navigation satellite system (GNSSglobal navigation satellite system (GNSS)) simulators and test equipment. Different types of systems are discussed, including radio frequency (RFradio frequency (RF)) and intermediate frequency (IFintermediate frequency (IF)) simulators; record and playback systems; and measurement simulators. The key features of each of these devices are examined, illustrating their various implementations, typical usage, and highlighting their individual benefits and drawbacks. The chapter concludes with an overview of considerations that should be borne in mind when selecting and using simulators and test equipment.

This chapter introduces the fundamental observation equations for multiconstellation global navigation satellite systems (GNSSglobal navigation satellite system (GNSS)s). It starts with an introduction of the basic observation equations for pseudorange, carrier-phase, and Doppler measurements. In the remainder of the chapter, the parameters used in modeling the basic observation equations are discussed. The parameters covered in the discussion are relativistic effects, atmospheric delays, the carrier-phase wind-up effect, antenna phase-center offset and variation, pseudorange and carrier-phase biases, and finally multipath errors and receiver noise.

This chapter introduces the concept of observation combinations, commonly used, for example, to compute positioning solutions with measurements from multiple frequencies or to study measurement noise, multipath, or ionospheric effects. Based on a generic parametrization for pseudorange and carrier-phase observations, a general expression for linear combinations is introduced. The impact of the coefficients on the properties and the noise of the combined observable is explained. The chapter covers combinations using measurements from a single satellite observed by one receiver. The discussion will then be extended to differential observations from two satellites, receivers and epochs.

The focus of this chapter is on the models for positioning. Since the global navigation satellite system (GNSS) observation equations are nonlinear in the position coordinates, the chapter is started with a section on the linearization of the observation equations for pseudorange (code) and carrier-phase. After that, absolute (point) positioning models are discussed, starting with the code-based single point positioning (SPP) model, followed by the model for precise point positioning (PPP), based on code and phase. The relative positioning models can be distinguished into code-dominated (differential GNSS or DGNSS) models and phase-dominated (real-time kinematic or RTK) models. For the latter type of models, a general multifrequency undifferenced model is presented, which may form the basis of both relative network model and the (absolute) model that enables PPP users to perform integer ambiguity resolution (PPP-RTK). After that the link is made between the undifferenced model and the single and double differenced versions of the positioning model and an overview is given of the various positioning concepts.

This chapter presents the estimation and filtering principles as used in global navigation satellite system (GNSSglobalnavigation satellite system (GNSS)) data processing. Estimation and filtering are concerned with retrieving or recovering parameters of interest from noisy measurements. The least-squares (LS) principle is the standard approach for estimating unknown parameters from uncertain data. Various forms of LS estimation, such as partitioned-LS, recursive-LS, constrained-LS, and nonlinear-LS, are discussed.The parameters of interest, as well as the dominant error sources, are often time varying. If these time variations can be modeled, the parameters can be resolved based on minimum mean squared error prediction, filtering, and smoothing techniques. Of the various such techniques, the Kalman filter is most prominent. It recursively estimates the state of a dynamic system. Different forms of the Kalman filter are discussed, together with its linkage to recursive smoothing techniques. Several GNSS examples are included in support of the general introduction on the principles and properties of LS estimation and Kalman filtering.

Global Navigation Satellite System (GNSSglobal navigation satellite system (GNSS)) carrier-phase integer ambiguity resolution is the process of resolving the carrier-phase ambiguities as integers. It is the key to fast and high-precision GNSS parameter estimation and it applies to a great variety of GNSS models that are currently in use in navigation, surveying, geodesy and geophysics. The theory that underpins GNSS carrier-phase ambiguity resolution is the theory of integer inference. This theory and its practical application is the topic of the present chapter.

Modeling errors, when passed unnoticed, may seriously deteriorate the final results of any estimation process. It is therefore of importance to have quality control procedures in place so as to be able to judge and validate the outcome of estimation. This chapter presents such methods for global navigation satellite system (GNSSglobal navigation satellite system (GNSS)) model validation and qualification. Since batch and recursive estimation are common methods of GNSS data processing, the validation and integrity monitoring of both will be discussed in this chapter.

Since its introduction in 1997, precise point positioning (PPPprecise point positioning (PPP)) offers an attractive alternative to differential global navigation satellite system (GNSSglobal navigation satellite system (GNSS)) positioning. The PPP approach uses undifferenced, dual-frequency, pseudorange and carrier-phase observations along with precise satellite orbit and clock products, for standalone static or kinematic geodetic point positioning with centimeter precision. This chapter introduces the PPP concept and specifies the required models needed to correct for systematic effects causing centimeter-level variations in the satellite-to-user range. For completeness, models and methods for processing single-frequency GNSS data are presented and specific aspects of GLONASSGlobal’naya Navigatsionnaya Sputnikova Sistema (Russian Global Navigation Satellite System) (GLONASS) (Global’naya Navigatsionnaya Sputnikova Sistema) and new GNSSs are also described. Furthermore, recent developments in fixing undifferenced carrier-phase ambiguities, which can considerably shorten or nearly eliminate the initial delay for PPP convergence, are highlighted. Existing web applications and real-time corrections services enabling post-mission and real-time PPP are presented. Finally, typical PPP precision and accuracy estimates are discussed, including the solution of station tropospheric zenith path delays and receiver clocks, with millimeter and nanosecond precision respectively.

This chapter describes the concepts of differential global navigation satellite system (DGNSSdifferential GNSS (DGNSS)) positioning focusing on practical details given that the fundamental concepts have been covered in prior chapters. The chapter starts with a review of the general concepts of DGNSS, including a quantitative discussion on the biases in DGNSS measurements. The next section focusses on code-based DGNSS positioning, presenting an overview of DGNSS services as well as a brief discussion on the format and latency of DGNSS corrections. A significant part of this chapter is devoted to carrier-phase dominated DGNSS, or real-time kinematic (RTKreal-time kinematic (RTK)) positioning. Besides a theoretical consideration that includes the Russian Global Navigation Satellite System (GLONASSGlobal’naya Navigatsionnaya Sputnikova Sistema (Russian Global Navigation Satellite System) (GLONASS)) and multi-GNSS RTK, the section provides examples of RTK positioning performance that are obtained in practice. The last section details on network RTK, which is an extension of the standard RTK technique to cover longer distances.

Attitude estimation is the process of determining the spatial orientation of an object. A system formed by multiple Global Navigation Satellite System (GNSSglobal navigation satellite system (GNSS)) antennas placed at known relative positions acts as an attitude sensor. This chapter provides an overview of practical applications of GNSS-based attitude determination, gives the principles of attitude representation and estimation, and reviews a constrained ambiguity resolution method to reliably fix the carrier-phase integer ambiguities and obtain precise attitude estimations.

This chapter discusses the role of global navigation satellite systems (GNSSglobal navigation satellite system (GNSS)s) and inertial measurements in the estimation of the state vector for a maneuvering system. The chapter considers the main objectives of accuracy, continuity, availability, and integrity; and, the contributions that the different types of sensors make toward achieving these objectives. The chapter includes an example design. Then, the chapter reviews the concepts of loose, tight, and ultratight or deeply coupled systems. Throughout, the advantages, disadvantages, and tradeoffs between alternative approaches are discussed.

This chapter presents an overview of applications of global navigation satellite systems (GNSSglobal navigation satellite system (GNSS)personalnavigation) relevant to the land and maritime environments. It focuses on the positioning performance requirements, technologies, developments, and trends for current and projected growth areas for GNSS in the land, rail, and maritime transport sectors. Representative applications including: personal navigation, location-based services, maritime, and land-based intelligent transport systems, railway logistics, and maritime operations are showcased as they encompass overlapping and related tasks such as fleet and asset monitoring, cooperative mobility, autonomous and precise navigation, vehicle and machinery control, and others.

The Global Positioning System (GPSGlobal Positioning System (GPS)) has been available for civilian use for the past three decades and is now extensively used in aviation to support multiple applications.This chapter describes how GNSS is used in aviation, the performance requirementsperformancerequirements that are being applied and the operational applications that have been enabled. It describes how conventional navigation has been gradually replaced by area navigation, the global introduction of Performance Based Navigation (PBNperformancebased navigation (PBN)integrity) and how the availability of GNSS has played a significant role in that evolution. The performance requirements for the different phases of flight are presented including the different methods by which the navigation integrity is ensured.The goal of this chapter is to provide the reader with an overview of how GNSS has been adopted in aviation and explain how it has been integrated onto the aircraft alongside other navigation systems. The regulatory and certification process is also described to introduce the mechanisms by which aircraft operators can get approval to use GNSS in their daily operations.

This chapter explains the fundamentals of ground-based augmentation systems (GBASground-basedaugmentation system (GBAS)GBASs). GBASs are fielded at airports to support civil aviation operations down to and including precision approach and landing. This chapter describes how GBAS generates differential corrections for Global Positioning System (GPSGlobal Positioning System (GPS)) pseudorange (L1 C/A-code) signals based on measurements taken at known (reference) locations, how reference measurements are monitored to protect against GPS and GBAS faults or anomalies, and what information is broadcast to users to support enhanced accuracy and integrity (or safety). The application of GBAS to civil aviation precision approach and landing is explained along with the key considerations in fielding GBAS reference equipment at airports. Augmentation systems that transmit additional global navigation satellite system (GNSSglobal navigation satellite system (GNSS))-like ranging signals to users are also briefly introduced.

Signals transmitted by global navigation satellite system (GNSSglobal navigation satellite system (GNSS)) satellites are not confined to the surface of the Earth but can likewise be used for navigation in space. Satellites in low Earth orbits, in particular, benefit from a similar signal strength and experience a full-sky visibility. On the other hand, the harsh space environment, long-term reliability requirements and the high dynamics of the host platform pose specific challenges to the design and operation of space-borne GNSS receivers. Despite these constraints, satellite manufacturers and scientists have early on started to exploit the benefits of GNSS technology. From the first flight of a Global Positioning System (GPSGlobal Positioning System (GPS)) receiver on Landsat-4, GNSS receivers have evolved into indispensable and ubiquitous tools for navigation and control of space vehicles.Following a general introduction, the chapter first describes the specific aspects of GNSS signal tracking in space and highlights the technological challenges of space-borne receiver design. Subsequently, the use of GNSS for spacecraft navigation is discussed taking into account both real-time navigation and precise orbit determination. Relevant algorithms and software tools are discussed and the currently achieved performance is presented based on actual missions and flight results. A dedicated section is devoted to the use of space-borne GNSS for relative navigation of formation flying satellites.The chapter concludes with an outlook on special applications such as spacecraft attitude determination, GNSS tracking of ballistic vehicles as well as GNSS radio science.

The International global navigation satellite system (GNSSglobal navigation satellite system (GNSS) ) Service (IGSInternational GNSS Service (IGS)) is an organization devoted to the generation of high-precision GNSS data and products; a service that benefits science and society. It is a voluntary federation of over 200 self-funding agencies, universities, and research institutions in more than 100 countries. Established in 1992 and formally launched on 1st January 1994, the IGS has delivered an uninterrupted time series of products that are utilized by a broad spectrum of users. IGS products have evolved over time, including the provision of GNSS data for constellations other than GPSGlobal Positioning System (GPS), and the addition of real-time GNSS data and products.This chapter provides an overview of the IGS, including a brief history and details of the current organization and its key components. The various products offered by the IGS are described and an outlook of future activities is given.

Many sophisticated Global Navigation Satellite System (GNSSglobal navigation satellite system (GNSS)) applications require high-precision satellite orbit and clock products. The GNSS orbits and clocks are usually derived from the analysis of tracking data collected by a globally distributed GNSS receiver network. The estimation process adjusts parameters for the satellite orbits, transmitter and receiver clocks, station positions, tropospheric delays, Earth orientation, intersystem and interfrequency biases, and carrier-phase ambiguities. The estimation requires detailed modeling of geophysical processes, atmospheric and relativistic effects, receiver tracking modes, antenna phase centers, spacecraft properties, and attitude control algorithms. This chapter describes precise orbit and clock determination of the GNSS constellations as performed by the analysis centers of the International GNSS Service, including models, estimation strategies, products, and the combination of orbit and clock solutions.

The Global Positioning System (GPSGlobal Positioning System (GPS)) became available as a civilian geodetic survey technology in the early 1980s. It has since revolutionized not only geodesy, but surveying operations as well. Global Navigation Systems (GNSSglobal navigation satellite system (GNSS)s) are today a fundamental tool for the land, engineering, and hydrographic surveyor. The majority of GNSS survey tasks relate to the determination of high-accuracy coordinates in a well-defined reference frame, typically using differential GNSS positioning techniques based on the analysis of carrier-phase measurements. Carrier-phase-based positioning is capable of distinct levels of accuracy – submeter, few decimeters, centimeter, and even subcentimeter – through a combination of special instrumentation, sophisticated software, and unique field operations. The evolution of GNSS from a geodetic surveying technology to a versatile surveying tool has seen precise positioning implemented in real-time, using ever shorter spans of measurements, and even when the user receiver is in motion. Furthermore, new techniques based on precise single-point positioning, as well as wide-area reference receiver networks, are starting to find wider use.

Continuous geodetic observations are fundamental to characterize changes ingeodesy space and time that affect the Earth system. The advent of global navigation satellite systems (GNSSglobal navigation satellite system (GNSS)s), starting with the Global Positioning System (GPSGlobal Positioning System (GPS)) in the early 1980s, has significantly increased the range of geodetic applications and their precision. Significant improvements have progressively been made in the GNSS software packages developed by research institutes, leading to the determination of high-precision geodetic parameters and their temporal variations. The proliferation of dense GNSS networks (local, national, continental and global), composed of continuously observing stations, allows for a variety of geodetic and Earth science applications. Most areas of science, Earth observation, georeferencing applications, and society at large, today depend on being able to determine positions to millimeter-level precision. Point positions, to be meaningful and fully exploitable, have to be determined and expressed in a well-defined reference frame. All current global and regional reference frames rely on the availability of the international terrestrial reference frame (ITRFInternational Terrestrial Reference Frame (ITRF)Reference FrameInternational Terrestrial), which is the most accurate realization of the international terrestrial reference system (ITRSInternational Terrestrial Reference System (ITRS)Reference SystemInternational Terrestrialglobalgeodetic observing system (GGOS)). One of the major modern achievements in geodesy today is the ability to determine highly precise global and regional terrestrial reference frames based on GNSS observations, fully connected to the ITRF. This chapter describes the use and applications of GNSS in geodesy, focusing on its role in the International Association of Geodesy’s (IAG’s) global geodetic observing system (GGOS) for monitoring our planet in space and time, GNSS-based reference frame implementation, Earth rotation and sea level monitoring.

Geodynamicgeodynamic studies rely on measurement of motions over time, such as displacements, displacement time series, or velocities for those sites that move steadily with time. Global navigation satellite systems (GNSSglobal navigation satellite system (GNSS)s) are widely used for geodynamics research, including studies of tectonic plate motions and plate boundary deformation, earthquakes and seismology, volcano deformation, surface loading deformation, and glacial isostatic adjustment. GNSS is an ideal tool for these studies because it can provide time series of millimeter-precision positions using inexpensive, portable and easily deployed equipment. This chapter illustrates and summarizes the important concepts and the basic computational models used to relate active processes within the Earth to surface deformation that can be observed using GNSS. These include conceptual models for the earthquake cycle, elastic dislocation theory, the Mogi volcanic source model, and surface loading computations. The chapter also summarizes important research results in all of these topics. Rapid and real-time applications of GNSS to use surface deformation for earthquake and tsunami warning are growing, and are likely to become even more important in the future, as will multi-GNSS observations to provide greater measurement accuracy.

Global navigation satellite system (GNSSglobal navigation satellite system (GNSS))-based atmosphere sounding techniques have become a widely recognized and operationally used remote sensing tool. A major milestone of this development was the beginning of the continuous use of GNSS data for improving regional and global forecasts in 2006. The principle behind these techniques is the utilization of atmospheric propagation effects on the GNSS signals on their way from the navigation satellites to receivers on the ground or aboard satellites. The atmosphere delays the time of arrival and introduces a curvature of the signal path. These effects can be accurately estimated and be used for the monitoring of the atmospheric variability. There are two different observation geometries. Therefore, we focus in the first part of this chapter on ground-based networks which are used to estimate the amount of water vapor above each receiver site. The second part deals with the use of radio occultation measurements from GNSS receivers aboard low Earth orbit satellites for global atmosphere sounding. We introduce and describe both techniques which provide observations suitable for the short-term weather forecasting and the long-term time series for climate research and monitoring.

Global navigation satellite system (GSSS)-based monitoring of the ionosphere is important in a twofold manner. Firstly, GNSS measurements provide valuable ionospheric information for correcting and mitigating ionospheric range errors or to warn users in particular in precise and safety of life (SoL) applications. Secondly, spatial and temporal resolution of ground- and space-based measurements is high enough to explore the dynamics of ionospheric processes such as the origin and propagation of ionospheric storms.It is discussed how ground- and space-based GNSSglobal navigation satellite system (GNSS) measurements are used to create global maps of total electron content (TECtotal electron content (TEC)) and to reconstruct the highly variable three-dimensional (3-D) electron density distribution on global scale under perturbed conditions. Thus, the monitoring results can be used for correcting ionospheric errors in single-frequency applications as well as for studying the driving forces of space weather-induced perturbation features at a broad range of temporal and spatial scales. Whereas large- and medium-scale perturbations affect accuracy and reliability of GNSS measurements, small-scale plasma irregularities and plasma bubbles have a direct impact on the continuity of GNSS availability by causing strong and rapid fluctuations of the signal strength, known as radio scintillations.It is discussed how better understanding of space weather-related phenomena may help to model and forecast ionospheric behavior even under perturbed conditions. Hence, ionospheric monitoring contributes to the successful mitigation of range errors or performance degradation associated with the ionospheric impact on a broad spectrum of GNSS applications.

This chapter discusses the use of properties of global navigation satellite system (GNSSglobal navigation satellite system (GNSS)global navigation satellite system (GNSS)reflectometry (GNSS-R)multistatic radar) signals after their reflection on the Earth’s surface. Global navigations satellite system reflectometry (or GNSS-R) is a multistatic radar that uses the GNSS constellations to extract information on the properties of the reflecting surfaces. Experiments have demonstrated that useful information can be extracted from such reflected signals. GNSS-R instruments have been installed in ground and coastal platforms, aircraft, stratospheric balloons, and spacecrafts. As a natural consequence it has been proposed by space agencies for the deployment of dedicated space missions. In the first part of this chapter the properties of the GNSS reflected signals on different components of the Earth’s surface are discussed, and the technical principles sustaining different types of GNSS-R instruments are presented. The second part of this chapter presents methods to retrieve geophysical information from the GNSS-R signals, results obtained in different experiments and plans for future space missions.

Time and navigation are intimately linked and rely on each other. Global navigation satellite system (GNSSglobal navigation satellite system (GNSS)) positioning is based on the measurement of time intervals needed by the signal to travel from satellites to the receiving station on the Earth or nearby. The precision of GNSS positioning is reached thanks to atomic frequency standards onboard the satellites and the possibility to determine their synchronization differences at the subnanosecond level. Time is thereby the core of GNSS. Inversely GNSS is widely used for accurate time and frequency dissemination, as well as for the comparison of distant clocks as needed for time and frequency metrology. All these aspects of using GNSS for time/frequency applications will be presented in this chapter.