1
Overview of the Global Positioning System

1.1 Introduction

A global navigation satellite system (GNSS) is a complex global network consisting of three segments: the space segment (satellite constellation), the control segment (ground‐based tracking stations), and the user segment (receiver equipment). The purpose of such a system is to obtain highly accurate positioning information of a receiver on the surface of the Earth.

The system operates via the use of a constellation of satellites. These satellites orbit in a configuration such that several of them are visible from any point on Earth at any given time. Each satellite continuously transmits a coded signal, which includes information that uniquely identifies the transmitting satellite as well as its location in space. A receiver on the surface of the Earth may pick up one of these signals and use it to determine the distance between its corresponding satellite and the receiver. Since an electromagnetic signal travels almost exactly at the speed of light, , through Earth's atmosphere, it is possible to calculate this distance by simply multiplying the time between transmission and reception by .

Using multiple signals simultaneously, the receiver may trilaterate its position by determining the , , and coordinates of its location on the surface of the Earth. In theory, only three satellites are needed to determine a receiver's position, since we appear to have only three unknowns: , , and . However, the clock in the receiver that is used to determine the time of reception is not synchronized to the satellite clocks. Since the distance calculation is dependent on the signal's travel time, any error in the timestamp at the moment of reception will translate to an error in the calculated distance as well. This receiver clock error is a fourth unknown in our system of equations, so we actually need a fourth satellite to determine our solution. A more rigorous mathematical analysis of this process will be explored in Section 2.4.

Several independent GNSS networks are currently in operation, including the United States' Global Positioning System (GPS), Russia's Globalnaya Navigazionnaya Sputnikovaya Sistema (GLONASS), and the European Union's Galileo. Several regional navigation satellite systems are also currently operational, such as China's BeiDou (planned to be expanded to a global network), India's Navigation with Indian Constellation (NAVIC), and Japan's Quasi‐Zenith Satellite System (QZSS).

Since its inception, GNSS has become a ubiquitous technology with far‐reaching applications. Among other things, GNSS has been applied to navigation, surveying and mapping, precision agriculture, logistical supply chain management, weather prediction, emergency and relief services, seismology, and even facilitation of stock trades. A brief discussion of some applications is presented in Section 1.2. It is clear that GNSS has played a significant role in shaping the modern world, and will likely continue to expand in utility as further improvements are made and new applications are devised.

Maintenance and improvement of GNSS is a tremendous global endeavor, resulting from the collaboration of numerous scientists and engineers in disparate fields, including aerospace engineering, electrical engineering, astronomy, electromagnetics, and geophysics. As a highly multidisciplinary field of study, the literature on GNSS is correspondingly broad. It is infeasible for an individual to be an expert in every facet of GNSS, and one may find the literature of a neighboring field impenetrable without at least a cursory understanding of its fundamental concepts.

The primary focus of this book is the atmospheric effects on the radiowave signals used by GNSS. In order to appreciate the challenges posed to GNSS positioning by atmospheric propagation effects, a basic understanding of GNSS fundamentals is necessary. To that end, this chapter and Chapter 2 aim to provide an overview of important concepts, many of which will be referenced throughout the remainder of this book.

This chapter's focus is a concrete example of an existing GNSS system: the United States' GPS. In particular, the system‐level design and logistics of operating the GPS service will be covered. Although the implementation details of each GNSS differ, the broad design considerations relevant to GPS are typical and so serve as a suitable example for GNSS design in general. Section 1.3 discusses the three major subsystems of GPS, also known as GPS segments. They are the space segment (the constellation of satellite vehicles), the control segment (a network of ground stations for monitoring and control), and the user segment (receivers which decode GPS signal data and provide services such as positioning and timing).

Section 1.4 briefly introduces the terminology related to the Keplerian orbits (a type of trajectory that includes GPS satellite orbits). This is relevant when discussing satellite ephemeris data, which is information encoded into a GPS signal regarding current and predicted trajectories of satellites in the constellation. Ephemeris data is part of the so‐called Navigation Message, which is discussed in Section 1.5 alongside other aspects of the GPS communication subsystem such as broadcast frequencies, modulation scheme, and ranging codes.

1.2 Applications of GNSS

The applications of GNSS are multitudinous and varied, and over the last few decades numerous industries have seen tremendous innovation by leveraging the positioning and timing information GNSS provides [2].

1.2.1 Applications of Standard GNSS Positioning

One of the most common uses of GNSS is for navigation. The average person is no doubt familiar with the utilization of GNSS for driving directions, but there are many other navigation applications as well. For example, aviation has been dramatically improved by GNSS. Accurate positioning information allows for safer takeoffs and landings by providing air traffic controllers with real‐time information on the locations of multiple aircraft. Pilots may also use positioning information to land in remote locations with limited ground‐based services, such as a medical evacuation helicopter attempting to rescue people from a mountain. GNSS has also been used to optimize flight routes, reducing travel time, and costs. Analogous marine applications have also been sought, providing efficient routes for ships and ensuring safe arrival and departure at ports. GNSS may even be used to aid spacecraft launch, and is helpful for monitoring the orbits of other space‐based vehicles such as telecommunication satellites.

Another major application of GNSS is surveying and mapping. Quickly and readily available positioning information has allowed surveyors to model the Earth to an unprecedented degree of accuracy and completeness. Prior to the advent of GNSS, surveying techniques required a line of sight from a ground station to the receiver, limiting their functionality to areas with good vantage points (such as hilltops or mountains), or wide open areas free of obstructions (such as plains). Conversely, as long as a GNSS receiver has a good view of the sky, it is able to communicate with satellites to determine its position. This expanded functionality allows surveyors to map many more areas, such as valleys, cities with tall buildings, and even coastal areas with limited access to land‐based reference stations.

GNSS has even aided in mapping the ocean floor, though indirectly. Ships utilize GNSS information to determine their location on the surface of the ocean, and sonar is used to map the local topography of the ocean floor and any submersed structures. This information is tremendously important for mariners, as it can alert them to the location of shoals or hazardous obstacles that must be avoided. Additionally, underwater construction projects such as bridge piers or oil rigs rely heavily on GNSS.

1.2.2 Applications of Centimeter and Millimeter‐Level Positioning Accuracy

A single GNSS receiver is able to determine its location on the surface of the Earth to an accuracy of less than 10 m. Through the use of differential positioning or real‐time kinematic (RTK) positioning, in conjunction with error correction techniques, it is possible to achieve centimeter or even millimeter‐level accuracy. Sources of error as well as mitigation techniques will be discussed in Chapter 2.

Centimeter‐level accuracy has allowed farmers to employ precision agriculture, in which entire fields of crops may be planted, watered, and harvested by autonomous vehicles. Additionally, this level of accuracy allows for better control of pesticide, herbicide, and fertilizer dissemination, producing a higher crop yield for the farmer as well as mitigating damage to the environment.

Construction has also seen an increase in automation from centimeter‐level GNSS positioning. Autonomous vehicles are able to bulldoze or grade an area to specified dimensions, or lay down concrete such as for curbs or sidewalks. This level of automation reduces time, cost, and the number of workers required to complete a construction project.

Self‐driving cars would also be infeasible without centimeter‐accurate GNSS. These positioning measurements, in conjunction with light detection...