Contents

9.1 Introduction 166

9.2 How GPS Works 166

9.3 The Main Components of GPS 167

9.4 GPS Signal Characteristics 168

9.5 The Primary Error Sources 170

9.5.1 Atmospheric and Environmental Effects 170

9.5.1.1 Ionosphere and Troposphere Delays 170

9.5.1.2 Relativistic Propagation Error 172

9.5.1.3 Multipath 172

9.5.2 Satellite and Receiver Clock Errors 172

9.5.3 Orbital Errors 172

9.5.4 Receiver Noise, Interchannel Bias, and Other Instrumental Biases 173

9.5.5 Antenna Phase Center Location 174

9.5.6 Reference Station Error 174

9.5.7 Number of Visible Satellites and Their Geometry 174

9.5.8 Interference and Jamming (Intentional Interference) 175

9.5.9 Intentional Degradation of the Satellite Signal 175

9.6 Mathematical Models of Pseudorange and Carrier Phase Observables 176

9.7 Positioning with GPS 179

9.7.1 Point versus Relative Positioning 180

9.7.1.1 Point (Absolute) Positioning 180

9.7.1.2 Relative Positioning 180

9.7.2 DGPS Services: An Overview 182

9.7.2.1 DGPS Services: Examples 183

9.7.2.2 DGPS Message Format 184

9.7.3 Network-Based Real-Time Kinematic GPS (RTK GPS) 187

9.7.4 Precise Point Positioning 189

9.8 Real Time versus Postprocessing 190

9.9 How Accurate is GPS? 191

9.10 GPS Instrumentation 192

9.11 GPS Modernization and Other Satellite Systems 194

9.12 GPS Mapping Project and Connection to Agricultural Geophysics 194

Acknowledgments 196

References 196

9.1 INTRODUCTION

Much of the geophysics equipment used today for agricultural applications has the capability to be integrated with Global Positioning System (GPS) receivers so that positional data can be obtained at the same time geophysical measurements are collected. Integration of GPS with resistivity and electromagnetic induction equipment already allows soil electrical conductivity to be mapped over large farm fields in just a few hours time. The GPS is a satellite-based, all-weather, continuous, global radionavigation and time transfer system, designed, financed, deployed, and operated by the U.S. Department of Defense (DOD). The system was originally intended for military applications, but in the 1980s, the U.S. government made it available to civilian users, free of any subscription fees or setup charges. The first GPS satellite was launched on February 22, 1978, and in 1993, the system was declared fully operational. During the following years, the breadth and the scope of the GPS applications exploded in the civilian market, taking advantage of the systems' full and sustained operability. As a result, radionavigation-based positioning and tracking is currently ever-present in a number of science, engineering, mapping, and everyday life applications.

GPS is an example of a ubiquitous technology responsible for the paradigm shift in contemporary navigation, positioning, surveying, and mapping techniques. As a result of progressive innovation and a significant drop in the price of the equipment in the last decade, GPS technology currently supports a variety of applications. These applications range from precise positioning, cadastral mapping and engineering, to remote sensing, environmental and GIS (geographic information system) surveys, and law enforcement. GPS radionavigation signals are used to navigate spaceships, aircrafts, and land-based vehicles, including transportation fleets and emergency cars, and to guide and track individual pedestrians. GPS is also used to monitor wildlife, track the race cars and paragliders, and monitor deformation of large structures. The system has also been effectively used in traffic monitoring, location-based services (LBS) and, in recent years, in monitoring the space weather and tropospheric conditions; and the number of new applications is still growing.

From the standpoint of agricultural geophysics, GPS is essential in providing a means for geo-registration (geolocation) of the ground-based sensors used to monitor soil conditions and locate subsurface features. This chapter will introduce the primary definitions, concepts, and mathematical models related to GPS positioning and sensor orientation applications. The overview of GPS design, implementation, and modernization is provided, followed by the primary positioning modes and associated mathematical models, including the real-time kinematic (RTK) and differential GPS (DGPS) concepts. The primary error sources, positioning accuracy, and basic aspects of GPS instrumentation are also addressed.

9.2 HOW GPS WORKS

The primary concept related to navigation with satellites is triangulation in space. The GPS satellites that serve as a space-based reference for the user's positioning solution transmit a continuous signal toward Earth during their 12-hour revolution around the globe. GPS receivers use signals from multiple satellites to determine the distances (or ranges) to the satellites that are subsequently used to triangulate the user's position coordinates (see Figure 9.1). The range observation is recovered by measuring the travel time of the signal between the satellite and the user's receiver. To perform a positioning or a navigation task, a GPS receiver must be locked onto the signal of at least three satellites to calculate a two-dimensional (2D) position (latitude and longitude); with four or more satellites in view, the receiver can determine three-dimensional (3D) position coordinates (latitude, longitude, and height) of the user. If continuous lock to multiple satellites is maintained, the receiver can provide an uninterrupted position solution, as well as additional information, such as speed, bearing, distance traveled, and distance to destination. The receiver can even provide location and directions to the nearest post office or gas station if it is equipped with suitable GIS databases and digital maps.

Unknown location of receiver r

Unknown location of receiver r

FIGURE 9.1 Determination of the position of user r by triangulation, using range measurements P to multiple satellites.

The concept of GPS technology was formulated with the following primary objectives:

Suitability for all classes of platforms, such as spaceborne, airborne, marine, land-based, and individual pedestrian, under a wide variety of dynamics

Availability any time, any weather, anywhere on Earth and its vicinity

Real-time positioning, velocity, and time determination capability

Providing the service to an unlimited number of users worldwide

Positioning on a single global geodetic datum (World Geodetic System, WGS84)

Redundancy provisions to ensure the survivability of the system

Restricting the highest accuracy to a certain class of users (military)

Low cost and low power users' unit

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