The Global Navigation Satellite System (GNSS): Positioning, Velocities, and Reflections

Since the inception and realization of the Global Positioning System (GPS) in the 1970–1980s, the Global Satellite Navigation System (GNSS) has become a ubiquitous tool in civil, business, and scientific life. Major breakthroughs in our understanding of dynamic Earth processes were only achievable through this precise positioning technology. While positioning is the chief objective of the system, the nature of its design requires satellite signals to traverse the ionosphere and the troposphere and results in signal reflections off the ground. In addition to tracking crustal dynamics, this system design enables the study of the atmosphere and local environmental sensing, impacting fields far beyond solid Earth research, including space physics, atmospheric science, glaciology, hydrology, and natural hazards. In this chapter I review some of the history of this technology and its impact on the Earth sciences. Using the example of GPS, I introduce how satellite positioning systems work and how we can infer precise positions from the signals broadcast by the satellites. For this, I give an overview on reference systems, different observation models, the predominant precise positioning strategies, and how the various error terms can be corrected. Once a solid understanding of precise positioning is developed, I present some of the complications that arise in high-rate (1 or more sample per second observations) subdaily and real-time kinematic positioning, which is of great utility in the characterization and monitoring of many natural hazards. GNSS enables observations beyond precise positioning. I provide background and observation models for instantaneous velocity estimations, useful in real-time applications particularly where precise orbits and initial positions are not available, and GNSS reflectometry, which allows to perform local environmental sensing around GNSS monuments, including the inference of snow depth or tidal heights. Throughout the chapter, each method is illustrated by a number of applications from either the literature or novel work. The focus is on some highlights from the last decade of geodetic work, with a slant toward examples from solid Earth and hydrologic research.