Though GNSS satellites are just some among many other satellites which are implemented for geophysical measurements, GNSS play a very significant and increasing role in collecting geophysical data. They have become an indispensable tool for many geophysical applications. Apart from this, GNSS satellites have also allowed spread spectrum radiowave technology to mature. This technology can be used on its own for atmosphere probing in other parts of the electromagnetic spectrum than GNSS’s. Together with other atmosphere sensing instruments, GNSS provides information for weather prediction and monitoring climate change. GNSS has also become an essential tool for estimation of the Earth’s rotation parameters. There are also emerging areas of paramount importance, which are yet under development, such as using GNSS signals for earthquake studies. Figure 11.1 depicts the subject of this chapter in relation to other chapters in this book.

Data from global reference networks are routinely used to monitor movement of the Earth’s tectonic plates relative to each other and to an inertial frame. In this task the same type of GNSS observables as described in Chapter 8 are used. Such observables are created using measurements from globally distributed networks of GNSS receivers, supplying various data in raw data formats, mostly in RINEX. The most prominent among such networks is the IGS network. There are also various regional and local networks, which can provide large volumes of geophysical information. Among such networks, for example, is the Geographical Survey Institute (GSI) network in Japan, which consists of more than 1200 high-end geodetic GNSS receivers. The data from these networks include code and carrier phase measurements, Doppler measurements, and signal-to-noise ratio. All measurements are delivered from dual frequency receivers using two frequencies (L1, L2). We consider implementation of data from such networks in Sections 11.1, 11.3, and 11.4.