Geodesy is the science of the study of Earth’s shape, gravity, and rotation. These properties change with time because Earth is a dynamic system comprising fluid atmosphere and oceans, mobile tectonic plates and active geological faults, changing distributions of ice, snow, surface and ground water, and numerous deep Earth processes.
Earth “shape” can be considered synonymous with “position” – the mathematical description using “3D coordinates” of the location of any point on or above the Earth’s surface with very high accuracy – for which GNSS is ideally suited. The focus of modern geodesy is the monitoring of changes in coordinates with time. This requires the definition, realization, and maintenance of high-fidelity, high-accuracy geodetic reference systems and their associated practical realizations as Terrestrial Reference Frames (TRF)– the one most relevant being the International Terrestrial Reference Frame (ITRF).
In the context of this chapter, GNSS geodesy comprises the following:
· Precise GNSS positioning methodologies – mathematical modeling of carrier-phase observations, operational procedures for determining sub-centimeter accurate coordinates, the hardware that track the GNSS signals, the measurement processing software, and so on.
· Ground infrastructure used to augment GNSS – extensive ground networks of permanent GNSS receivers which substantially improve GNSS positioning accuracy, and which, in addition, are vital to both the definition of national TRFs as well as providing a means of connecting to the ITRF.
· GNSS services to support GNSS geodesy – to overcome systematic errors of GNSS positioning such as satellite orbit and clock errors, satellite signal biases, and atmospheric signal disturbances, such as provided by the International GNSS Service (IGS).
· TRFs defined at the global, regional‚ or national level in which the precise coordinates are expressed.
This chapter is organized into three parts. The first part is an introduction to the principles of GNSS geodesy. The International Association of Geodesy (IAG) is the scientific association that organizes the space geodetic services that support GNSS geodesy. An important IAG services is the IGS, as discussed in the second part of the chapter. The third part describes one of the most important products of GNSS geodesy, the ITRF.
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