Modern geodesy as discussed in this volume started with the development of distance measurement using propagating electromagnetic signals and the launch of Earth-orbiting satellites. With these developments, space-based geodesy allowed global measurements of positions, changes in the rotation of the Earth, and the Earth’s gravity field. These three areas (positioning, Earth rotation, and gravity field) are considered the three pillars of geodesy. The accuracy of current measurement systems allows time variations to be observed in all three areas. Also the complexity of problems is such that each of the pillars interacts with each other and also with many other branches of Earth Science. This interaction is most apparent in the role that water plays in modern geodetic measurements. Every chapter in this volume mentions the role of water. It is critical because it can move rapidly and over large distances; it can exist in all three phases, gas, fluid, and solid; and modern geodetic methods are accurate enough that their measurements are sensitive to its effects. In its vapor form, its refractive properties delay microwave signals propagating through the Earth’s atmosphere. For geodetic positioning, this is a noise source but it is a signal for metrological applications. In the liquid form, it forms oceans that affect both the tidal signal and the rotation of the Earth. Also in liquid form, its mass changes the gravity field as it is moved through the hydrologic cycle. In solid form, it has a gravitational and deformation signal that changes if melting of the ice unloads the surface of the Earth. The interactions between the pillars include the elastic loading effects of changing mass loads that can be seen in the gravity field and in the positions of ground stations. The movement of water to and from the oceans can be seen with altimeter satellites whose orbital information is derived from measurements from ground stations whose positions are affected by the changing mass load. In modern, time-dependent geodetic data analysis these interactions need to be accounted for. The common interface between the geodetic methods is the coordinate systems and reference frames used to analyze data. Coordinate systems and the associated reference frames form a core theme of the chapters in this volume. Two other unifying themes are the measurement systems of geodesy that are used again throughout the volume, and interplay of errors in measurements, signals, and noise. In this chapter weexamine these three themes.
