middle of continents moves up and down during the day by over 30 centimeters in response to lunar and solar tides; tectonic plates shift and collide; earthquakes and volcanoes disrupt the landscape by many meters; storms batter shorelines; ocean currents, hurricanes, and monsoons move enormous masses of air and water around the planet; and deep in the Earth’s mantle and core, convection cells move continents and power the geodynamo, which generates our protective magnetic field. Instead of spinning smoothly and steadily like a well-balanced top, the Earth wobbles in complex ways, and its spin rate (and, as a result, the length of day) changes over time scales as short as hours, while slowing down over long periods of time.1 To account for this continuous movement, we must continuously redraw the reference lines, and in turn continuously recalculate our position on Earth.

High-precision geodesy helps us to quantify and respond to local and regional problems by allowing us to “see” what we cannot sense directly. For example, the depletion of underground aquifers or oil and gas reserves can cause local subsidence, or sinking of the land, disrupting sewers and other underground utilities. On a much larger scale, geodesy enables us to monitor global climate change as it is reflected in ice sheet melting and sea level change. These trends, which only can be measured precisely with geodesy, ultimately may have significant—or potentially catastrophic—impacts, causing loss of life and billions of dollars of damage to homes, businesses, and the environment.

WHAT IS THE GLOBAL PRECISE GEODETIC INFRASTRUCTURE?

Geodesy is the science of measuring and understanding three fundamental parameters of the Earth—its shape, rotation and orientation, and gravity field—and their change over time. These parameters carry fundamental information about the planet and its workings. Today this is no longer a three-dimensional problem, but really a four-dimensional problem in which temporal changes in these quantities are tracked. Geodesists do this using an infrastructure based on precisely located positions of a set of reference (“fiducial”) points on the Earth’s surface. Using these reference points, geodesists create a terrestrial reference system (or spatial reference system)—a common coordinate framework for which scientists have determined, by calculation, all the reference points’ exact coordinates at a given time. The primary realization of the global spatial reference system is the International Terrestrial Reference Frame (ITRF). The ITRF and other terrestrial reference frames are established by equipping selected reference points with some combination of radio telescopes, laser ranging systems, Global Navigational Satellite Systems receivers (GNSS, a general term for systems like the Global Positioning System, or GPS) and radio beacons, and sometimes gravimeters. In addition, data from observations of Earth-orbiting satellites, the moon, and distant extragalactic objects known as quasars are incorporated. This combination of ground-based instruments and satellites constitutes the precise, global geodetic infrastructure.

A common spatial reference frame is both an important theoretical concept and a practical tool. Using geopositioning, one can locate a point or an object as it moves within a terrestrial reference frame. Beyond navigation, the ability to track the real-time location of mobile devices equipped with built-in GNSS/GPS receivers has sparked rapid growth in location-based services. Utility companies, for example, equip field crews with smart phones, enabling real-time access to dynamic maps of underground cables and pipes that can be updated “on the fly” as workers move around a field site. Developments in location-based services may drive revenues of more than $12.7 billion by 2014, according to a report published by Juniper Research (Wauters, 2010). The accuracy of geopositioning, and the scientific and societal applications that rely on it, depend on the continued existence of a reliable technological and scientific infrastructure.

1

Keeping time of day, measured by the rotation of the Earth relative to the Sun, synchronized with more accurate atomic clocks is the reason for introducing the occasional “leap second.”



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