The Contribution of GNSS CORS Infrastructure to the Mission of Modern Geodesy

Geodesy is the science of measuring and mapping the geometry, orientation and gravity field of the Earth including the associated variations with time. Geodesy has also provided the foundation for high accuracy surveying and mapping. Modern Geodesy involves a range of space and terrestrial technologies that contribute to our knowledge of the solid earth, atmosphere and oceans. These technologies include: Global Positioning System/Global Navigation Satellite Systems (GPS/GNSS), Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI), Satellite Altimetry, Gravity Mapping Missions such as GRACE, CHAMP & GOCE, satelliteborne Differential Interferometric Synthetic Aperture Radar (DInSAR), Absolute and Relative Gravimetry, Precise Terrestrial Surveying (Levelling & Traversing). A variety of services have been established in recent years to ensure high accuracy and reliable geodetic products to support geoscientific research. The reference frame defined by Modern Geodesy is now the basis for most national and regional datums. Furthermore, the GPS/GNSS technology is a crucial geopositioning tool for both Geodesy and Surveying. There is therefore a blurring of the distinction between geodetic and surveying GPS/GNSS techniques, and increasingly the ground infrastructure of CORS receivers attempts to address the needs of both geodesists and other positioning professionals. Yet Geodesy is also striving to increase the level of accuracy by a factor of ten over the next decade in order to address the demands of “global change” studies. The Global Geodetic Observing System (GGOS) is an important component of the International Association of Geodesy. GGOS aims to integrate all geodetic observations in order to generate a consistent high quality set of geodetic parameters for monitoring the phenomena and processes within the “System Earth”. Integration implies the inclusion of all relevant information for parameter estimation, implying the combination of geometric and gravimetric data, and the common estimation of all the necessary parameters representing the solid Earth, the hydrosphere (including oceans, ice-caps, continental water), and the atmosphere. This paper will describe the background to the establishment of GGOS, and discuss the important role to be played by GPS/GNSS infrastructure in realising the GGOS mission. The Mission of Geodesy According to Helmert’s classical definition, geodesy is the “science of the measurement and mapping of the Earth’s surface” by direct measurements, such as terrestrial triangulation, levelling, and gravimetric observations, and, in the past 50 years, also with space techniques, based primarily on the tracking of a wide range of artificial Earth satellites. Following Seeber (2003), the primary mission of Geodesy can be defined as: • Establishment of geodetic reference frame and determination of precise global, regional and local 3D positions, • Determination of the Earth’s gravity field and its related models, such as geoid, • Measurement and modelling of geodynamical phenomena, such as crustal deformation, polar motion, Earth rotation, tides, etc. Since the launch of the first artificial satellite, SPUTNIK, in 1957, geodesy has evolved into a combination of a number of geosciences and engineering sciences. Several important research discoveries gave rise to the development of satellite geodesy. For example, by analysing SPUTNIK’s radio-signals it was discovered that the observed Doppler shift could be converted to a useful navigation information if the satellite’s location in space were known. An increasingly important contribution of satellite geodesy is satellite-based navigation, facilitated through radio-navigation signals transmitted by Earth orbiting satellites as in the Global Positioning System (GPS). GPS is now not only an indispensable tool for space geodesy, but it has also revolutionised surveying and navigation, and is increasingly used for personal positioning. Nowadays the phrase “Global Navigation Satellite System” (GNSS) is used as an umbrella term for all current and future global radio-navigation systems. Although GPS is currently the only fully operational GNSS, the Russian Federation’s GLONASS is being replenished and will be fully operational by 2010, the European Union’s GALILEO will be deployed and operational by 2013, and China’s COMPASS is likely to also join the “GNSS Club” by the middle of the next decade. The International Association of Geodesy (IAG – http://www.iag-aig.org) has established services for all the major satellite geodesy techniques: International GNSS Service (IGS – http://www.igs.org), International Laser Ranging Service (ILRS – http://ilrs.gsfc.nasa.gov), and the International DORIS Service (IDS – http://ids.cls.fr), as well as for the International Very Long Baseline Interferometry Service (IVS – http://ivscc.gsfc.nasa.gov/). These services generate products for users, including precise orbits, ground station coordinates, Earth rotation values, and atmospheric parameters. All have networks of ground tracking stations that also are part of the physical realisation of the International Terrestrial Reference System. For satellites to serve as space-based reference points, their trajectories (or orbits) must be known. The precise computation of orbits is accomplished by geodetic techniques combined with orbital mechanics. Precision Orbit Determination (POD) also requires the use of accurate geodetic models for gravity, tides and geodetic reference frames. At the same time, POD’s sensitivity to various types of geodetic information makes it a powerful tool in the refinement of geodetic models. Consequently, a satellite can be considered as a probe or sensor, moving in the Earth’s gravity field, along an orbit disturbed by the gravitational attraction. Thus, measurements of directions, ranges, and range-rates between terrestrial tracking stations (and sometimes also from other orbiting satellites) and the satellite “targets” provide, in addition to navigation parameters, a wealth of information about the size and the shape of the Earth and its gravity field. The first example of gravitational mapping by the use of satellites was the 1958 determination of Earth flattening from measurements to the EXPLORER-1 and SPUTNIK-2 satellites, followed by the discovery of the pear-shape of the Earth in 1959. The latest generation of gravity mapping missions, CHAMP, GRACE and GOCE, now are determining the features of the gravity field down to 100km or so in size, on a monthly basis, monitoring changes in the gravity field with time due to mass transports such as due to the Water Cycle. This is an exciting time for gravimetric geodesy with the recent establishment of the International Gravity Field Service (IGFS – http://www.igfs.net) and the release of the most detailed and precise model, EGM2008, of the Earth’s gravity field ever. Aside from their navigation and precise positioning function, geodetic satellites serve as remote sensing tools. An example is satellite radar altimetry, a remote sensing technology that can measure the ocean surface topography (TOPEX/Poseidon, Jason-1, etc) or ice topography (e.g., ICESat, CRYOSAT). Variations in ocean surface topography indicate gravitational variations due to undersea features, such as seamounts or trenches, as well as ocean circulation features from small eddies to basin-wide gyres. The altimeters can also be used to measure parameters such as wave height, wave direction, and wave spectra. Other geodetic remote sensing tools include differential interferometric synthetic aperture radar (DInSAR) satellites such as TerraSAR-X, ALOS, Radarsat-2, and others. These can detect changes in the shape of the topography as small as a centimetre with spatial scales of just a few metres. Modern Geodesy is now equipped with an array of space technologies for mapping (and monitoring changes) the geometry of the surface of the solid Earth and the oceans, as well as its gravity field. However the fundamental role of geodesy continues to include the definition of the terrestrial and celestial reference systems. These reference systems are the bedrock for all operational geodetic applications for national mapping, navigation, spatial data acquisition and management, as well as the scientific activities associated with geodynamics and solid Earth physics, mass transport in the atmosphere and oceans, and global change studies. The International Celestial Reference System (ICRS) forms the basis for describing celestial coordinates, and the International Terrestrial Reference System (ITRS) is the foundation for the definition of terrestrial coordinates. The definitions of these systems include the orientation and origin of their axes, the scale, physical constants and models used in their realisation, such as, for example, the size, shape and orientation of the reference ellipsoid that approximates the Earth’s surface and the Earth’s gravitational model. The coordinate transformation between these two systems is described by a sequence of rotations that account for precession, nutation, Greenwich Apparent Sidereal Time (GAST), and polar motion, which collectively account for variations in the orientation of the Earth’s rotation axis and its rotational speed. GAST and polar motion are monitored by geodetic techniques, while precession and nutation are described by respective models (McCarthy and Petit, 2003). While a reference system is a mathematical abstraction, its practical realisation through geodetic observations is known as a reference frame (or datum). The conventional realisation of the ITRS is the International Terrestrial Reference Frame (ITRF), which is a set of coordinates and linear velocities (due mainly to crustal deformation and tectonic plate motion) of well-defined fundamental stations (e.g. networks of stations of the IGS, ILRS, IDS, IVS), derived from space-geodetic observations collected at these points. The ICRS is realised through the International Celestial Reference Frame (ICRF), which is a set of estimated position coordinates of extragalactic reference radio sources. At present, the ICRF is determined by Very Long Baseline Interferometry (VLBI), while the ITRF is accomplished by a combination of several independent space-based geodetic techniques, including VLBI, Satellite Laser Ranging (SLR), GNSS, and Doppler Orbitography by Radiopositioning Integrated on Satellites (DORIS). The ITRF and ICRF are defined by the International Earth rotation and Reference systems Service (IERS – http://www.iers.org/). Geodesy is facing an increasing demand from science, engineering, the Earth observation community, and society at large for improved accuracy, reliability and access to geodetic services, observations and products. Thus, a challenge that geodesy is now facing is to maintain the ITRF at the level that allows, for example, the determination of global sea level change at the sub-millimetre per year level, determination of the glacio-isostatic adjustments due to deglaciation since the Last Glacial Maximum and to modern mass change of the ice sheets, at the mm-level accuracy, precoand post-seismic displacement fields associated with large earthquakes at the sub-centimetre accuracy level, early warnings for tsunamis, landslides, earthquakes, and volcanic eruptions, mmto cm-level deformation and structural monitoring, etc. In response, the IAG is currently in the process of establishing the Global Geodetic Observing System (GGOS), which will unify all the geometric and gravity services of the IAG, in order to address the demands for improved geodetic products by society and the scientific community (Tregoning and Rizos, 2007; Drewes, 2005). GNSS will play a vital role in GGOS.