Geodesy is a suite of powerful Earth-observation techniques, associated methodologies, and analysis tools that today are making a vital contribution to science and society. Yet geodesy is not a new, child-of-technology sciaence. It dates back hundreds of years — some would claim thousands of years, and that the ancient Greeks and other pre-Christian cultures shaped its direction. This is illustrated by its classical definition as the science of measuring and mapping the geometry, orientation in space, and gravity field of the Earth; these days we also include their variations over time. At a practical level, geodetic practice forms the foundation for surveying, navigation, and mapping, and the digital datasets underpinning these activities.
What has enabled geodesy to change from an esoteric natural science that underpins the making of maps to today’s cutting-edge geoscience? There are a number of reasons for this transformation. Firstly, modern geodesy relies on space technology, and enormous strides have been made in accuracy, resolution, and coverage due to advances in satellite sensors and an expanding portfolio of satellite missions. Secondly, geodesy can measure Earth parameters that no other remote-sensing technique can, such as the position and velocity of points on the surface of the Earth and the shape and changes of the Earth’s ocean and land surfaces, and it can map the spatial and temporal features of the gravity field.
These geodetic parameters are in effect the “fingerprints” of many dynamic Earth phenomena, including those that we now associate with global change (due to anthropogenic as well as natural causes). The challenge is to invert the outward expressions of these global-change phenomena in order to measure and monitor over time the underlying physical causes.
Finally, what relentlessly drives geodesy into the future is the innovative use of signals transmitted by global satellite navigatiaon systems such as GPS and GLONASS.
Space-geodetic techniques such as GNSS, satellite, and lunar-laser ranging; very-long-baseline interferometry; Doppler orbitography and radiopositioning integrated by satellite (DORIS); satellite sea and ice altimetry; satellite gravity mapping; and satellite interferometric synthetic aperture radar mapping have revolutionized the geosciences. They have significantly improved our understanding of how the solid Earth, atmosphere, and oceans work as a system, giving us new insights into atmospheric and oceanic circulation, the global water cycle, the waxing and waning of ice and glaciers, sea-level rise, global tectonic motion and local earthquake fault mechanisms, to name a few of geodesy’s Earth-observation applications.
Global Geodetic Observing System. GNSS today plays a crucial role in geodesy; however, we will see a massive increase in capability. Geodesy strives to increase the level of accuracy in the determination of these geodetic parameters by a factor of 10 over the next decade.
The Global Geodetic Observing System (GGOS) is an important component of the International Association of Geodesy (IAG). GGOS will integrate all geodetic measurements in order to monitor the phenomena and processes within the Earth system at far higher fidelity than at present. This integration implies the inclusion of all relevant information for parameter estimation, 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. GGOS is geodesy’s contribution to the Global Earth Observing System of Systems (GEOSS) initiative.
Although GPS is popularly associated with the WGS84 datum, an important GNSS contribution to geodesy is its role in the definition of the International Terrestrial Reference Frame (ITRF, itrf.ensg.ign.fr). In addition, high-accuracy differential GNSS techniques — which have been refined over several decades — provide the day-to-day means of determining point coordinates in the ITRF. This reference frame is nowadays the basis for most national and regional datums for mapping and science.
The International GNSS Service (IGS, igs.org) was established in 1994 as an IAG service to the geosciences, providing high-accuracy orbit and clock products as well as open (and free) access to measurements made by a dense ground network of continuously operating GPS/GNSS tracking stations. The IGS therefore supports ITRF maintenance and densification. The IGS nowadays supports many more user communities, such as navigation, surveying, machine guidance, atmospheric remote sensing, and others, both directly and indirectly.
GNSS’s utility includes the role that it plays in precise orbit determination of Earth observation, geodetic, and environmental satellites. GPS receivers onboard almost all such satellites ensure that the data from the satellite sensors can be correctly processed and interpreted. Consider how sea-level rise is measured by satellite-borne radar altimeters. The measurement of the time taken for a radar pulse from satellite to the ocean surface and back is made by the altimeter and converted to distance, but it is knowing where the satellite is in three dimensions to centimeter accuracy that allows the ocean surface to be mapped to extraordinary resolution. Millions of such measurements, over many years, referenced to the ultra-stable ITRF, enable scientists to determine with confidence the 3D position of a grid of points on the ocean surface and its rate of change, not just as a single average rise in sea height, but in its full spatial complexity.
The Challenge. Can GNSS and the IGS rise to the GGOS challenge? Although GPS is currently the only fully operational GNSS, the Russian Federation’s GLONASS is being replenished, and the IGS currently also generates GLONASS products. The European Union’s Galileo is planned to be deployed and operational by 2014 (although that date may slip several years), and China’s Compass is likely to also join the club by 2020, after first deploying a regional navigation satellite system by 2012. Together with dozens more satellites from other countries and agencies, it is likely that the number of GNSS satellites useful for geodesy will increase to almost 150, with perhaps six times the number of broadcast signals on which geodetic measurements can be made.
Simultaneously, the IGS is evolving to a multi-GNSS service, and at the same time improving the quality and timeliness of its products. Real-time IGS products will soon be available to all users.
In summary, geodesy faces an increasing demand from science, engineering applications, the Earth-observation community, and society at large for improved accuracy, reliability and access to geodetic services, measurements, and products. Thus, geodesy must maintain the ITRF at the level that allows, for example, the determination of global sea-level change at the sub-millimeter 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 millimeter-level accuracy; pre-, co-, and post-seismic displacement fields associated with large earthquakes at the sub-centimeter accuracy level; early warnings for tsunamis, landslides, earthquakes, and volcanic eruptions; millimeter- to centimeter-level deformation and structural monitoring; and more.
In response, the IAG established in 2007 the GGOS, to unify all the geometric
and gravity services of the IAG so as to support the ambitious goals of modern geodesy. Through the IGS, GNSS will play an indispensible role in GGOS. However, the Earth-observing techniques of modern geodesy are but one — albeit under-appreciated — set of applications of GNSS technology. As GNSS performance improves, and as it becomes more and more pervasive, our society’s reliance on this critical utility grows exponentially.
CHRIS RIZOS is professor and head of the School of Surveying & Spatial Information Systems, University of New South Wales, Sydney, Australia. He is vice president of the International Association of Geodesy. He will assume the presidency from mid-2011 for a four-year term.