GPS World, January 2011

EXPERT ADVICE Why Geodesy Matters Now More than Ever Chris Rizos G eodesy 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 childof technology science It dates back hundreds of years some would claim thousands of years and that the ancient Geodetic practice forms the foundation for surveying navigation and mapping and the digital datasets underpinning these activities 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 todays 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 Earths 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 globalchange 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 geodesys 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 www ggos org is an important component of the International Association of Geodesy IAG www iag aig org 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 geodesys contribution to the Global Earth Observing System of Systems GEOSS www earthobservations org 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 GPS World January 2011 www gpsworld com 8