Geodesy Grand Challenges

“.‘down’ is n ot an y chan ce direction but where wha t ha s weightan d wha t is made of Ea rth a re ca rried.”Aristotle, Physics (ca. 350 BCE)Introduction:The Science of GeodesyA Foundation for Innovation: Grand Challenges in GeodesyThe earliest geodetic measurementswere obtained over two thousand yearsago, when Eratosthenes establishedthat our planet has a spheroidal shapeand described Earth in terms of asingle number: its size.Figure 01 Geodetic observing systems have advanced an extremely broad range of applications within Earth sciences.This figure illustrates a few of these applications. These and others are discussed in this document.4Since the time of Eratosthenes,geodesy has grown into the science ofobserving and understanding Earth’stime-varying shape, gravity field, androtation. What makes this scienceso powerful is the immense rangeof phenomena that can be studiedusing these observations. Moderngeodesy targets the study of processesas diverse as deformation of Earth’ssurface, redistribution of mass withinand on the surface of the solid Earth,oceanic and atmospheric circulation,changes in sea level; and variationsin the flow and mass balance ofglaciers. In addition, since geodetictechniques often use electromagneticsignals propagating through theatmosphere of Earth, modern geodesyprovides information on atmospherictemperature and water vapor, and onionospheric electron density. Recentstudies suggest that geodesy mightbe used to study snow accumulation,vegetation structure, biomass andcarbon sequestration, and soilmoisture. Thus, in the early twentyfirst century, the goal of geodesy has

evolved to include the study of thekinematics and dynamics of, andinteraction among, the solid Earth,cryosphere, hydrosphere, atmosphere,and biosphere (Figure 01).To achieve these broad goals, geodesyuses a rich assortment of highaccuracy measurement techniques thatreveal the dynamic Earth system ata wide range of spatial and temporalscales. Current applications of thesetechniques include:yy R eflecting laser light frommirrored, orbiting satellites toreveal changes in the geocenterand the orientation of Earth inspace.yy U sing superconducting magnets tolevitate a metal sphere in a nearlyperfect vacuum to measure theFigure 02acceleration due to gravity.Extremely accurate intersatellite range-rate measurements by the Gravity Recovery and Climateyy Reflecting microwave signals offExperiment (GRACE) tandem satellites have proEarth’s surface from a spaceborneduced the first ever maps of global time-variableor airborne radar to detectgravity. Some of the most important targets forGRACE are the major ice-mass complexes inmotions of Earth’s crust andGreenland and Antarctica. This figure shows thatice sheets, measure sea-surfaceice-mass loss from Greenland (top) and Antarcticaheight and ocean currents, and(bottom) has been accelerating.characterize biomass.yy M easuring the distance betweentwin satellites with micronaccuracy to track the movement ofmass within the interior of Earthand water and ice on the surface ofEarth (Figures 02, 03).yy S urveying glaciers with spacebornelasers to track ice motions andmonitor loss of ice mass due toclimate change.yy O bserving quasars at the edgeof the universe with giant radiotelescopes to reveal the dynamicsand shape of Earth’s core.yy C ombining optical and radarimages from satellites and aircraftto track horizontal motions overdays to decades.yy S canning the local environmentwith ground-based lasers andilluminating the ground withairborne lasers to create imagesthat reveal the dynamic forces thatshape Earth’s surface (Figure 04).yy U sing global and regionalnetworks of GNSS1 instrumentsfor an increasing variety ofhigh-precision Earth scienceapplications, a short list of whichincludes detailed measurementof tectonic plate motion anddeformation, volcano monitoring,and glacier flow.Figure 03Geodetic observing systems such as GRACEenable us to observe the movement of massnear the surface of Earth for the first time. Thisis a map of the mean rate of change of massnear Earth’s surface, expressed as the rate ofequivalent water depth, based on GRACE data.The GRACE data in this map, for example, revealongoing mass changes (mostly mass loss) to icesheets in Greenland, Antarctica, and Alaska; GIAin North America and northern Europe; groundwater changes in South America and Africa; andother mass motions. 101 monthly gravity fieldsfrom the period 2002–10 were combined tomake this map.5

These measurements would not beuseful without the mathematicaltechniques necessary to analyze thedata, the computational capacity ofmodern cyberinfrastructure, andthe contributions of engineers andcomputer scientists. The informationthat is sought must often be obtainedfrom the observations using intricateinversion methods. In fact, the famousmathematician and physicist Gaussinvented the least-squares methodin the eighteenth century in partto invert data from triangulationnetworks, which were the state-ofthe-art geodetic system of his time.Since Gauss’s era, the method of leastsquares has become the basis for awide variety of inversion techniquesthat are used in many different fields.Theoretical models that expressgeodetic observations in termsof physical parameters are oftenquite complex, even more so whenobservations have been made fromplatforms that are themselves inmotion with respect to the Earth, orwhen long-term changes may affectthe observing system itself.Geodesy provides a rich toolbox ofapplications for cutting-edge researchin other scientific fields, such asearthquake physics, volcanology,geodynamics oceanography,atmospheric and climate science,hydrology, glaciology, geomorphology,ecosystem science, as well as physicsand astronomy. In addition to theseresearch applications, geodesy maybe used to study natural hazardsand, potentially, to provide earlywarning of earthquakes, tsunamis,landslides, and volcanic eruptions;and to study how the coasts respondto sea-level change and to storms.Given the current trend of innovationwithin geodesy, the range of geodeticapplications will continue to increase.2The relationships between geodesyand these disparate fields attest to thestrength of geodesy as a discipline.Figure 04Airborne Laser Swath Mapping (ALSM) producesimages of the terrain with unprecedented resolution. In (a), an unfiltered ALSM image of the SouthFork Eel River in northern California shows heavilyforested terrain. In (b), a filtering process has beenused to extract a “bare-Earth” image from theALSM data set, revealing subtle landslide featuresthat are hidden beneath the forested terrain.12 Throughoutthis document, the more general term “Global Navigational Satellite System” (GNSS), which includes all satellite navigation systems such as GPS. In some instances, the more specific “Global Positioning System” (GPS) is used where itis more appropriate. In the context of this document “geodetic science” involves research relevant to geodetic theory or observation, including: satellite orbit determination; rotational dynamics; electromagnetic wave propagation; signal detection; causes and modes ofcrustal deformation; inversion theory and error analysis; gravity and potential theory; and reference systems. The term “geodetic application” refers to application of geodetic science to other fields of research, such as those listed above.6

Within these fields, geodesy enablestransformative observations anddiscoveries. Recent examples include:yy M easurement of present-dayinstantaneous velocities of Earth’stectonic plates.yy F irst global determination ofthe mass balance of Earth’s greatice sheets, and the observationthat the mass loss in Greenlandand Antarctica may be rapidlyaccelerating.yy D iscovery of periodic, slowaseismic slip in subduction zonesin Japan, Cascadia, Mexico, andaround the world.Figure 05One of the targets for the EarthScope PlateBoundary Observatory (PBO) is the integratedvelocity fields (and their temporal variability)for North America. This figure shows the meanhorizontal crustal velocity field from PBO GNSSsolutions through November 2009. For clarity, error ellipses are not shown, but all velocityestimates with formal uncertainties greater than 1mm/yr are omitted. Data from PBO sites in Alaskaand CONUS east of 100 W are not shown.yy P recise measurements of secularand transient deformation dueto active seismogenic faults (inparticular, major plate boundaryfaults such as the San Andreasfault in California), with directrelevance to seismic hazardestimates.yy A ccurate determination ofpresent-day global sea-level rise,which is a sensitive indication ofclimate change caused by meltingof glaciers and ice sheets as well aschanges to the thermal and salinityconditions of the ocean.3yy U nexpected, sudden and dramaticaccelerations and decelerations ofdeep outlet glaciers in Greenlandand elsewhere.yy D etermination of the crustaldeformation field of NorthAmerica with unprecedentedaccuracy, and spatial and temporalresolution (Figure 05).3yy E xtraordinary images obtainedfrom LiDAR (Light Detectionand Ranging) mapping of activefaults, which has enabled newinsights into the slip distributionof recent earthquakes that are notpossible with any other approach.yy D etection of magmatic activity atdozens of volcanoes worldwidethat were previously thought to bedormant.yy E stablishment of internationalgeodetic services for thedevelopment of standards, models,and documentation, includingthe International DORIS Service(IDS), International GNSSService (IGS), the InternationalLaser Ranging Service (ILRS),and the International VLBIService (IVS). These servicescoordinate data analysis of globalnetworks and make data and dataproducts freely available.yy D evelopment of the GlobalGeodetic Observing System(GGOS), recently adopted asa permanent component of theInternational Association ofGeodesy (IAG).yy E stablishment of UNAVCOfor focused support of geodeticapplications by providing stateof-the-art geodetic equipment,facilities, engineering, and dataservices for projects located allover the world. Geodetic determination of sea level change featured prominently in the Fourth Assessment Report of the Inter-governmental Panel on Climate Change (IPCC).7

yy I nferences of the mechanicalproperties of damage zonesaround major crustal faults frommeasurements of small strain orseismic shaking induced by nearbyearthquakes (Figure 06).yy E stablishment of NCALM for theacquisition of airborne LiDAR dataand the Open Topography portalto integrate and distribute highresolution topography data and tools.This document addresses two differentperspectives of “geodetic science” and“geodetic applications” by embracingboth and recognizing their power incombination. It is organized aroundEarth science applications that mightemerge and evolve over the next 5–10years. To that end, this documentidentifies seven “Grand Challenges” forEarth science. These challenges werechosen because they are fundamental tounderstanding Earth, they are importantto a global society that increasinglydepends on this understanding forits safety and prosperity, and they canbe significantly transformed by theapplication of geodetic observations. Therecommendations that follow the GrandChallenges are intended to positionthe field of geodesy for a decade ofinnovation, collaboration, and scientificachievement.8Figure 06The recent occurrence of moderate and greatearthquakes within high-rate GNSS networks drivesnew capabilities for understanding their impact, withimplications for early detection and warning. A newapproach for deriving displacements relies on a jointstochastic filtering of a high-rate GNSS displacementtime series (the determination of which is itself a recent geodetic advance) and very-high-rate accelerometer data. This approach has the advantages inherentto GNSS, namely accurate determination of the staticoffset during an earthquake with no clipping for largedisplacements, and takes advantage of the higherobservation rate provided by accelerometers. Aboveis a detail from the April 4, 2010 MW 7.2 El MayorCucapah earthquake vertical displacement record.

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“If there is ma gic on this p lan et, it is con ta in ed in wa ter.”Loren Eiseley, The Immense Journey (1957)SECTION01Where is the Water?A Foundation for Innovation: Grand Challenges in GeodesyWater is arguably the fundamental component of the Earth system. It enables life, moves energy through the Earth system, andreshapes our planet. Water is exchanged on a variety of timescales among the oceans, atmosphere, cryosphere, and lithosphere.As Earth responds to climate change, water in the Earth system responds and redistributes itself in a variety of ways. Waterformerly locked up in the ice sheets melts and joins the oceans or is stored on the continents. Precipitation patterns continueto change. Rainfall is reduced in some areas, causing drought, while formerly arid regions may have abundant rainfall. Sea levelsrise and ocean circulation changes. Thus, the monitoring of the temporal changes in Earth’s water reservoirs is fundamental tounderstanding the planet-scale impact of climate change.Various impacts of the redistribution of water on Earth can be monitored by geodetic observational systems. The redistributionof water can be determined directly by estimating changes in Earth’s gravitational field as it responds to the moving water massand to the deformation of the solid Earth caused by moving water (Figure 07). We can measure the height of the oceans andthe ice sheets using laser and radar altimetry, the velocity of glaciers using Interferometric Synthetic Aperture Radar (InSAR),sub-pixel optical and SAR pixel tracking and GNSS, and the response of the solid Earth due to the weight of the redistributedwater using GNSS. Changes in the amount of water contained in the atmosphere can be measured by the delays they cause toelectromagnetic signals used by geodetic measurement systems.Reflections of electromagnetic signalsoff of Earth’s surface can inform us evenabout the amount of water containedon and within the ground. Geodesy thusprovides the precise tools to monitor thesmall, but very important, changes wesee in the water reservoirs of Earth as itresponds to climate change.Figure 07Data from GRACE satellite gravity observations indicate that California’s Central Valley (left) is losinggroundwater (right) at a rate of 31 3 mm/yr from 2004 -10. This amount of water is nearly the capacityof Lake Mead, the largest reservoir in the United States. The Central Valley is a major agricultural producer,and depends to a great extent on groundwater for irrigation. Geodetic measurements such as these raisequestions regarding the sustainability of groundwater depletion on this scale.10In this section, we present three GrandChallenges focused on issues relatingto water and climate. The title of thissection is “Where is the Water?” becausethat is an aspect of this problem thatgeodesy, through its sensitivity to massredistribution and accurate distancemeasurements, is uniquely positioned toanswer.

Grand Challenge 1Will the global population have enough water to sustain itself?Fresh water is the fundamental buildi

scientific fields, from ground water systems and fault dynamics to mapping the speed of ice flows and the amount of water vapor in the atmosphere. Widespread recognition that technology-driven science is a national asset in a global economy has further strengthened public investment in exploring these phenomena and their relevance to society.