Geodesy in the 21St Century

Eos, Vol. 90, No. 18, 5 May 2009

Volume 90 number 18 5 MAY 2009 EOS, Transactions, American Geophysical Union pages 153–164

geophysical discoveries, the basic under- Geodesy in the 21st Century standing of earthquake mechanics known as the “elastic rebound theory” [Reid, 1910], PAGES 153–155 Geodesy and the Space Era was established by analyzing geodetic measurements before and after the 1906 San From flat Earth, to round Earth, to a rough Geodesy, like many scientific fields, is Francisco earthquakes. and oblate Earth, people’s understanding of technology driven. Over the centuries, it In 1957, the Soviet Union launched the the shape of our planet and its landscapes has developed as an engineering discipline artificial satellite Sputnik, ushering the world has changed dramatically over the course because of its practical applications. By the into the space era. During the first 5 decades of history. These advances in geodesy— early 1900s, scientists and cartographers of the space era, space geodetic technolo- the study of Earth’s size, shape, orientation, began to use triangulation and leveling mea- gies developed rapidly. The idea behind and gravitational field, and the variations surements to record surface deformation space geodetic measurements is simple: Dis- of these quantities over time—developed associated with earthquakes and volcanoes. tance or phase measurements conducted because of humans’ curiosity about the For example, one of the most important between Earth’s surface and objects in Earth and because of geodesy’s application to navigation, surveying, and mapping, all of which were very practical areas that ben- efited society. Today, geodesy is no different. The field is now concerned with changes in the shape of Earth’s surface, because small detectable changes are associated with issues of great societal impact such as ice melting, sea level rise, land subsidence, and aquifer depletion. For example, the rate of polar ice melt may be estimated from combined satellite gravity and ground Global Positioning System (GPS) measurements. Global estimates of sea level changes are measured by altimetry satellites within entire ocean basins. Human-induced depletion of aquifers is reflected in subsidence measured by synthetic aperture radar (SAR) satellites. Twenty-first-century geodetic studies are dominated by geodetic measurements from space. Space geodesy uses a set of techniques relying on precise distance or phase measurements transmitted or reflected from extraterrestrial objects, such as quasars, the Moon, or artificial satellites. Early space geodetic measurements, beginning in the 1980s, had accuracy levels between 5 and 10 cen- timeters. These measurements were conducted across the entire globe and yielded the first direct observations of tectonic plate motion. Further improvements to space geodetic technologies have increased the accuracy to subcentimeter levels. Today, space geodetic observations can detect small movements of the Earth’s solid and fluid surfaces as well as changes in the atmosphere and ionosphere. Hence, geodesy has many applications in a variety of fields Fig. 1. Space geodetic applications in various Earth science disciplines at global and continental extending well beyond its traditional role in scales. SE stands for solid Earth, and Hydro represents hydrology. Sources for this figure are pro- solid Earth sciences (Figures 1 and 2). vided in the electronic supplement to this Eos issue (http://www.agu.org/eos_elec/). Additional information can be found at http://www.unavco.org/geodesy21century.

By S. Wd o w i n s k i a n d S. Er i k ss o n Eos, Vol. 90, No. 18, 5 May 2009

shows how far and how rapidly space geodesy has advanced since its beginnings about 50 years ago.

Types of Modern Geodetic Missions Space-based geodetic observations can be categorized into four basic techniques: positioning, altimetry, interferometric synthetic aperture radar (InSAR), and gravity studies. Precise positioning is the fundamen- tal geodetic observation required for surveying and mapping. Instead of the traditional triangulation and leveling networks that require line of sight (LOS) between measurement points, space geodetic methods use LOS between the measurement points and celestial objects or satellites. For example, to measure changes in distance across the San Andreas Fault, scientists used radio telescopes at Vandenberg (California) and Yuma (Arizona) to detect the phase delay in the arrival of quasar signals between the two sites. Similar distance changes have also been determined by satellite laser ranging (SLR) and more recently and more precisely by GPS. As a result of these techniques, relative positioning can be achieved over very large distances in which the precision is almost independent of the distance between the two measurement points. Building on this idea, scientists have developed advanced positioning techniques through Global Navigation Satellite Systems (GNSS). GNSS encompasses the various satellite navigation systems, such as the United States’ GPS, Russia’s Globalnaya Navigat- sionnaya Sputnikovaya Sistema (GLONASS), and Europe’s Galileo. Although these satellite systems were designed mainly for navigation, they were found to be very useful for precise positioning, with accuracy levels of less than a centimeter. GNSS also provides very high temporal resolution measurements (second by second, or even faster), yield- ing key observations of time-dependent processes in the lithosphere, atmosphere, and Fig. 2. Space geodetic applications in various Earth science disciplines at local scales. SE stands ionosphere. for solid Earth, GT stands for geotechnical, and Hydro stands for hydrology. Sources for this figure By contrast, rather than measuring three- are provided in the electronic supplement to this Eos issue (http://www.agu.org/eos_elec/). dimensional (3-D) changes by positioning Additional information can be found at http://www.unavco.org/geodesy21century. techniques, altimetry involves only changes in surface elevation. Space-based radar and laser altimetry techniques accurately measure the satellite’s height above the Earth’s space (satellites, the Moon, or quasars) are developed. In parallel, clever utilization of surface, which is then converted to the sur- very precise, easily repeated, and obtain- observables from nongeodetic missions face’s height above a reference ellipsoid. able for almost any surface location. Conse- such as GPS or SAR satellites resulted in Altimetry measurements are conducted by quently, positioning, surface elevation, and very precise positioning or change measure- releasing pulses toward the Earth’s surface gravity field and their changes with time ments—scientists could now detect earth- every several milliseconds, resulting in cir- can be determined precisely with global quake and magma-induced crustal deforma- cular ground measurements (footprints) coverage. tion, subsidence, glacial movements, and along the satellite track. Because of the The first generation of space geodetic wetland surface water level changes. large footprint (diameter > 75 meters), altim- observations relied on existing astronomical During the short history of the space geo- etry measurements are useful for measur- equipment such as the radio telescope, and detic era, innovative development of space ing flat surfaces. Radar altimetry was actu- on analysis of early satellite orbits. These technologies has yielded numerous geo- ally designed for measuring the height of observations yielded the first space-based detic methods (see Table S1 in the elec- ocean water surfaces but also was found measurements of tectonic plate motion tronic supplement to this Eos issue (http:// to be useful in measuring changes in ice and Earth’s gravity field. Dedicated satel- www.agu.org/eos_elec/)). A quick look cap elevation and water levels in rivers and lites for geodetic measurements were soon into the main space geodetic technologies lakes. Eos, Vol. 90, No. 18, 5 May 2009

Similar types of data, not measured from (Chandler wobble). These properties are the Gravity Recovery and Climate Experi- satellites, involve airborne light detection essential for precise determination of sat- ment (GRACE) satellite enables estima- and ranging (lidar) and terrestrial laser ellite orbits used in a variety of application of large-scale regional water budget scanning (TLS), both of which measure 3-D tions, such as weather forecasting and GPS changes (Figure 1e) and changes in polar positions of a large number of points located navigation. ice mass due to glacier melting. Changes in on surfaces facing the instrument. Airborne The global coverage of altimetry satel- ice cap elevation are also monitored by the lidar is widely used to measure elevation, lites allows decadal observations of sea level polar-orbiting Ice, Cloud, and Land Eleva- whereas TLS measures small-scale struc- change over entire ocean basins, comple- tion Satellite (ICESat) and other altimetry tures and detailed surface features. menting tide gauge records, many of which satellites. Geoid and elevation changes over A powerful method to detect surface span the past 100 years, acquired only along polar ice caps include both ice changes and change is InSAR. This method compares coastlines. In addition, GPS measurements GIA response to current and past melting. pixel-by-pixel SAR phase observations of help improve the relative sea level change Ground-based GPS measurements are essen- the same area acquired from roughly the record by monitoring the subsidence or tial for determining mass changes due to GIA- same location in space to produce digi- uplift of gauge stations, which can decouple induced crustal uplift, allowing improved esti- tal elevation models (DEMs) or surface dis- the relative movement of land from sea. mates of polar ice cap melt rate. placement maps with high spatial resolution Accurate determination of the Earth’s In addition to monitoring changes to the (typically from 5 to 100 meters). Such maps, shape by measuring the geoid (Figure 1b) surface of Earth, many continental-scale termed interferograms, are obtained from is another important contribution using pre- atmospheric and ionospheric phenomena repeat orbit observations and can reach cise tracking of altimetry and gravity mis- can be measured through satellite geodesy. centimeter-level precision. sion satellites. Altimetry measurements over GPS and other GNSS techniques can monitor In addition, satellite orbits are very sensi- oceans combined with gravity models can changes in atmospheric precipitable water tive to lateral variations in the Earth’s grav- yield detailed information on the bathym- (Figure 1f) and ionospheric total electron ity field. Precise measurements of satellite etry of the ocean floor. For example, they content (TEC). Such changes can be moni- orbits by ranging (distance) and other tech- can help detect the location of unknown tored because signals transmitted from the nologies yield accurate determination of the seamounts (Figure 1c), which show up as GNSS satellites are sensitive to the water con- geoid shape and its variations over time. The localized gravity anomalies reflecting lateral tent in the atmosphere and to TEC in the ion- geoid is defined as the equipotential surface mass differences between seamounts and osphere. Precipitable water retrievals have of Earth’s gravity field that best fits global ocean water. been shown to improve the representation mean sea level. It describes the mass distri- of atmospheric water vapor in numerical bution within the Earth, from which one can Applications on a Continental Scale weather prediction systems, increasing their infer Earth’s dynamic structure. The new ability to forecast heavy rain and hurricane generation of gravity satellites are also very Relative motion of tectonic plates produces intensity. TEC retrievals can help scientists sensitive to short-term changes in the geoid deformation along plate boundaries through forecast adverse space weather. reflecting near-surface mass redistribution associated volcanoes and earthquakes. Geo- such as ice melt or large-scale seasonal detic studies can determine the details of Satellite Geodesy on Local Scales changes in water budget. Geoid measure- earthquake- and magma-induced deforma- ments are also crucial for calibrating GPS- tion on a local scale. Integrating these details Changes in the Earth’s surface at local determined height with a standard mean sea for a larger geographic area reveals the full scales (<100 kilometers) are best measured level datum. picture of plate boundary deformation. by a combination of high temporal reso- Space geodesy provides observations at For example, the geodetic component of lution (GPS) and high spatial resolution various spatial and temporal scales with a EarthScope—the Plate Boundary Observa- (InSAR) observations. In tectonically active corresponding variety of applications. High tory, funded by the U.S. National Science areas, utilization of both techniques has spatial resolution measurements and global Foundation—has expanded on existing revealed details of crustal deformation dur- coverage provide the means to investigate local GPS networks to provide continental- ing, after, and between moderate and large localized, continental, and global-scale pro- scale coverage of the western United States. earthquakes (Figure 2a) as well as magma- cesses. The precise and repetitive nature The broad and dense geographic coverage induced deformation beneath volcanoes of satellite orbits enables reliable, repeated of EarthScope, with its high-quality, continu- and continental rifts (Figure 2b). In the cry- data acquisition, with temporal resolu- ous data, is ideal for measuring volcanic and osphere, these two methods provide a new tion of seconds (GNSS techniques) to tens plate tectonic motions, strain accumulation understanding of the kinematics of glacier of days (altimetry, InSAR, and gravity tech- on faults, earthquake surface displacement, flow (Figure 2c). niques). Some measurements began in the and postseismic deformation on timescales InSAR is very effective in detecting local- 1980s, providing scientists with data sets of of seconds to decades. ized subsidence and uplift of land surfaces repeated observations that span decades. Geodetically observed vertical move- in response to natural or anthropogenic ments by GPS in Europe and North America causes. Some of the more successful appli- Global-Scale Applications reflect glacial isostatic adjustment (GIA, pre- cations are monitoring subsidence due viously known as postglacial rebound; see to compaction of sediments (Figure 2d), The different geodetic measuring tech- Figure 1d) due to the melt of ice caps fol- aquifer-system response to groundwater niques, along with varying spatial and tem- lowing the last glacial period (~110,000 to pumping and artificial recharge (Figure 2e), poral accuracies, allow for geodetic insight 10,000 years ago) and corresponding mass extraction of fluids in oil fields, and excava- on a global scale. adjustment in the newly unloaded mantle. tion in mines and tunnels. InSAR and GPS In addition to broadly monitoring plate These observations provide excellent con- are also powerful tools in studying surficial motion (Figure 1a), independent geodetic straints on mantle viscosity. GIA also has processes such as landslides (Figure 2f) and measurements have revealed congruence a global-scale contribution as it changes soil moisture content. of short-term plate motions with those on the Earth’s dynamic oblateness (J2), which Space geodesy also measures changes in geological timescales and provide better reflects changes in the latitudinal distribu- water surfaces on continents. InSAR com- constraints for quantitative plate motion tion of mass within the Earth. bined with water level gauges provides infor- models. The same positioning techniques Shorter-timescale seasonal and multi- mation on wetland water level changes (Fig- have been used to monitor Earth’s rotation, year redistribution of water and ice mass are ure 2g). Altimetry observations are used for including variation in rotation rates affect- expressed by small but detectable changes detecting water levels in rivers and lakes ing the length of each day and pole motion in the geoid shape. The high precision of (Figure 2h), especially in remote regions. Eos, Vol. 90, No. 18, 5 May 2009

The observations are local at points where beyond these traditional solid Earth haz- applications, geodesy today brings scientists ground tracks of satellite orbits intersect riv- ards. Sea level rise, glacial melting, and hur- together for interdisciplinary research that ers or lakes and can be combined to provide ricane forecasts are of immediate interest helps mitigate the influence of the forces of regional information. to communities around the world, particu- nature on our growing population as well as Though not space-based, airborne lidar larly in the context of global climate change. the effect of the population on Earth’s frag- and TLS instruments are very effective geo- Geodesy can also reveal the overlapping ile surface. detic techniques—both are used in geo- threats from multiple hazards—for example, For more information on space geo- morphological studies, such as determining in areas of coastal subsidence such as Bang- detic techniques and their applica- fault slip rates across offset topographic fea- kok, Thailand, the effect of continued sea tions, please visit http://www.unavco.org/ tures. Airborne lidar can map subtle eleva- level rise amplifies flooding hazards. geodesy21century. tion changes in extremely flat areas, such as One of the greatest global challenges of south Florida, where it is used for determin- the 21st century is securing fresh water for the Reference ing flood zones. increasing worldwide population and for sus- taining natural ecosystems. Geodetic moni- Reid, H. F. (1910), The California Earthquake of Societal Implications toring of subsidence in depleted aquifers and April 18, 1906, vol. 2, The Mechanics of the Earth- water level changes in wetlands, rivers, and quake, Report of the State Investigation Commis- Natural hazard mitigation, the effects of lakes yields important constraints for hydro- sion, Carnegie Inst. of Wash., Washington, D. C. global warming, and optimum use of water logical models that can serve as decision sup- resources are areas of major concern for port tools for water resource managers. Author Information humankind today. The implications of space The varied scales and high precision geodesy when applied to natural hazards of space geodetic observations are help- Shimon Wdowinski, Division of Marine Geology associated with earthquakes and volcanoes ing to push the frontiers of knowledge and Geophysics, University of Miami, Miami, Fla.; are well known in the geoscience commu- regarding many Earth processes. Because and Susan Eriksson, UNAVCO, Boulder, Colo.; nity, but space geodesy also has an impact space geodetic measurements have many E-mail: Eriksson@unavco .org

NASA Mission to Explore Forcing forcing. Following their temporal development across a hemisphere of the Earth, of Earth’s Space Environment GOLD measurements are expected to resolve critical aspects of the forcings that PAGE 155 storms. The relative importance of each forc- drive the transition region. Using these two ing source varies with time, geographic loca- key parameters, theories and models of TI The Global-Scale Observations of the tion, and altitude; consequently, our under- forcing are tested and understanding of the Limb and Disk (GOLD) mission has been standing of the TI region is best advanced system is advanced. selected as a mission of opportunity by by considering its global-scale behavior, an The GOLD mission will address the follow- NASA’s Small Explorer program. This mis- approach often used in modeling. ing four key science questions, which are a sion, with an anticipated 2014 launch date, GOLD will provide, for the first time, a subset of the overarching question: is an opportunity to significantly advance near-simultaneous global-scale “snapshot” 1. How do geomagnetic storms alter the thermosphere-ionosphere (TI) science and of the temperature and composition in the temperature and composition structure of to provide answers to key elements of an lower thermosphere, allowing one to see the thermosphere; how does the low-latitude overarching question for heliophysics sci- how these two major parameters, shown nighttime ionosphere respond to geomag- ence: What is the global-scale response of in Figure 1, react to external and internal netic storms; and is the initial state of the the thermosphere and ionosphere to forcing (e.g., by geomagnetic storms or atmospheric tides) in the integrated Sun-Earth system? Understanding the response of the TI region to forcing is important for scientific as well as societal reasons. Scientifically, understanding how Earth’s TI responds to forcing provides insights into the response of similar regions on other planets. Societal impacts that arise from an inadequate understanding of this region include unnecessary delays in air travel and unanticipated interruptions in satellite services such as the Global Position- ing System. The GOLD mission promises to lead to a decrease in such problems.

Scientific Objectives Fig. 1. Storm-time changes in thermospheric temperature and composition. Differences between The TI region contains the transition storm-time and quiet-time calculations from the National Center for Atmospheric Research’s between the plasma-dominated region of the thermosphere-ionosphere electrodynamics general circulation model (TIEGCM) are shown (left) atmosphere and the neutral, fluid-dominated for the temperature (K) and (right) for the column density ratio of atomic oxygen to molecular atmosphere at lower altitudes. External forc- nitrogen (O/N2 , in percent). Both are on a constant-pressure surface at an altitude of approxi- ing by the solar extreme ultraviolet (EUV) nor- mately 160 kilometers. After averaging from an intrinsic, approximately 625-square-kilometer mally dominates in this region, but internal resolution to the 5º × 5º grid typically used by the TIEGCM, the Global-Scale Observations of the forcing from magnetosphere-ionosphere (MI) Limb and Disk (GOLD) mission is expected to resolve differences of less than 15 K in tempera- coupling or from atmospheric tides can have a ture or less than 10% in O/N2 column density in 2-hour averages. The spatial resolution and the critical or even dominant influence, as MI cou- viewing geometry used for these images approximate those of the GOLD imager. pling frequently does during geomagnetic