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 mea- surements 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 subsid- ence 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 tech- niques relying on precise distance or phase measurements transmitted or reflected from extraterrestrial objects, such as quasars, the Moon, or artificial satellites. Early space geo- detic measurements, beginning in the 1980s, had accuracy levels between 5 and 10 cen- timeters. These measurements were con- ducted across the entire globe and yielded the first direct observations of tectonic plate motion. Further improvements to space geo- detic technologies have increased the accu- racy 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. WDO W IN S KI AND S. ERIK ss ON Eos, Vol. 90, No. 18, 5 May 2009 shows how far and how rapidly space geod- esy 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 syn- thetic aperture radar ( InSAR), and gravity studies. Precise positioning is the fundamen- tal geodetic observation required for sur- veying and mapping. Instead of the tradi- tional triangulation and leveling networks that require line of sight (LOS) between measurement points, space geodetic meth- ods use LOS between the measurement points and celestial objects or satellites. For example, to measure changes in dis- tance across the San Andreas Fault, scien- tists used radio telescopes at Vandenberg (California) and Yuma (Arizona) to detect the phase delay in the arrival of quasar sig- nals between the two sites. Similar distance changes have also been determined by sat- ellite 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 measure- ment points. Building on this idea, scientists have developed advanced positioning techniques through Global Navigation Satellite Systems (GNSS). GNSS encompasses the various sat- ellite navigation systems, such as the United States’ GPS, Russia’s Globalnaya Navigat- sionnaya Sputnikovaya Sistema ( GLONASS), and Europe’s Galileo. Although these satel- lite systems were designed mainly for navi- gation, 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 pro- cesses 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 mea- sure 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 applica- tion 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-