Satellite-based geodesy - CiteSeerX

Satellite-based geodesy Dorota A. Grejner-Brzezinska1,2 and Chris Rizos2 1

Satellite Positioning and Inertial Navigating (SPIN) Laboratory The Ohio State University Columbus, OH 343210 USA 2

School of Surveying & Spatial Information Systems The University of New South Wales Sydney, NSW 2052 AUSTRALIA

1. Geodesy and its primary mission According to Helmert’s classical definition, geodesy is the “science of the measurement and mapping of the Earth’s surface” by direct measurements, such as terrestrial triangulation, leveling, and gravimetric observations, and, in the past 50 years, also with space techniques, based primarily on the tracking of a wide range of artificial Earth satellites. Following Seeber (2003), the primary mission of Geodesy can be defined as: • • •

Establishment of geodetic reference frame and determination of precise global, regional and local 3D positions, Determination of the Earth’s gravity field and its related models, such as geoid1, Measurement and modeling of geodynamical phenomena, such as crustal deformation, polar motion, Earth rotation, tides, etc.

Modern geodesy provides the foundation for all observations related to global change within the ‘Earth system’. The importance of Earth observations, provided with increasing spatial and temporal resolutions, better accuracy, and with decreasing turn-around time, should be seen in the context of not only scientific understanding of the Earth system, but also in support of fundamental societal activities, such as managing natural resources, environmental protection, human health monitoring, disaster prevention and emergency response. 2. How space-based methods support the primary mission of geodesy Since the launch of the first artificial satellite, SPUTNIK, in 1957, geodesy has evolved into a combination of a number of geosciences and engineering sciences. Several important research discoveries gave rise to the development of satellite geodesy. For example, by 1

Geoid - essentially, the figure of the Earth abstracted from its topographical features; the particular equipotential surface that coincides with mean sea level in the absence of currents, air pressure variations, etc., and that may be imagined to extend through the continents. This surface is everywhere perpendicular to the force of gravity.

analyzing SPUTNIK’s radio-signals it was discovered that the observed Doppler shift could be converted to a useful navigation information if the satellite’s location in space were known. An increasingly important contribution of satellite geodesy is satellite-based navigation, facilitated through radio-navigation signals transmitted by Earth orbiting navigation satellites such as the Global Positioning System (GPS). GPS is now not only an indispensable tool for space geodesy, but it has also revolutionized surveying and navigation, and is increasingly used for personal positioning. Nowadays the phrase Global Navigation Satellite System (GNSS) is used as an umbrella term for all current and future global radio-navigation systems. Although GPS is currently the only fully operational GNSS, the Russian Federation’s GLONASS is being replenished and will be fully operational by 2010, the European Union’s GALILEO will be deployed and operational by 2013, and China’s COMPASS is likely to also join the ‘GNSS Club’ by the middle of the next decade. For satellites to serve as space-based reference points, their trajectories (or orbits) must be known. The precise computation of orbits is accomplished by geodetic techniques combined with orbital mechanics. The International Association of Geodesy (IAG – http://www.iagaig.org) has established services for all the major satellite geodesy techniques: International GNSS Service (IGS – http://www.igs.org), International Laser Ranging Service (ILRS – http://ilrs.gsfc.nasa.gov), and the International DORIS Service (IDS - http://ids.cls.fr). These services generate products for users, including precise orbits, ground station coordinates, Earth rotation values, and atmospheric parameters. All have networks of ground tracking stations that also are part of the physical realization of the ITRF (see section 3). Precision Orbit Determination (POD) also requires the use of accurate geodetic models for gravity, tides and geodetic reference frames. At the same time, POD’s sensitivity to various types of geodetic information makes it a powerful tool in the refinement of geodetic models. Consequently, a satellite can be considered as a probe or sensor, moving in the Earth’s gravity field, along an orbit disturbed by the gravitational attraction. Thus, measurements of directions, ranges, and range-rates between terrestrial tracking stations (and sometimes also from other orbiting satellites) and the satellite ‘targets’ provide, in addition to navigation parameters, a wealth of information about the size and the shape of the Earth and its gravity field. The first example of gravitational mapping by the use of satellites was the 1958 determination of Earth flattening from measurements to the EXPLORER-1 and SPUTNIK-2 satellites, followed by the discovery of the pear-shape of the Earth in 1959. The latest generation of gravity mapping missions, CHAMP, GRACE and GOCE, now are determining the features of the gravity field down to 100km or so in size, on a monthly basis, monitoring changes in the gravity field with time due to mass transports such as due to the Water Cycle. This is an exciting time for gravimetric geodesy with the recent establishment of the International Gravity Field Service (IGFS - http://www.igfs.net) and the release of the most detailed and precise model, EGM2008, of the Earth’s gravity field ever.

Aside from their navigation and precise positioning function, geodetic satellites serve as a remote sensing tool. An example is satellite radar altimetry2, a remote sensing technology that can measure the ocean surface topography (TOPEX/Poseidon, Jason-1, etc) or ice topography (e.g., ICESat, CRYOSAT). Variations in ocean surface topography indicate gravitational variations due to undersea features, such as seamounts or trenches, as well as ocean circulation features from small eddies to basin-wide gyres. The altimeters can also be used to measure parameters such as wave height, wave direction, and wave spectra. Other geodetic remote sensing tools include differential interferometric synthetic aperture radar (DInSAR) satellites such as TerraSAR-X, ALOS, Radarsat-2, and others. These can detect changes in the shape of the topography as small as a centimeter with spatial scales of just a few meters. 3. Terrestrial & celestial reference systems Modern Geodesy is now equipped with an array of space technologies for mapping (and monitoring changes) the geometry of the surface of the solid Earth and the oceans, as well as its gravity field. However the fundamental role of geodesy continues to include the definition of the terrestrial and celestial reference systems. These reference systems are the bedrock for all operational geodetic applications for national mapping, navigation, spatial data acquisition and management, as well as the scientific activities associated with geodynamics and solid Earth physics, mass transport in the atmosphere and oceans, and global change studies. The location of points in three-dimensional space are most conveniently described by Cartesian coordinates, X, Y and Z. Since the start of the Space Age such coordinate systems are typically geocentric, with the Z axis aligned with the Earth’s conventional or instantaneous rotation axis. Because the Earth’s center of mass is located at one focus of a satellite’s orbital ellipse, this point is the natural origin of a coordinate system defined by the satellite-based geodetic methods. The International Celestial Reference System (ICRS) forms the basis for describing celestial coordinates, and the International Terrestrial Reference System (ITRS) is the foundation for the definition of terrestrial coordinates. The definitions of these systems include the orientation and origin of their axes, the scale, physical constants and models used in their realization, such as, for example, the size, shape and orientation of the reference ellipsoid that approximates the Earth’s surface and the Earth’s gravitational model. The coordinate transformation between these two systems is described by a sequence of rotations that account for precession3, nutation4, Greenwich apparent sidereal time (GAST), and polar motion, which collectively account for variations in the orientation of the Earth’s rotation axis and its rotational speed. GAST and polar motion, also referred to as Earth Rotation 2

An altimeter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth underwater. 3 Mean secular component of the rotational motion of the Earth rotation axis due to the gravitational attraction of the Sun and the Moon. 4 Periodic component of the rotational motion of the Earth rotation axis due to the gravitational attraction of the Sun and the Moon.

Parameters, are monitored by geodetic techniques, while precession and nutation are described by respective models (McCarthy and Petit, 2003).

Figure 1. Combination and integration of the geodetic observation techniques (Plag et al., 2007). While a reference system is a mathematical abstraction, its practical realization through geodetic observations is known as a reference frame (or datum). The conventional realization of the ITRS is the International Terrestrial Reference Frame (ITRF), which is a set of coordinates and linear velocities (due mainly to crustal deformation and tectonic plate motion) of well-defined fundamental stations (e.g. networks of stations of the IGS, ILRS, IDS), derived from space-geodetic observations collected at these points. The ICRS is realized through the International Celestial Reference Frame (ICRF), which is a set of estimated position coordinates of extragalactic reference radio sources. At present, the ICRF is determined by Very Long Baseline Interferometry (VLBI), while the ITRF is accomplished by a combination of several independent space-based geodetic techniques, including VLBI, Satellite Laser Ranging (SLR), GNSS, and Doppler Orbitography by Radiopositioning Integrated on Satellites (DORIS). The ITRF and ICRF are defined by the International Earth Rotation and Reference System Service (IERS). Combination and integration of the geodetic observation techniques are shown in Figure 1.

Geodesy is facing an increasing demand from science, engineering, the Earth observation community, and society at large for improved accuracy, reliability and access to geodetic services, observations and products. Thus, a challenge that geodesy is now facing is to 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 the mm-level accuracy, pre- co- and postseismic displacement fields associated with large earthquakes at the sub-centimeter accuracy level, early warnings for tsunamis, landslides, earthquakes, and volcanic eruptions, mm- to cm-level deformation and structural monitoring, etc. In response, the IAG is currently in the process of establishing the Global Geodetic Observing System (GGOS), which will unify all the geometric and gravity services of the IAG, in order to address the demands for improved geodetic products by society and the scientific community (Tregoning and Rizos, 2005; Drewes, 2005). 3.1. National mapping datums Before satellite-based positioning systems were first used in the 1960’s, national datums were non-geocentric. Typically a reference system that best-fitted the shape of the Earth within the region covered by that datum was established; examples are the North American Datum 1927 (NAD27) and the Australian Geodetic Datum (AGD) 1966. A unified World Geodetic System became essential for several reasons, such as (1) beginning of international space science and astronautics, (2) the lack of inter-continental geodetic ties, (3) the inability of the existing geodetic systems to provide a worldwide geo-data basis, and (4) need for global maps for navigation and aviation, and military operations. Particularly since the advent of GPS, national datums around the world have been progressively redefined as geocentric reference systems, providing much higher reference frame accuracy, and ensuring that national geo-databases are compatible with GPS-derived coordinates. ITRF now provides the foundation for all national and regional reference frames, and facilitates a link of the local frames to each other. 3.2. World Geodetic System (WGS84) WGS84 is a particular realization of a terrestrial reference system, created and maintained by the U.S. Department of Defense (http://earthinfo.nga.mil/GandG/publications/tr8350.2/tr8350_2.html); it is a geocentric reference system used by GPS. Current geodetic realizations of WGS84 and the ITRF are consistent with each other at the few centimeter level globally. 4.

Future developments and trends

The advent of space-based geodetic techniques put geodesy in transition by extending the scope of geodetic observing systems from providing the reference frame, as well as methodology and tools for positioning and navigation, to a global dynamic system for Earth monitoring (Plag and Pearlman, 2007). It also modernized the primary mission of geodesy of establishing the reference frame by facilitating the development of geocentric global datums. This transition is therefore from a utility serving other geosciences, to a “system of systems”

that consists of the sensors (satellites), global reference frames, dynamic models and observing systems that together create consistent data sets that are relevant to nearly all societal activities and numerous scientific and engineering applications. With the expansion of Earth observing satellite-based systems, geodesy will continue to transform from a local service, aiding mappers and surveyors, to a global one, supporting geophysical, oceanographic, atmospheric and environmental science communities. Among the important applications that are possible only due to the availability of space-based geodetic techniques the following must be mentioned, as they will continue growing are: sea level monitoring and early tsunami warning, and atmospheric/climate remote sensing. The increasing accuracy of navigation and positioning using satellite-based geodetic methods will continue to create new applications in transport, commerce (location based services or infomobility, also known as telegeoinformatics), precision agriculture, construction, asset and natural resources monitoring, etc. Since the GNSS observations are inherently calibrated with respect to atomic time scales, GNSS will continue supporting time transfer and providing a time-base for synchronization of computer networks, telecommunication switches, electric grids, etc. Bibliography 1. Drewes H. (editor): Journal of Geodynamics, The Global Geodetic Observing System, Vol. 40 (4-5), November-December 2005, pp. 355-502. 2. McCarthy D.D. and G. Petit (editors): IERS Conventions. IERS Technical Note No. 32, 2003 (http://tai.bipm.org/iers/conv2003/conv2003.html). 3. Plag H-P., G. Beutler, R. Forsberg, C. Ma, R. Neilan, M. Pearlman, R. Richter and S. Zerbini: Linking the Global Geodetic Obseving System (GGOS) to the Integrated Global Observing Strategy Partnership (IGOS-P) through the theme “Earth system Dynamics,” in Dynamic Planet, Monitoring and Understanding a Dynamic Planet with Geodetic and Oceanographic Tools (Tregoning. P and C. Rizos, editors), IAG Symposium, Cairns, Australia, 22-26 August, 2005, Springer, Part VI, GGOS: Global Geodetic Observing System, 2007, pp. 727-734. 4. Seeber G.: Satellite geodesy, 2nd edition, Walter de Gruyter Berlin New York, 2003. 5. Tregoning. P and C. Rizos (editors): Dynamic Planet, Monitoring and Understanding a Dynamic Planet with Geodetic and Oceanographic Tools, IAG Symposium, Cairns, Australia, 22-26 August, 2005, Springer, Part VI, GGOS: Global Geodetic Observing System, 2007, pp. 701-766. Additional Readings 1. Keller, W.: Satellite Geodesy (http://www.unistuttgart.de/gi/geoengine/sat_geod/satgeodesy.pdf). 2. National Geodetic Survey: Geodesy for the Layman (http://www.ngs.noaa.gov/PUBS_LIB/Geodesy4Layman/TR80003D.HTM). 3. Ashkenazi, V.: Geodesy and Satellite Navigation: A Lighthearted Look at a Strange Relationship (http://www.ife.unihannover.de/mitarbeiter/seeber/seeber_65/pdf_65/ashk2.pdf)