Journal of Geodynamics 58 (2012) 44–61 Contents lists available at SciVerse ScienceDirect Journal of Geodynamics j ournal homepage: http://www.elsevier.com/locate/jog Review Geocenter motion and its geodetic and geophysical implications a,∗ b c Xiaoping Wu , Jim Ray , Tonie van Dam a Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA b NOAA National Geodetic Survey, Silver Spring, MD 20910, USA c University of Luxembourg, Luxembourg, Luxembourg a r t i c l e i n f o a b s t r a c t Article history: The horizontal transport of water in Earth’s surface layer, including sea level change, deglaciation, and Received 4 July 2011 surface runoff, is a manifestation of many geophysical processes. These processes entail ocean and atmo- Received in revised form 12 January 2012 sphere circulation and tidal attraction, global climate change, and the hydrological cycle, all having a Accepted 14 January 2012 broad range of spatiotemporal scales. The largest atmospheric mass variations occur mostly at synoptic Available online xxx wavelengths and at seasonal time scales. The longest wavelength component of surface mass transport, the spherical harmonic degree-1, involves the exchange of mass between the northern and southern Keywords: hemispheres. These degree-1 mass loads deform the solid Earth, including its surface, and induce geo- Geocenter motion center motion between the center-of-mass of the total Earth system (CM) and the center-of-figure (CF) of Satellite geodesy the solid Earth surface. Because geocenter motion also depends on the mechanical properties of the solid Reference frame Surface mass variation Earth, monitoring geocenter motion thus provides an additional opportunity to probe deep into Earth’s Glacial isostatic adjustment interior. Most modern geodetic measurement systems rely on tracking data between ground stations and satellites that orbit around CM. Consequently, geocenter motion is intimately related to the realization of the International Terrestrial Reference Frame (ITRF) origin, and, in various ways, affects many of our measurement objectives for global change monitoring. In the last 15 years, there have been vast improve- ments in geophysical fluid modeling and in the global coverage, densification, and accuracy of geodetic observations. As a result of these developments, tremendous progress has been made in the study of geo- center motion over the same period. This paper reviews both the theoretical and measurement aspects of geocenter motion and its implications. © 2012 Elsevier Ltd. All rights reserved. Contents 1. Introduction . 45 2. Theory of geocenter motion due to a degree-1 load .. 46 2.1. Elastic Earth deformation . 47 2.2. Viscoelastic Earth deformation. 48 3. Geophysical causes. 49 3.1. Seasonal and interannual surface mass transport .. 49 3.2. Non-seasonal mass transport . 50 3.3. Long-term mass transport processes .. 51 4. Geodetic implications . 52 4.1. Geocenter motion and reference frame ... 52 4.2. Geocenter motion and sea level . 52 5. Measurements .. 53 5.1. Translational approaches . 53 5.2. Degree-1 and inverse approaches ... 54 5.3. Geocenter velocity. 56 6. Discussion .. 56 6.1. Prospects for geodetic measurements . 56 ∗ Corresponding author. E-mail address: xp [email protected] (X. Wu). 0264-3707/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jog.2012.01.007 X. Wu et al. / Journal of Geodynamics 58 (2012) 44–61 45 6.2. Status and prospects for global fluid models . 58 6.3. Prospects for theoretical developments and routine geocenter monitoring . 59 7. Conclusions . 59 Acknowledgements . 60 References . 60 1. Introduction In most cases, however, the non-tidal predictions tend to be less reliable. By and large, the data assimilating or forward-fitting mod- The center-of-mass (CM) of the total Earth system, that is the els are incomplete, non-unique, and highly uncertain over certain solid Earth and its fluid envelope, is usually referred to as the geo- spatiotemporal domains due to poorly known driving forces, inac- center (Petit and Luzum, 2010). The geocenter is used to describe curately modelled process dynamics and parameters, as well as Earth’s motion in inertial space and serves as the orbital center poor input data coverage and accuracy. There is also a general lack for all Earth satellites. In this paper, the term “geocenter motion” of realistic uncertainty assessments for the models. As we enter the is defined as the motion of the CM with respect to the center- age of millimeter accuracy in geodesy, the use of models to esti- of-figure (CF) of the solid Earth surface (e.g., Ray, 1999 and the mate geocenter motion is not sufficiently precise for many modern contributions therein). (The reader should bear in mind that other geodetic applications. conventions/definitions exist and are present in the literature.) Yet Geocenter motion can also be directly observed. Observing it is the solid Earth (and its CF origin) that actually moves within the geocenter motion is theoretically simple, i.e., tracking satellites reference frame if the origin is defined at CM. This is the convention orbiting around CM from stations located on Earth’s surface. In that is generally recommended for most physical modeling, such as reality, actual measurements of such small motions are very chal- in the IERS Conventions (Petit and Luzum, 2010) for general rela- lenging due to the stringent consistency and accuracy requirements tivistic formulations. Furthermore, the origin of the secular ITRF that must be provided by the tracking data themselves and the (Altamimi et al., 2007, 2011) is a long-term average of satellite CM kinematic/dynamic modeling of the geodetic systems. In fact, mea- realizations. In any case, the concept of relative motion is important suring geocenter motion is at the forefront of high-precision global because CM can only be observed from points fixed to the Earth’s geodetic endeavors and has become a robust performance indica- surface via geodetic satellite data, integrated over certain discrete tor for various geodetic techniques (Stolz, 1976; Vigue et al., 1992; time intervals. Eanes et al., 1997; Watkins and Eanes, 1997; Pavlis, 1999; Bouille Ever since the publication of the review article of Farrell (1972), et al., 2000; Cretaux et al., 2002; Moore and Wang, 2003; Collilieux geocenter motion has been recognized as a direct consequence et al., 2009; Kang et al., 2009; Cheng et al., 2010; Rebischung and of degree-1 (n = 1) surface or internal (Greff-Lefftz and Legros, Garayt, 2010). 1997) loading and involves a special case of the load-induced Another approach to observing geocenter motion was first rec- deformation in the spherical harmonic domain. This unique, very ognized by Blewitt et al. (2001). The authors demonstrated the long-wavelength component is important (critically so in many possibility of observing the n = 1 deformation and its annual vari- cases) for understanding global mass redistribution processes such ability using a set of globally distributed.