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Concept# World Geodetic System

Summary

The World Geodetic System (WGS) is a standard used in cartography, geodesy, and satellite navigation including GPS. The current version, WGS 84, defines an Earth-centered, Earth-fixed coordinate system and a geodetic datum, and also describes the associated Earth Gravitational Model (EGM) and World Magnetic Model (WMM). The standard is published and maintained by the United States National Geospatial-Intelligence Agency.
History
Efforts to supplement the various national surveying systems began in the 19th century with F.R. Helmert's famous book Mathematische und Physikalische Theorien der Physikalischen Geodäsie (Mathematical and Physical Theories of Physical Geodesy). Austria and Germany founded the Zentralbüro für die Internationale Erdmessung (Central Bureau of International Geodesy), and a series of global ellipsoids of the Earth were derived (e.g., Helmert 1906, Hayford 1910/ 1924).
A unified geodetic system for the whole world became essential in the 1950s for

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Accretion disks surrounding compact objects, and other environmental factors, deviate satellites from geodetic motion. Unfortunately, setting up the equations of motion for such relativistic trajectories is not as simple as in Newtonian mechanics. The principle of general (or Lorentz) covariance and the mass-shell constraint make it difficult to parametrize physically adequate 4-forces. Here, we propose a solution to this old problem. We apply our framework to several conservative and dissipative forces. In particular, we propose covariant formulations for Hooke???s law and the constant force and compute the drag due to gravitational and hard-sphere collisions in dust, gas, and radiation media. We recover and covariantly extend known forces such as Epstein drag, Chandrasekhar???s dynamical friction, and Poynting-Robertson drag. Variable-mass effects are also considered, namely, Hoyle-Lyttleton accretion and the variable-mass rocket. We conclude with two applications: (1) The free-falling spring, where we find that Hooke???s law corrects the deviation equation by an effective anti???de Sitter tidal force and (2) black hole infall with drag. We numerically compute some trajectories on a Schwarzschild background supporting a dustlike accretion disk.

1999

Mathilde Edmée Lucette Radiguet de la Bastaie

The 2006 Slow Slip Event in Guerrero, Mexico, is one of the world's largest SSE observed up to now. Thanks to the development of the Guerrero GPS network, the 2006 event was recorded at 15 continuous GPS stations that allowed a detailed study of the surface displacements produced by the event. Temporal variations of the displacement vectors, observed during the 2006 SSE, suggest a complex spatio-temporal evolution of slip on the plate interface. We developed a new method to model the spatial and temporal evolution of slip on the fault plane. We assume a functional form for the slip function on the fault plane, similar to the slip function defined by Liu et al. (2006) for kinematic source inversion of regular earthquakes. This slip function is parameterized by the slip amplitude, the initiation time of the slip, and the slip duration, which is the combination of an accelerating and decelerating rise time. The fault plane is divided in 42 subfaults of 78 km x 25 km each. The Green functions are calculated for a layered elastic half space. First, forward modeling tests show that the geodetic data are sensible to the location of the initiation slip patch, the propagation of the rupture and the duration of the slip. Second, using the same parameterization for the slip function, we invert for the slip amplitude, slip duration and rupture propagation velocity. We perform a linearized least-square inversion in the frequency domain. This new inversion procedure provides a continuous image of the spatial and temporal variatio

2009