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Physical Geodesy

The chair Physical Geodesy is led by Prof. Dr. Frank Flechtner in the framework of a joint professorship between TU Berlin and the GFZ German Research Centre for Geosciences since March 2013. At GFZ he is head of Section 1.2 "Global Geomonitoring and Gravity Field".

Overview

One of the most important physical quantities known from lessons at school, the gravity acceleration at the Earth surface (g ≈ 9.81m/s²), is far from being a constant value. In that case our home planet would be a sphere and the masses inside would be equally, radial symmetric, distributed. This is certainly not the case and makes gravity to one of the most important quantities in physics and geodesy. Additionally, most of geodetic measurements are depending on gravity, as all instruments which have to be leveled, have to be aligned to the local plumb line.

The fundamental task of physical geodesy is to measure, describe and model the gravity field of the Earth from gravity observations performed at the Earth surface or in space. Only with a precise knowledge of the gravity field many processes inside and at the Earth surface can be understood. The precise knowledge of the representation of the global gravity field can be used as a reference both for physical as also for geometric questions in geodesy.

An important contribution is the temporal variation of the gravity field which reflects mass transports in system Earth caused by

  • Variations in sea level and ocean surface currents,
  • Variations in the continental hydrological cycle,
  • Melting of ice in the large glacier systems, or
  • Mass displacements in tectonically active regions.

Further on, Physical Geodesy enables the definition of global height systems for the precise representation of topography, sea surface or bathymetry.

The methods of Physical Geodesy can also be applied for the modeling of the gravity field of other stellar objects. For example, the motion of artificial satellites around planets and moons can be used to analyze their gravity field and to derive information on their inner structure or origin.

The US-German GRACE (Gravity Recovery and Climate Experiment) mission was dedicated to analyze large-scale temporal variations of the Earth's gravity field. For this, a pair of twin satellites was in a polar orbit with an initial altitude of 500 km (end of mission ca. 320 km) between March 2002 and June 2017. The co-orbiting satellites, which are separated by approximately 220 km, are sensitive to variations in the gravity field. The resulting changes in the inter-satellite distance, which were observed by a microwave tracking system with a precision of a tenth of the thickness of a human hair, were used at the GRACE Science Data System at JPL (Pasadena, California), UTCSR (Austin, Texas) and the GFZ German Research Center for Geosciences in Potsdam to derive each month a map of the global gravity field. The actual 6th version (RL06) of reprocessed GRACE gravity maps shows a significantly decreased noise and higher spatial resolution compared to precursor gravity models.

Between March 2009 and November 2013 the ESA mission GOCE (Gravity field and steady-state Ocean Circulation Explorer) flew in an extremely low orbit of only 250 km (at the end only 235 km). The primary mission goal was to derive a static (or mean) gravity field of the Earth with high resolution and unprecedented accuracy. For this the satellite was equipped with six accelerometers which directly observe the gravity gradient in all three spatial directions.

GRACE-FO (GRACE Follow-on) has continued the GRACE time series starting in May 2018. GRACE-FO has been currently implemented and will be operated and evaluated jointly by NASA and GFZ. To improve the inter-satellite distance measurement and consequently the accuracy of the derived gravity field models, a Laser Ranging Interferometer (LRI) will be operated in parallel to the GRACE microwave tracking system as a demonstrator for Next Generation Gravity Missions (NGGM) which will be solely based on Laser technology and (likely) multiple pairs on different orbits.

Current terrestrial methods are based on aero-gravimetry data of the German HALO (High Altitude and LOng Range Research Aircraft) experiment. Together with supra-conducting and absolute gravimeter observations and altimetry data from various satellite missions the terrestrial data are used for combination with the mentioned GRACE/GOCE satellite data and generation of gravity models with ultra-high spatial resolution (less than 10 km).

The themes and methods of Physical Geodesy are conveyed in the framework of the master study “M.Sc. Geodesy and Geoinformation Science” within the module “SGN Physical Geodesy”. The addressed topics are

  • Fundamentals and basic ideas of Physical Geodesy
  • Fundamentals in potential theory, spherical harmonics and geodetic boundary value problems
  • Gravity field of the Earth: normal potential and normal gravity, linear model of Physical Geodesy, and geoid determination
  • Gravity observations (absolute, relative, on moving platforms, continuous) and gravity reductions
  • Physically and geometrically defined height systems
  • Space- based methods and new satellite missions

For self-study the following literature is recommended.

  • Hofmann-Wellenhof, B. und Moritz, H. (2006): Physical Geodesy, second, corrected edition. Springer Wien New York
  • Heiskanen, H., Moritz, H. (1985): Physical Geodesy, Nachdruck TU Graz
  • Torge, W. (2003):Geodäsie, Walter de Gruyter Verlag, Berlin New York
  • Torge, W. (1989):Gravimetrie, Walter de Gruyter Verlag, Berlin New York

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Contact

Chair of Physical Geodesy
Technische Universität Berlin
Institute of Geodesy and Geoinformation Science
Faculty VI
sec. H 12
Main Building
Room H 5121
Straße des 17. Juni 135
10623 Berlin
+49 30 314 23205
+49 30 314 21973 (Fax)