Institut für Erdmessung

Institut für Erdmessung Webseite

Projekts:

• High resolution analysis of GRACE sensor data
• Global gravity field modeling using satellite data
• Modellierung des Einflusses atmosphärischer Turbulenz auf GNSS-Phasenbeobachtungen

Tamara Bandikova and Jakob Flury

The Gravity Recovery and Climate Experiment (GRACE) is a joint mission of the National Aeronautics and Space Administration (NASA) and the German Aerospace Center (DLR). Since its launch in 2002, this gravity field satellite mission successfully provides unique information about the global Earth’s gravity field and its spatial and temporal variations. GRACE consists of two identical satellites flying in the polar orbit around the Earth in a mutual distance of approximately 220 km carrying out ?m precise inter-satellite ranging.

As the satellite constellation and sensor equipment is unique, extensive sensor and data analysis is needed. The research focus at Institute für Erdmessung (IfE) lies in the sensor analysis of GRACE mission, especially in the analysis of accelerometer data and attitude variations. The aim of our work is to understand and reduce disturbance effects in gravity field determination and to support the mitigation of noise sources for next generation missions.

Almost nine years of observations represent a huge amount of data which allow a high resolution analysis of long-time series of onboard sensor and instrument data. For this data processing, the RRZN compute servers are used with a great advantage.

Publications:

• Bandikova T, Flury J, Ko U-D: Impact of spacecraft attitude variations on the GRACE scientific results.
  Poster. EGU General Assembly, 2.-7.5.2010, Vienna.
• Bandikova T, Flury J, Ko U-D: Analysis of systematic effects in GRACE inter-satellite pointing.
  Poster. GRACE Science Team Meeting, 11.-12.11.2010, Potsdam.

Contact information:

• Prof. Dr.-Ing. Jakob Flury
• Dipl.-Ing. Tamara Bandikova

Further information:

• Institut für Erdmessung
• Centre for Quantum Engineering and Space-Time Research - QUEST

Majid Naeimi

The determination of the Earth's gravity field is one of the major topics in geodesy and geophysics. The Earth's gravity field is dependent on the mass structure and distribution within the Earth system. Since there is only insufficient information available about the density of the Earth, gravity observations are used to determine the Earth's gravity field, including terrestrial and airborne observations as well as satellite gravimetry. The determination of the gravitational potential of the Earth from space is possible since the early 1970s, and especially the current satellite missions CHAMP, GRACE and GOCE are providing precise and homogeneous observations of the Earth’s gravity.

A common and well-known method to represent the gravity field of the Earth is the so-called spherical harmonic analysis. The resulting models are available in the form of Stokes' coefficients. The necessary computations are quite laborious and time/memory consuming. For example, to develop the Earth’s gravity field up to degree and order 120 in terms of spherical harmonics, based on 3 months of satellite observations with a sampling rate of 5 seconds, a linear system of equations has to be solved with more than 1.5 million data points and over 14500 unknowns. The system of equations is solved by least-squares adjustment, where the normal equation matrix (size in this example 14500 x 14500) has to be inverted. Thus the computations cannot be implemented on usual computers and PCs, and therefore the RRZN compute servers were employed to run the MATLAB programs for computing the Earth gravity models. A typical gravity modeling needs 10 to 20 Gigabytes of memory and takes 4-5 hours to be finished. The figure below shows the Earth’s gravity as computed from satellite observations.

Publications:

• Naeimi, M., Mueller J., Flury, J.: On the spatial pattern of semi-annual signals present in GRACE gravity fields.
  Poster, EGU General Assembly, Wien, 21.04.2009.
• Naeimi M, Mueller J, Flury J.: Temporal mass variations in Northern India as observed by GRACE.
  Poster AGU 2009 Joint Assembly, The meetings of Americas, 24-27 May 2009, Toronto, Ontario, Canada.
• Naeimi M, Optiga Team: Comparing different approaches for processing GRACE level-1 data.
  Poster, EGU General Assembly, Wien, May 2010.
• Müller, J., Naeimi, M., Gitlein, O., Timmen, L., Denker, H.: A land uplift model in Fennoscandia combining GRACE and absolute gravimetry data.
  Physics and Chemistry of the Earth, 2010.

Contact:

• M.Sc. Majid Naeimi
• Dr.-Ing. Heiner Denker

Further information:

• Institut für Erdmessung
• Centre for Quantum Engineering and Space-Time Research -- QUEST

Markus Vennebusch und Steffen Schön

Signale von Navigationssatelliten (wie z.B. des Global Positioning Systems GPS) werden zur Positionierung von Objekten und zur präzisen Vermessung in regionalen oder globalen Netzen verwendet. Dass dabei die Laufzeitverzögerung der Signale aufgrund von troposphärischen Effekten einen großen Einfluss hat, ist bereits seit Beginn von satellitenbasierten Vermessungen bekannt. Der Einfluss der Troposphäre wird gewöhnlich in einen gut zu modellierenden `trockenen' Anteil und einen variablen (und daher zu bestimmenden) `feuchten' Anteil unterteilt. Dieser variable Anteil ist von periodischen (z.B. jährlichen oder täglichen) meteorologischen Effekten gekennzeichnet; er weist aber auch hochfrequente Variationen auf, die Periodenlängen im Bereich von Minuten bis Sekunden besitzen. Diese hochfrequenten Variationen werden beispielsweise durch Konvektion oder Wind-Scherung verursacht und bewirken so Fluktuationen des Brechungsindexfeldes, die wiederum hochfrequente Fluktuationen der Phasenlage und der Amplitude der elektromagnetischen Trägerwelle des GPS-Signals erzeugen.

Basierend auf der Theorie der Wellenausbreitung in turbulenten Medien (`Turbulenztheorie') kann der Einfluss atmosphärischer Turbulenz stochastisch modelliert werden. Dazu müssen Varianz-Kovarianz-Matrizen aufgestellt werden, die je nach Szenario mehrere Millionen Einträge aufweisen, von denen jeder über ein analytisch nicht lösbares Mehrfachintegral über einen aufwändigen Integrationskern berechnet wird. Diese Berechnungen werden seit 2009 mit dem Cluster-System des RRZN durchgeführt.

Das Projekt wird gefördert durch die Deutsche Forschungsgemeinschaft (SCHO 1314/1-1).

Veröffentlichungen:

• Vennebusch M., S. Schön und U. Weinbach (2010): Temporal and spatial stochastic behaviour of high-frequency slant tropospheric delays from simulations and real GPS data, Advances in Space Research, Special Issue: GNSS Remote Sensing, Elsevier, DOI 10.1016/j.asr.2010.09.008
• Vennebusch M. und S. Schön (2009): Generation of slant tropospheric delay time series based on turbulence theory, Conference proceedings, Geodesy for Planet Earth - IAG 2009, Buenos Aires, (accepted for publication in IAG Symposia Series).

Ansprechpartner:

• Prof. Dr.-Ing. Steffen Schön
• Dr.-Ing. Markus Vennebusch

Weitere Informationen:

• Institut für Erdmessung