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Research Projects
Turbulenz Untersuchungen und verbesserte Modellierung der atmosphärischen Refraktion mit VLBI und GNSS

Research Projects | Positioning and Navigation

Turbulence investigations and improved modelling of atmospheric refraction with VLBI and GNSS

Leaders:  Prof. Dr.-Ing. Steffen Schön
Email:  schoen@ife.uni-hannover.de
Team:  Dipl.-Ing. Franziska Kube
Year:  2012
Sponsors:  DFG (SCHO 1314/3-1)
Is Finished:  yes

Motivation

Progress in improving the quality of space-geodetic observations for precise positioning and monitoring Earth rotation is hampered by insufficient knowledge of the temporal and spatial refractivity variations in the neutrosphere. Today, only the effects showing annual to hourly long periodic variations are considered. The short periodic fluctuations in the range of minutes to sub-second that are caused by turbulence in the first few kilometres above the ground, i.e. the atmospheric boundary layer, are still neglected. These refractivity fluctuations induce phase fluctuations of wavefronts passing through the turbulent medium. They form a significant error source for electromagnetic GNSS and VLBI wave propagation. Turbulent processes can be best described stochastically. Following the widely accepted Kolmogorov turbulence model, refractivity fluctuations with scales inside the inertial subrange, i.e. between an inner and outer scale length, can be described by specific power-law processes. Since the resulting phase fluctuations are non-stationary, dedicated analysis tools are applied such as temporal or spatial structure functions, i.e. variances of the process of first order differences. The prevailing turbulent regime is then characterized by the shape of the structure function yielding typical slopes in a log-log-plot of 5/3 or 2/3.

Goals

  • Improved characterization of refractivity fluctuations and determination of turbulence parameters

  • Enhanced modelling of neutrospheric refraction effects

Methodology

 

The basis for reaching these goals is the exciting opportunity of a unique small-scale observing network with two techniques in the radio frequency domain in parallel, namely VLBI and GPS. A cluster of geodetic radio Telescopes and continuous operating GNSS stations at the Geodetic Observatory Wettzell and the Wettzell footprint network opens up numerous unique opportunities for studies of variability of water vapour in the atmosphere and the resulting refraction effects. Besides existing methods like temporal and spatial structure functions, new analysis concepts have to be developed - not only to determine the structure constant and power law exponent but also further relevant atmospheric turbulence parameters, like e.g., the scale length or the wind direction and speed. Furthermore we will develop operational models to use actual turbulence information for an improved estimation of tropospheric delays for both VLBI and GNSS observations.