Ray-traced delays in the atmosphere for geodetic VLBI

Geometry of a 1D ray-tracing method for a receiver located at P1 and the upper limit of the troposphere at Pk. Points P1 and P2 are two sample points of the ray path. Figure taken from Nilsson et al. (2014).

Tropospheric delay modeling is the major error source in the analysis of geodetic VLBI observations, and in order to comply with the ambitious GGOS accuracy goals of 1 mm in position and 0.1 mm/year in velocity for the terrestrial reference frame, it is imperative to improve existing tropospheric correction models. The determination of slant delays by ray-tracing through high-resolution data from numerical weather models (see the illustration on the right) is the most rigorous approach possible today, suited to replace conventional mapping function strategies in VLBI data analysis.

Project RADIATE VLBI aims to determine ray-traced delays for the whole history of about five million geodetic VLBI observations since 1979 from operational analysis and re-analysis data of the European Centre for Medium-range Weather Forecasts (ECMWF) as well as for all upcoming VLBI observations from forecasting data to have the delays available in real-time. Computed on the supercomputers of the ECMWF with ray-tracing algorithms that have been developed at the Vienna University of Technology in the last years, our results are then used in global VLBI solutions to assess the impact on terrestrial and celestial reference frames as well as on Earth orientation parameters. Furthermore, "forecasted" ray-traced delays will be used in near real-time applications, in particular for the analysis of Intensive sessions.

For a limited time span of three years, we will also calculate ray-traced delays which are uniformly distributed over the sky above all geodetic VLBI sites with a 6-hourly time resolution. These products facilitate improvements of tropospheric blind models, e.g. by adding an azimuth-dependence to the mapping function coefficients or higher order spherical harmonics to the horizontal tropospheric gradients. All these new models will again be validated in global VLBI solutions against the results from standard and observation-wise approaches.


T. Nilsson, J. Böhm, D. D. Wijaya, A. Tresch, V. Nafisi, H. Schuh. Path Delays in the Neutral Atmosphere, in J. Böhm and H. Schuh (eds): Atmospheric Effects in Space Geodesy, Springer Verlag, ISBN 978-3-642-36931-5, pp. 73-136, 2013.

V. Nafisi, M. Madzak, J. Böhm, A. A. Ardalan, H. Schuh. Ray-traced tropospheric delays in VLBI analysis, Radio Science, Vol. 47, RS2020, doi:10.1029/2011RS004918, 2012.