Raytraced Delays in the Atmosphere for Geodetic VLBI

Radiate: FWF P25320

Project-Leader: Johannes Böhm (1 May 2013 – 30 June 2017)

Summary for public relations work:

While atmospheric effects on the propagation of radio waves take place in the whole atmosphere, the troposphere as the lowest part plays a key role with its weather phenomena and the highly variable distribution of humidity. Modelling the delays in the troposphere of the signals from extragalactic radio sources in the analysis of geodetic Very Long Baseline Interferometry (VLBI) observations and of the signals from Global Navigation Satellite Systems (GNSS), such as the U.S. Global Positioning System (GPS) or the European Galileo, is a major error source influencing the accuracy of these space geodetic techniques. In particular, insufficient models may affect station coordinates and the terrestrial reference frame. The goal of the project RADIATE VLBI was to challenge the existing tropospheric delay models, thereby developing improved and new methods for the tropospheric calibration.

We used the concept of “ray-tracing" to determine values of those delays through applying electromagnetic wave equations onto data of numerical weather models provided by the European Centre for Medium-range Weather Forecasts (ECMWF). In the first part of project RADIATE VLBI we developed a sophisticated and fast ray-tracing program called RADIATE. This program was then used to determine slant tropospheric delays for all (more than 10 million) geodetic VLBI observations and to derive improved tropospheric delay models, such as the Vienna Mapping Function 3 (VMF3).
We found that the application of ray-traced delays in VLBI analysis in particular improves the solution of VLBI sessions with a small number of observations or if no tropospheric gradients are estimated. The newly developed VMF3 slightly improves the accuracy of the terrestrial reference frame with an impact on station heights up to two millimetres. Ray-traced delays and tropospheric gradients, the latter derived together with VMF3, do have a significant impact on the celestial reference frame with source declination changes up to more than 50 microarcseconds. In parallel to 6-hourly coefficients of the VMF3, we also derived so-called empirical delay models such as GPT3, which is fully consistent with VMF3 but only contains annual and semi-annual terms.
Although we could confirm that existing tropospheric delay models are already at a very high level of accuracy, we do provide new models (VMF3, GPT3, gradients) which improve the accuracy of geodetic parameters. However, it should be stressed that further research in tropospheric delay modelling is required to reach the goal of one millimetre accuracy in daily station coordinates. On the other hand, all models developed in project RADIATE VLBI may be implemented in positioning and navigation devices with GNSS receivers such as smartphones.

Further reading:

  • Hofmeister A., Böhm J. (2017). Application of ray-traced tropospheric slant delays to geodetic VLBI analysis, Journal of Geodesy Vol. 91(8), doi:10.1007/s00190-017-1000-7, pp. 945-964.
  • Landskron D., Böhm J. (2017). VMF3/GPT3: Refined Discrete and Empirical Troposphere Mapping Functions, Journal of Geodesy, doi: 10.1007/s00190-017-1066-2.