The Joint Working Group "Intra- and Inter-Technique Atmospheric Ties" was approved by the International Association of Geodesy’s (IAG) executive committee in December 2019, for the term 2019-2023. It falls under Commission 1 (Reference Frames) and is joint with Sub-Commission 4.3 (Atmosphere remote sensing), and GGOS (Global Geodetic Observing System). The activities of this WG act as a follow-on to IAG JWG 1.3 (chair: Robert Heinkelmann, vice-chair: Jan Douša, term: 2015-2019), and are coordinated by Kyriakos Balidakis (chair) and Daniela Thaller (co-chair).

The differences between atmospheric parameters (mainly zenith delays and gradients) at co-located stations that observe nearly simultaneously, and stem from external systems (e.g., meteorological sensors or weather models) are understood as atmospheric ties. Atmospheric ties mainly exist because of differences in (i) the observing frequency, (ii) the relative position, and (iii) the observing system set-up.

The acquisition of accurate atmospheric delay corrections is of paramount importance for mm-level positioning employing space geodetic techniques. Atmospheric delay corrections may stem from dedicated instruments such as water vapor radiometers, meteorological sensors, numerical weather models, or from the geodetic data itself. While the latter is fairly common for modern GNSS and VLBI, observation geometry and accuracy limitations inherent to other systems such as SLR and DORIS impede the accurate atmospheric parameter estimation, thus hindering among else positioning. To this end, it might be useful to compare and combine atmospheric parameters at co-located sites, in a manner similar to the combination of station and satellite coordinates, as well as Earth rotation parameters (via local, space, and global ties, respectively). The multi-technique combination is indispensable to the distinction between real signals and undesired technique-specific artefacts. Nowadays, the multi-technique combination is facilitated by the increasing investments in state-of-the-art geodetic infrastructure at co-located sites. However, a host of systematic and random errors render the combination via atmospheric ties a difficult task. Moreover, since atmospheric delays are dependent upon essential climate variables (pressure, temperature, and water vapor), differences in long-term atmospheric delay time derivatives at co-located stations might offer an insight into local climate change.

The purpose of this working group is to answer the questions:

(i) How can one relate atmospheric (electrically neutral) parameter estimates and the time derivatives thereof that refer to different place, time, and observing system? What are the limits in distance, time lag, and observing system?

(ii) What is the optimal way to combine atmospheric parameters?

(iii) What is the benefit from including atmospheric ties in a multi-technique terrestrial reference frame combination?

(i) Comparison of atmospheric (electrically neutral) delay estimates from single-technique geodetic analysis (GNSS, SLR, VLBI, and DORIS)

(ii) Comparison of atmospheric delays from state-of-the-art meso-β scale weather models (e.g., ERA5 and MERRA2), and high-resolution runs utilizing the Weather Research and Forecasting (WRF) Model

(iii) Assessment of spatial and temporal correlation between atmospheric parameters

(iv) Assessment of multi-technique combination employing atmospheric ties on the single site and global TRF level

Members (in alphabetical order): Kyriakos Balidakis, David Coulot, Mateusz Drożdżewski, Claudia Flohrer, Changyong He, Robert Heinkelmann, Chaiyaporn Kitpracha, Frank Lemoine, Lisa Lengert, Tobias Nilsson, Arnaud Pollet, Marcelo Santos, Benedikt Soja, Krzysztof Sośnica, Daniela Thaller, Xiaoya Wang, Dudy Wijaya, and Florian Zus

Further Details: ftp://ftp.gfz-potsdam.de/pub/home/kg/kyriakos/iag_jwg_atmospheric_ties/

Selected References:

Krügel, M., D. Thaller, V. Tesmer, M. Rothacher, D. Angermann, and R. Schmid (2007) Tropospheric parameters: combination studies based on homogeneous VLBI and GPS data, J. Geod., doi: 10.1007/s00190-006-0127-8

Pollet, A., D. Coulot, O. Bock, and S. Nahmani (2014) Comparison of individual and combined zenith tropospheric delay estimations during CONT08 campaign, J. Geod., doi: 10.1007/s00190-014-0745-5

Rothacher, M., D. Angermann, T. Artz, W. Bosch, H. Drewes, M. Gerstl, R. Kelm, D. König, R. König, B. Meisel, H. Müller, A. Nothnagel, N. Panafidina, B. Richter, S. Rudenko, W. Schwegmann, M. Seitz, P. Steigenberger, S. Tesmer, V. Tesmer, and D. Thaller  (2010) GGOS-D: homogeneously reprocessing and rigorous combination of space geodetic techniques. J. Geod. doi: 10.1007/s00190-011-0475-x

Thaller, D.  (2008) Inter-technique combination based on homogeneous normal equation systems including station coordinates, Earth orientation and troposphere parameters. PhD thesis. Scientific Technical Report STR08/15, Deutsches GeoForschungsZentrumPotsdam (GFZ), Germany. doi: 10.2312/GFZ.b103-08153

Thaller, D., V. Tesmer, R. Dach, M. Krügel, M. Rothacher, and P. Steigenberger (2008) Combining VLBI Intensive with GPS Rapid solutions for deriving a stable UT time series, IVS 2008 General Meeting Proceedings

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