JWG C.8: Optimal processing and homogenization of GNSS-PWclimate data records

Chair: Olivier Bock (France)
Vice chair: Galina Dick (Germany)
Affiliations: Commission 4, IGS, GGOS

Introduction

Water vapor plays a key role in the Earth’s climate and weather as a dominant greenhouse gas and the most efficient actor of heat transfer from the surface to the atmosphere and from low to high latitudes. As the climate warms, the amount of water vapor in the atmosphere is expected to rise. This implies a cascade of changes in the global and regional water cycles and in weather extremes. Monitoring and understanding the spatial and temporal variability and changes of water vapor are thus of crucial importance. Not surprisingly, water vapor is one of the five priority Essential Climate Variables (ECVs) targeted by the Global Climate Observing System (GCOS). GNSS-Precipitable Water (PW) is currently one of the constituent observing techniques of the GCOS Reference Upper Air Network (GRUAN) and is called to contribute more massively to GCOS thanks to its widely expanding networks covering all the continents.In accordance with IAG’s general missions, the objectives of this WG are to coordinate the collection, processing, qualification, and interpretation of GNSS-PW observations within the geodetic community and provide scientific guidance on the usage of these data to the climate community. The motivations for this WG are the following. There are currently no clear guidelines in the geodetic practice on how to produce the most accurate and homogeneous tropospheric parameters (ZTDs, gradients) and convert ZTD to PW. A substantial part of the issues is related with the data processing schemes which are usually tuned to produce accurate station positions and/or satellite products (orbits, clocks), in which the tropospheric parameters are of secondary interest. Whilst station heights and ZTDs are tightly correlated parameters, ZTDs are more prone to absorb short-term observation and modeling errors, and an optimal positioning scheme does not guarantee optimal ZTD estimates. A dedicated tuning of processing models and settings would thus be beneficial to produce more accurate tropospheric products from GNSS measurements (with application also to DORIS and VLBI). Another issue for climateapplications comes from the presence of inhomogeneities in the GNSS time series due to instrumental changes, changes in the antenna’s environment, or changes in the processing and post-processing procedures. The latter can be circumvented by considering products obtained with fixed processing and post-processing schemes, including consistent and homogeneous auxiliary data. The impact of instrumental changes is handled during the data processing thanks to station metadata (e.g. antenna and radome information provided in site-logs). The other nuisances can be corrected in post-processing homogenization procedures. The elaboration of consistent and valuable GNSS climate data records (CDRs) needs to tackle all these issues and in addition to provide uncertainty estimates associated with each observation.

Objectives

• Bring together experts from the geodetic community to establish optimal processing and post-processing strategies with the aim of minimizing biases and inhomogeneities in tropospheric products (ZTDs, PWs, gradients) and providing realistic uncertainty estimates;
• Provide recommendations on optimal processing and post-processing procedures to produce "climate-grade" tropospheric products, and coordinate their implementation with the IGS Tropo WG and ACs in the framework of future reprocessing campaigns;
• Check with the IGS Infrastructure Committee, IGS ACs, and GGOS that the station metadata information in IGS site-logs and RINEX files is consistent and up to date;
• Evaluate and stimulate the development of Quality Check/Quality Assurance(QC/QA) methods and tools for the analysis and interpretation of GNSS ZTD and PW time series (e.g. outlier detection and screening methods, bias and inhomogeneity detection and correction methods, homogenization tools, noise structureanalysis, etc.);
• Provide recommendations on QC/QA methods and tools for the elaboration of consistent GNSS-PW CDRs from reprocessed ZTD solutions;
• Coordinate the intercomparison and qualification of relevant tropospheric datasets (e.g. IGS reprocessings) by comparison with reference observations (e.g. fromGRUAN);
• Cooperate with GCOS and the Copernicus Climate Change Service (C3S) to establish a repository of qualified GNSS-PW CDRs (e.g. GRUAN and IGS as reference CDRs and other GNSS networks as baseline CDRs)
  

Program of Activities

• Organize regular meetings with WG members to share scientific results and coordinate collaborative work;
• Elaborate and disseminate recommendations and associated tools for the optimal processing and post-processing/homogenization of GNSS tropospheric products;
• Meetings with IGS and GGOS on related topics;
• Meetings with GCOS and Copernicus to establish a repository of qualified GNSS-PW climate data records;
  

Members

• Samuel Nahmani (France)
• Arnaud Pollet (France)
• Paul Rebischung (France)
• Pierre Bosser (France)
• Florian Zus (Germany)
• Markus Bradke (Germany)
• Tzvetan Simeonov (Germany)
• Jonathan Jones (UK)
• Kalev Rannat (Estonia)
  
• Hannes Keernik (Estonia)
• Katarzyna Stepniak (Poland)
• Anna Klos (Poland)
• Johannes Boehm (Austria)
• Alvaro Santamaria (France)
• Sylvain Loyer (France)
• Sharyl Byram (USA)