Radiate ORD

In the troposphere (strictly speaking in the neutral atmosphere up to about 80 kilometres), signals from Global Navigation Satellite Systems (GNSS) such as the U.S. Global Positioning System (GPS) or the European system Galileo are slower than velocity of light. If not corrected for those tropospheric delays, the range measurements between ground stations and GNSS satellites would be too long by up to 20 metres, thereby significantly degrading the positioning accuracy. Along those lines, errors in the delay models propagate into geodetic parameters such as station coordinates, thus limiting high- accuracy applications like reference frame realisation where we are striving for millimetre-accuracy, but also navigation tasks with GNSS with decimetre or metre-accuracies. Reference frames of highest accuracy are critical for many applications, e.g. for the observation of sea level rise. Technische Universität Wien (TU Wien) world-renowned for determining and providing the most accurate tropospheric delay models globally available. These models comprise the Vienna Mapping Functions (VMF1), which are determined from operational data of the European Centre for Medium- Range Weather Forecasts (ECMWF) for specific geodetic sites and global grids with a time resolution of six hours, as well as empirical ("blind") models, such as the Global Mapping Functions (GMF) and the Global Pressure and Temperature (GPT) series, which do not need further input parameters. The main objective of project Radiate ORD as submission to the Call on "Open Research Data Initiatives" by the Austrian Science Fund (FWF) is to make the current service fully adhere to open access policy. Consequently, Radiate ORD will focus on the consolidation and solidification of all processes necessary to generate and provide the research data following the guidelines of the open access policy. These steps include the improvement and modernisation of the present data flow with stable backup procedures. Furthermore, the interface to the repository of open research data at TU Wien has to be improved, comprising the addition of metadata and the development of clear strategies for versioning and updating the data to allow for full and easy open access. Beyond that, we will explore the possibility of moving the generation of the VMF1 coefficients to the ECMWF, either as parameters determined a posteriori from archived data or even as regular parameters of the operational runs at ECMWF. We will also add new tools and parameters to the suite of products already existing. For example, we will make our ray-tracing program for the determination of tropospheric delays from numerical weather models openly accessible as web-tool, and we will provide VMF1 coefficients for optical techniques like Satellite Laser Ranging (SLR) to allow for a consistent tropospheric delay modeling across optical and microwave techniques.

Electromagnetic signals from satellites or from faraway quasars are delayed and bent on their way through the Earths atmosphere. This is, e.g., dependent on the water content in the respective layers of the troposphere. Data from numerical weather models are ideally suited for determining this delay with very high precision. A technique referred to as ray- tracing allows to transform the meteorological quantities from the weather model into the respective troposphere delay of an electromagnetic wave. Furthermore, so-called mapping functions and gradient models can downscale these troposphere delays to arbitrary elevation and azimuth angles. Mapping functions and gradient models can be determined from various sources, however, those determined from numerical weather models are considered as the most accurate ones. The research division Higher Geodesy at TU Wien has been playing a major role in the development of such models for many years. The central point of this project is, as the name ORD = open research data implies, the public and free provision of all data and program codes. For this purpose we have initiated the website http://vmf.geo.tuwien.ac.at , where we offer the entirety of our troposphere products by our research division for free download. In the course of the predecessor project RADIATE VLBI, the ray-tracing program RADIATE as well as the models Vienna Mapping Functions 3, Global Pressure and Temperature 3 and GRAD were developed, which utilize numerical weather models by the European Centre for Medium-range Weather Forecasts (ECMWF) and can be used for space geodetic techniques such as GNSS, VLBI or DORIS. In the frame of the project RADIATE ORD, these models have first undergone a detailed analysis, before they were published on our website. Every day, the latest model coefficients are determined through automated processes. By August 2019, there is an average of 400 visitors and 30 GB of download per day from our server. Furthermore, a new model for troposphere delays at optical wavelengths, as are used in Satellite Laser Ranging (SLR), named Vienna Mapping Functions for optical frequencies (VMF3o) was developed. For the determination of the model coefficients of VMF3o, the program RADIATE was extended by a new module for ray-tracing in the optical frequency range and VMF3o was integrated to the daily, automated processes. In the sense of the open data guideline we have also made the program code of RADIATE publicly available. Additionally, users can request ray-traced delays for an arbitrary observation geometry, which are then automatically determined and sent to them by email. Apart from that, articles have been published which emphasize that the application of models such as VMF3 and GRAD as well as of the ray-traced delays indeed lead to improved VLBI products.