<p>Accurate positioning for activities such as navigation, mapping, and surveying rely on permanent stations located all over the world and continuously tracking Global Navigation Satellite Systems (GNSS, such as Galileo, GPS, GLONASS).&#160;<br>The Royal Observatory of Belgium maintains repositories containing decades of observation data from hundreds of GNSS stations belonging to Belgian and European networks (e.g., the EUREF public repository).&#160;<br>However, current procedures for accessing GNSS data do not adequately serve user needs. For example, in the case of the EUREF repository, despite the fact that its GNSS data originate from a significant number of data providers and could be handled in different ways, provenance information is lacking and data licenses are not always available.<br>In order to respond to user demands, GNSS data and the associated metadata need to be standardised, discoverable and interoperable i.e., made FAIR (Findable, Accessible, Interoperable, and Re-usable). Indeed, FAIR data principles serve as guidelines for making scientific data suitable for reuse, by both people and machines, under clearly defined conditions.&#160;<br>We propose to identify existing metadata standards that cover the needs of the GNSS community to the maximum extent and to extend them and/or to develop an application profile, considering also best practices at other GNSS data repositories.&#160;</p><p>Here we present two proposals for metadata to be provided to the users when querying and/or downloading GNSS data from GNSS data repositories.&#160;<br>We first consider metadata containing station-specific information (e.g., station owner, GNSS equipment) and propose an extension of GeodesyML, an XML implementation of the eGeodesy model aligned with international standards such as ISO19115-1:2014 and OGC's GML. The proposed extension contains additional classes and properties from domain specific vocabularies when necessary, and includes extra metadata such as data license, file provenance information, etc. to comply with FAIR data principles. All proposed changes to GeodesyML are optional and therefore guarantee full backwards compatibility.&#160;</p><p>Secondly, we consider metadata related to GNSS observation data i.e. RINEX data files. We propose an application profile based on the specifications of the Data Catalog Vocabulary (DCAT), a RDF vocabulary that, by design, facilitates the interoperability between data portals (supporting DCAT-based RDF documents) and enables publishing metadata directly on the web by using different formats.<br>In particular, our proposal (GNSS-DCAT-AP) includes new recommended metadata classes to describe the specific characteristics of GNSS observation data: the type of RINEX file (e.g., compression format, frequency); the RINEX file header and information regarding the GNSS station including the GNSS antenna and receiver; the software used to generate the RINEX &#160;file. Additional optional classes allow the inclusion of information regarding the GNSS antenna, receiver and monument associated with the GNSS station and extracted from the IGS site log or GeodesyML files</p>
<p>Global Navigation Satellite Systems (GNSS) are a widely spread cost effective technique for geodetic applications and monitoring the Earth&#8217;s atmosphere. Therefore, the density of the GNSS networks have grown considerable since the last decade. Each of the networks collects huge amounts of data from permanently operating GNSS stations. The quality of the data is variable, depending on the evaluated time period and satellite system. Conventionally, the quality information is extracted from daily estimates of different types of GNSS parameters such as number of data gaps, multipath level, number of cycle slips, number of dual frequency observations with respect to the expected number, and from their combinations.</p><p>The EUREF Permanent GNSS Network Central Bureau (EPN CB, Bruyninx et al., 2019) is operationally collecting and analysing the quality of more than 300 GNSS stations and investigates the main reason of any quality degradation. EPN CB is currently operating a semi-automatic (followed by a manual) data-monitoring tool to detect the quality degradations and investigate the source of the problems. In the upcoming years, this data-monitoring tool will be used to also monitor the GNSS component of the European Plate Observing System (EPOS) expected to include more than 3000 GNSS stations. This anticipated inflation of GNSS stations to be monitored will make it increasingly challenging to select the high quality GNSS data. EPN CB&#8217;s current system requires time-consuming semi-automatic inspection of data quality and it is not designed to handle the larger amounts of data. In addition, the current system does not exploit correlations between the daily data quality, time series and the GNSS station metadata (such as equipment type and receiver firmware) often common to many stations.</p><p>In this poster, we will first present the currently used method of GNSS data quality checking and its limitations. Based on more than 20 years of GNSS observations collected in the EPN, we will show typical cases of correlations between the time series of data quality metrics and GNSS station metadata. Then, we will set up the requirements and design the new GNSS data quality monitoring system capable of handling more than 300 stations. Based on the collected EPN samples and the typical cases, we will introduce ongoing improvements taking advantage of artificial intelligence techniques, show the possible design of the neutral network, and present supervised training of the neutral network.</p><p>Bruyninx C., Legrand J., Fabian A., Pottiaux E. (2019) GNSS Metadata and Data Validation in the EUREF Permanent Network. GPS Sol., 23(4), https://doi: 10.1007/s10291-019-0880-9</p>
<p>The IGS (International GNSS Service) site log format is the worldwide standard for exchanging GNSS station metadata. It contains, among other things, a description of the GNSS site and its surroundings, the contact persons, and an historical overview of the GNSS equipment. This information is valuable for reliable GNSS data analysis and interpretation of the results.</p><p>This IGS site log is also used within the EUREF Permanent Network (EPN, Bruyninx et al., 2019) and the GNSS component of the European Plate Observing System (EPOS, https://www.epos-eu.org/). However, due to their specific needs, these networks collect additional GNSS metadata. For example, within the EPN, individual receiver antenna calibration values are collected, as well as the information on the data provided by the station. EPOS is collecting in addition data licences. Within the Creative Commons permitted licence scheme, two licences will be adopted by EPOS, CC:BY and CC:BY:NC. Both licenses require that the data user acknowledges (cites) the data owner. To facilitate this data citation, EPOS recommends attributing Digital Object Identifiers (DOI) to the GNSS data and therefore also includes the DOI in the collected GNSS station metadata.</p><p>Many IGS and EPN stations also contribute to EPOS and therefore it is imperative to harmonize the collection and distribution of the additional metadata. The GeodesyML (http://geodesyml.org) format already allows including more metadata compared to the IGS site log format. In this poster, we will review the challenges and propose how to tackle them. We will finish by showing the choices made within the &#8220;Metadata Management and Distribution System for Multiple GNSS networks&#8221; (M<sup>3</sup>G) which collects and disseminates GNSS station metadata within both the EPOS and EPN networks.</p><p>Bruyninx C., Legrand J., Fabian A., Pottiaux E. (2019) GNSS Metadata and Data Validation in the EUREF Permanent Network. GPS Sol., 23(4), https://doi: 10.1007/s10291-019-0880-9&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;</p>
<p>Nowadays, we measure positions on Earth&#8217;s surface thanks to Global Navigation Satellite Systems (GNSS) e.g. GPS, GLONASS, and Galileo. Activities such as navigation, mapping, and surveying rely on permanent GNSS tracking stations located all over the world.<br>The Royal Observatory of Belgium (ROB) maintains and operates a repository containing data from hundreds of GNSS stations belonging to the European GNSS networks (e.g. EUREF, Bruyninx et al., 2019).&#160;</p><p>ROB&#8217;s repository contains GNSS data that are openly available and rigorously curated. The curation data include detailed GNSS station descriptions (e.g. location, pictures, and data author) as well as quality indicators of the GNSS observations.</p><p>However, funders and research policy makers are progressively asking for data to be made <em>Findable, Accessible, Interoperable, and Reusable (FAIR)</em> and therefore to increase data transparency, discoverability, interoperability, and accessibility.</p><p>In particular, within the GNSS community, there is no shared agreement yet on the need for making data <em>FAIR</em>. Therefore, turning GNSS data <em>FAIR</em> presents many challenges and, although <em>FAIR</em> data has been included in EUREF&#8217;s strategic plan, no practical roadmap has been implemented so far. We will illustrate the specific difficulties and the need for an open discussion including also other communities working on <em>FAIR</em> data.</p><p>For example, making GNSS data easily <em>findable</em> and <em>accessibl</em>e would require to attribute persistent identifiers to the data. It is worth noting that the International GNSS Service (IGS) is only now beginning to consider the attribution of DOIs (Digital Object Identifiers) to GNSS data, mainly to allow data citation and acknowledgement of data providers. Some individual GNSS data repositories are using DOIs (such as UNAVCO, USA).&#160; Are DOIs the only available option or are there more suitable types of URIs (Uniform Resource Identifiers) to consider?</p><p>The GNSS community would greatly benefit from <em>FAIR</em> data practices, as at present, (almost) no licenses have been attributed to GNSS data, data duplication is still an issue, historical provenance information is not available because of data manipulations in data centres, citation of the data providers is far from the rule, etc.</p><p>To move further along the path towards <em>FAIR</em> GNSS data, one would need to implement standardised metadata models to ensure data <em>interoperability</em>, but, as several metadata standards are already in use in various scientific disciplines, which one to choose?</p><p>Then, to facilitate the <em>reuse</em> (and long-term preservation) of GNSS data, all metadata should be properly linked to the corresponding data and additional metadata, such as provenance and license information. The latter is a good example up for discussion: despite the fact that &#8216;CC BY&#8217; license is already assigned to some of the GNSS data, other licenses might need to be enabled.</p><p>&#160;</p><p>Bruyninx C., Legrand J., Fabian A., Pottiaux E. (2019) &#8220;GNSS Metadata and Data Validation in the EUREF Permanent Network&#8221;. GPS Sol., 23(4), https://doi: 10.1007/s10291-019-0880-9&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;</p>
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