A New Collaboration for a Web-Services Enabled Geodesy Seamless Archive
The NASA ROSES ACCESS Program has funded UNAVCO, CDDIS and SOPAC, to expand and modernize metadata exchange definitions and technologies first developed in the late 1990s as the GPS Seamless Archive Centers (see the old GSAC Web pages for historical reference). The resulting system provides comprehensive user access to a broad range of space geodetic data including GPS/GNSS, DORIS, SLR, and VLBI. The new Web Services enabled Geodesy Seamless Archive (GSAC-WS software package) is now available.
The University of Nevada, Reno (UNR) is a fourth partner in the GSAC-WS project and will test the Web Services by incorporating them into their daily GNSS data processing scheme. The UNR effort will include quality assessment of current and legacy data.
About GSAC, an Open-source Software Package for Geodesy Repositories
GSAC is a free, open-source software package for data repositories for site-located instruments and their data files, especially geodesy repositories. GSAC provides a complete set of the latest web services for search, discovery, and download of geodesy data in existing repositories. Any organization with geodesy data files, and information about the data files and related monuments (stations) and instruments in a database, can implement a data repository with web services by utilizing GSAC code. GSAC uses the latest software technology to permit installation of a web site with the latest web services.
The GSAC software, from UNAVCO and partners SOPAC and CDDIS, is a package for comprehensive user access to a broad range of geodetic data, including GPS/GNSS, DORIS, SLR, and VLBI. GSAC is a suite of open source code which provides a web browser-based UI for data search, data file downloading, a web services REST API, and a client process to work with the API. GSAC is ready-to-use middleware between your database and file system on one hand and a remote user needing to query metadata or access data files on the other. GSAC uses your database, data files, and web server, and creates web services for data discovery and data downloads from your repository. There is also a GSAC command line client, a Java-based client program for accessing a GSAC repository through its API, that allows users to do programmatic searches of a GSAC repository and to download files.
GSAC software supports single and federated repositories. You can provide a data repository with no ties to other organizations or archives. GSAC can be as simple or as complex as your data holdings. GSAC becomes particularly powerful when used in the federated mode for search and download from several different GSAC-enabled data repositories. A federated GSAC system dynamically queries two or more participating GSAC repositories directly through their locally implemented GSAC APIs. A federated GSAC does not maintain its own database to copy other GSAC data holdings, nor does it mirror collections of data files. GSAC software was designed from the beginning to support simultaneous queries and results from multiple cooperating GSAC repositories.
Like other web services, GSAC accepts an incoming URL-based request, handles the request, and returns a result. The result may be metadata information, or data file access information.
The GSAC result return format for searches for site metadata is user selectable. GSAC offers site query results in web pages (HTML), and in geodesy formats SOPAC XML site log, SINEX, GAMIT station.info files; as two csv formats; as GSAC JSON and XML formats designed for computer use; and as a plain text file for human visual checks. You can see Samples of GSAC site queries' results (files generated) in differing formats.
From data file searches you can see a web page with a list of FTP or HTTP URLs for file downloads from your server, or get a file which is a wget script for FTP or HTTP of all files discovered by one file search, suitable for computerized file downloading of multiple files. Your data files must be available from a server, and the URLs for FTP or HTTP downloads of files from the server must be available to GSAC in some way. GSAC also supports data downloads using Java Webstart.
A GSAC implementation publishes on its web site the repository information in XML, a capabilities document for the API search, on the Information web page on each GSAC web site. This information is used by federated GSACs to work with other GSAC repositories, and it is used by the GSAC client.
The GSAC API uses the same capabilities, listed in the capabilities document and readable in the Information web page on each GSAC web site. These are used by the GSAC client program, and can be used by other programs, and by the URL line in a browser, to specity GSAC use and results.
Modern web services call for professional software engineering; GSAC provides a complete package of web services. Most of the coding required for web services is creating a browser-based UI, an API interface with published capabilities, request handling, and output handling. GSAC has all the code for these functions, plus a client program, and geodesy formats for results. GSAC software is developed, maintained, and supported by UNAVCO, assisted by SOPAC and CDDIS.
For more about GSAC-WS and its operation at your geodesy data repository, see the GSAC introduction, UNAVCO GSAC WS: Web Services for Geodesy Data Repositories.
We have fielded these GSAC-WS implementations:
UNAVCO's Federated Repository provides for processing searches to UNAVCO, SOPAC, and CDDIS Repositories in parallel, with aggregation of the results. Other collaborations of GSAC repositories may create their own federated GSAC services, independent of UNAVCO.
UNAVCO is also implementing the new IGS Site Log GSAC and the EA (Expanded Analysis) GSAC now:
- UNAVCO IGS Site Log GSAC
- UNAVCO EA (Expanded Analysis) GSAC
What is needed to Install a GSAC Data Repository?
You can install GSAC web services for a data repository when you have:
- metadata about sites and instruments at monuments and their data files, in a database (presently Oracle, Postgres, or MySQL).
- usually, but optionally, data files from the instruments. GSAC can provide discovery and download of files if desired, if you have a FTP download service set up for the data files.
- a web server. Running GSAC with the Tomcat web server and servlet container is common, but Tomcat is optional.
- the GSAC-WS software package provided by UNAVCO (free and open source code in the GSAC package from SourceForge).
- A Linux system to build GSAC-WC, with related software tools (bash, Java 1.6, svn, and ant), and with access to your database.
For more about installation, refer to the GSAC Installation page.
