At the time of this abstract submission, Unidata recently received word that its THREDDS (THematic Real-time environmental Distributed Data Services) initiative would become part of the NSDL (National Science, Math, Engineering, and Technology Education Digital Library) sponsored by the National Science Foundation.

The overarching goal of THREDDS is to provide students, educators, and researchers with coherent access to a large collection of real-time and archived data sets from a variety of environmental data sources at a number of distributed server sites. The data sets will be conveniently accessible from an collection of THREDDS-enabled data analysis and display tools ranging from "thin" web-based clients that allow the learner to browse and manipulate data using the processing power on the servers, to "thick" clients that harness the computing power and flexibility of the user's own workstation while accessing data from a collection of remote servers. THREDDS will provide real-time data delivery via reliable, event-driven "push" technology as well as transparent access to data sets using "pull" systems that make it possible to access data on remote servers as if they were on the users' own computer. The system will be built on a set of software components and data servers, most of which either are already in operation or are under development.

The heart of THREDDS, however, is metadata contained in publishable inventories and catalogs (PICats). Based on the eXtensible Markup Language (XML), PICats can be created in many different ways. Sites receiving real-time environmental data will instrument decoders to create PICats describing data products as they arrive. Crawlers will be implemented to create PICats by traversing existing retrospective data collections. Since PICats do not have to reside on the server with the data, researchers will be able to create PICats for research publications that point to datasets residing on several data servers. Educators will incorporate PICats of illustrative datasets into educational modules that also include tools for data analysis and visualization. Indeed students will eventually be able to use PICats to point to datasets related to their research projects, just as they now use URLs to point to relevant documents. Since they are text-based, PICats can be "harvested" and indexed in digital libraries using specialized tools that make use of the internal structure and semantic content as well as by tools similar to those used by current document search engines.

This paper presents an overview of THREDDS and an update on the current status.