Space: A Fundamental Part of the Infosphere

XML can help expose data semantics and customize data dissemination

April 2002

Space is a fundamental component

If you've ever listened to a weather report, watched a live news broadcast from around the world, or used a
hand held navigation device while hiking, you have enjoyed some of the benefits that space has to offer. In only 40 years we have gone from launching the first man-made satellite to integrating space into almost every aspect of our lives. In fact, we take for granted all that space has to offer.

In the context of military warfare, space is also relatively new. The Gulf War, called the first space war, showcased the value of information and connectivity available via space. Space data is used to plan military operations, direct precision guided munitions, and provide warning. Space communications systems also help provide worldwide connectivity. Today, space services are widely used across academic, civil, commercial, and military communities.

Challenges

In order to achieve the vision of an Infosphere, we need to identify enabling technologies and processes. Space systems experience many of the same challenges as airborne or terrestrial systems (e.g., efficiently distributing data and understanding what that data means). At the Electronic Systems Center's Strategic and Nuclear Deterrence Directorate (ESC/ND), we examined how to apply an international Internet data exchange standard called Extensible Markup Language (XML) to the military space arena using two tasks. The first task developed an approach for capturing space data structure, content, and meaning using the World Wide Web Consortium (W3C) emerging standard called XML Schema. The second task demonstrated how XML could be used to web enable space data exchange–a step that will make space data available via the Infosphere.

What is XML?

XML is a family of new technologies and open standards for web-based information management. The power of XML lies in its simplicity, its support in the commercial community, and its relationship to the Internet. XML has its root in other markup languages (e.g., Hypertext Markup Language (HTML) and Standard Generalized Markup Language (SGML)) that deal with data format. XML allows a user, or community of users, to define a set of markup tags that capture the inherent structure of the data. The components of the structure are called elements, and these elements are constructed in a hierarchical form. Unlike HTML, which uses tags to define data presentation (e.g., <I>print in italics</I>), XML uses tags to describe data content (e.g., <Horse>Mustang</Horse>). This provides a mechanism for coupling the meaning of the data with the data itself, and makes that meaning available to services like search engines. The benefit is that now when you search the web for information on 'Mustangs', search engines can distinguish between the cars and the horses because the element description, or tag, references either cars or horses.

It's a matter of semantics

Unfortunately, tagging data does not guarantee interoperability. Widespread interoperability demands the ability to not only transmit data across the enterprise, but also interpret that data correctly. Correct interpretation of data requires understanding both the structure of the data, and the meaning of the data (i.e., data semantics). XML Schema offers one approach for capturing both the data structure and some data semantics by utilizing the capabilities native to information technology standards.

An XML Schema file specifies the template for the XML tagged data file. The element tag hierarchy describes the overall structure and content. Additionally, XML Schema has a predefined "documentation" tag associated with every component that is intended to capture human readable information. This tag provides the mechanism for capturing human readable data semantics. By employing a convention that requires all components of the schema to populate the "documentation" tag with a definition, and any other elaborating information needed to understand the data in the given context, the meaning or semantics of the data are specified as well as the structure of the data. The power of this approach lies in its simplicity. Because XML Schema files are "well-formed" XML documents (i.e., they comply with XML syntax rules), the same tools that operate on the XML data files may also be used to operate on XML Schema files. Therefore, data consumers can extract the semantic information captured in the schema in much the same way as they extract the data from the XML data files.

An economic advantage provided by XML Schemas is the ability to manage the XML data specification. The XML Schema files provide the template that the XML data files reference, and the data file contents are validated against the rules described in the schema files. If the rules for data content change, one must update the schema and corresponding data files. However, the tools for data creation, viewing, and validation remain the same. This is a significant improvement over the past approach where the documentation of the data exchanged was kept separately (e.g., in an Interface Control Document). This separation resulted in multiple interpretations of the specification and inconsistent implementations of the specification. Worse yet, these interface inconsistencies can typically be found only during integration. XML Schemas provide the developers with a common template that can be used during design and development.

Demonstrating XML's applicability to space?

In addition to using XML to capture domain semantics, ESC/ND examined how XML might be applied in order to distribute high-quality satellite location data. Today, this data is sent to only two users, the National Reconnaissance Office (NRO) and National Aeronautics & Space Administration (NASA). High quality location data is needed for things like calculating where objects are in space so that the Shuttle and International Space Station do not collide with them. There are many potential users who would like to receive this location data. The obstacle impeding the distribution of this data is the analyst intensive formatting process required to create and distribute the data. In an XML-based approach, the data could be calculated once, XML tagged, and then distributed over the Infosphere using many possible avenues. Infosphere services such as Publish and Subscribe could be used to allow users to get the data they need when they need it. Further, the XML tagged data can be easily transformed into a user preferred format. This ability was demonstrated using Extensible Stylesheet Language Transformations (XSLT) that transformed a single XML data file into multiple, user-specific formats. This ability to customize the format vastly decreases the bandwidth needed to distribute the data.

Conclusion

In the same way that space data has evolved to become an integral part of everyday life, it is also evolving to be a necessary element of the Infosphere. To achieve the vision of an Infosphere, all systems must achieve semantic interoperability. XML is a foundational technology that can help expose data semantics and customize data dissemination. Using data exchange standards, such as standard message sets, is not new to military command and control. In fact, the government has achieved the current level of interoperability by defining and maintaining these standards. However, now is the time for the government to capitalize on the tremendous commercial investment being made in this open standard and to achieve new levels of interoperability by inserting information technologies like XML. Another step in this evolution is to capture data semantics precisely enough so that computers can understand them. This is an area of research known as the Semantic Web.

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