Space Surveillance Ontology Captured in an XML Schema

1. Space Surveillance Ontology Captured in an XML Schema Daniel L. Brandsma May 23, 2001 Organization: D540 Project: 03004000

2. Outline • Assumes everyone is aware of what XML is? • Ontology task background information • Ontology details • What is an ontology? • The ontology development process • Why Space Surveillance? • Our ontology approach using XML Schema • Space Surveillance ontology contents • Conclusions

3. Ontology Task Background Information • U.S. Air Force responsible for strategic defense C2 systems • $1.5 Billion contract awarded in Sept. ‘00 • MITRE briefed USAF on need to architect systems that satisfy current C2 requirements, but also scale to meet emerging requirements • Capturing domain semantics vital to interoperability • Task: • Develop preliminary space surveillance ontology • Provide insight into applicability of XML in space domain • Develop process needed to describe space information objects using XML • Use process as pathfinder for cross-community standardization of military space data • Investigate and leverage related efforts

4. Ontology Semantics Meaning Content Syntax Structure What is an ontology? Must also understand what the data means. For interoperability, understanding data syntax is not enough. Ontology = the definitions of the information elements, the individual data items, and the associated inter-relationships

5. Ontology Development Process • Developed preliminary ontology approach • Researched relevant XML efforts • Selected space surveillance as target domain • Selected XML Schema rather than Document Type Definition (DTD) vocabulary • Participated in XML training • Developed XML ontology approach using only XML Schemas • Researched XML tools • Developed space surveillance schema • Developed ontology demonstration approach • Researched registration process of government-wide XML Registry • Documented Space Surveillance Ontology

6. Why Space Surveillance? • Key mission for United States Space Command (USSPACECOM) • Prerequisite for all other operations that use space assets • A current catalog of where things are in space is a foundation for the military space domain • Data content and usage well understood • Effort not complicated by trying to define how data to be used • Data content, although vital, does not have stringent delivery time requirements • XML tags add overhead • Didn’t want value of XML based approach obscured by stringent performance requirements • Same mission area as related XML demonstration effort

7. Space Surveillance Ontology Approach • Decided to use newer XML schema vocabulary over DTDs • Document Type Definitions (DTDs) in wide use today but being supplanted by emerging World Wide Web Consortium (W3C) standard called XML Schema • XML Schema supports strong data typing, “inheritance”, and cross-namespace linking • Invented new approach for developing the space surveillance ontology using only XML Schema • Approach could be implemented across broad spectrum of domains

8. Space Surveillance Ontology XML Schema Approach

9. .xsl Space Surveillance Ontology Contents Data Transformations XML Schema Data Presentations Data structure Meta-data Definitions Database Instance of data content Data View 1 .html PredefinedQuery (e.g., XSQL) .xsd .xml Data View 2 XSL Processor .txt XML Validator Data Meta-data Meta- data Definitions .html .xml .html

10. Space Surveillance Ontology Contents • Architecture • SpaceSurveillancesData container element holds 5 major elements • Each major element in separate file • Contains relevant element and attribute declarations • Contains relevant type definitions • SpaceSurveillanceGlobals file holds type definitions and attribute declarations that apply across all major elements • To facilitate reuse • To isolate items that may change as schema evolves • Metrics • 93 elements • 39 user defined types • 34 attributes (1 attribute group used 16 times) Schema developed in only 2-3 weeks

11. Space Surveillance Schema Major Element Relationships Satellite and Elset data are used to generate tasking Satellite Tasking Element Set Elsets and tasking info Observations are are sent to sensors used to update Elsets Sensor Observation Sensors collect observations

12. Ontology DemoSensor Tasking Schema

13. Ontology DemoSensor Tasking Element Descriptions

14. Lessons Learned During Development • An ontology can capture needed data semantics • The most difficult part of XML schema development is determining the major information objects • XML is not about terseness • Our ontology approach is “pushing the technology envelope” • XML Schemas are superior to DTDs • Schema development is relatively easy with strong domain knowledge available • Reuse is powerful and easy to do with XML schemas • XML Schema definition language is flexible • Incremental integration eased development • Use of optional elements can be powerful • It is relatively easy to transform XML tagged data

15. Conclusions • XML is a technology of choice for interoperability - but it is only one aspect of the total solution • Internet standard for data structure and data exchange • Enjoys tremendous commercial support (e.g., Sun, MS,…) • Provides data transparency • Does not define semantics or address data storage • Capturing domain semantics is vital to interoperability • MITRE analyzed ability of XML to capture semantics and developed results applicable across many domains • XML Schema vocabulary preferred approach (over DTDs) • Namespaces enable of standard data definitions • Expect near-term USAF guidance to C2 programs requiring definition of information objects using XML Schema and registration of tags in XML Registry