Frank J. LoPinto
Computer Sciences Corporation
10110 Aerospace Road
Lanham-Seabrook, Maryland 20706
Phone: (301) 794-1486
Fax: (301) 459-4482
Email: LoPinto@niccolo.gsfc.nasa.gov
Donald Sawyer
NASA Goddard Space Flight Center
AbstractMetadata is information about data. Data without metadata is generally useless. An image of the Earth from space or a bubble chamber photograph may be beautiful but without metadata (e.g., where was the spacecraft, what was in bubble chamber) the data will yield few insights. What we need is a way to link data and its associated metadata such that researchers around the world, now and in the future, can access and understand the information generated by our scientific instruments.
The Consultative Committee on Space Data Systems (CCSDS) is an organization whose members are the space agencies from around the world. CCSDS conducts its work through three technical panels. Panel 1 deals with Telemetry, Tracking, and Command. Panel 2 deals with Standard Data Interchange Structures. Panel 3 deals with Cross Support Operations. The focus of this paper is to describe the work of Panel 2 and specifically the Standard Formatted Data Unit (SFDU) Structures and Construction Rules. Though SFDU "officially" stands for Standard Formatted Data Unit, we sometimes referred to it as the "Standard For Data Understanding."
The SFDU Recommendation is a packaging standard. It describes a method of encapsulating data within Label Value Objects (LVOs) and then relating LVOs to each other. There are no restrictions whatsoever on the format of the encapsulated data. The header part of the LVO provides information that can be used to find a description of the encapsulated data. One can think of an SFDU as a well organized, machine readable UNIX tar file (though the standard is independent of UNIX or any other operating system). The SFDU is a root directory. It contains files (simple LVOs) and subdirectories (compound LVOs). Instead of "readme" files containing some (possibly) useful descriptions, the SFDU either contains or points to machine readable Data Description Units (DDUs). DDUs describe the syntax of the data (i.e., how to read it), the semantics of the data (i.e., what physical quantities were measured), and supplementary data (e.g., a photograph of the apparatus, data analysis software, technical papers). Naturally, there are no restrictions whatsoever on the format of these descriptions.
NASA has developed the SFDU Science Data User's Workbench to make it easy for people to create SFDUs and to decode and examine them. The Workbench is an object oriented, distributed application that runs on UNIX workstations networked with TCP/IP. It provides a graphical, drag-and-drop user interface and has been demonstrated to members of the space science community in the United States and in Japan and Europe. It has also led to new software engineering insights as we use SFDUs to encode messages between objects, to organize and launch distributed applications, and to classify and retrieve software components in reuse libraries.