SAFE Information Model
A SAFE product, which can of course also be considered an “XFDU package”, wraps or references EO data and associate them with information expressed in EO vocabulary. The primary objective of SAFE is to hold the so called “Level-0” (or “L0”) data which is close to the telemetry level but it has, moreover, been qualified for the packaging of higher levels products, e.g. the ENVISAT GOMOS Level 1 and 2 products have already been successfully implemented in SAFE. All SAFE products contain the following metadata:
- Acquisition period : the acquisition period metadata that provide the time extents of all the data contained in the SAFE product. It is mainly dedicated to allow a fast time ordering and framing of the overall contained data.
- Platform/Sensor identification : the platform identifies the system (satellite/aircraft) that acquired the EO data wrapped by the SAFE product. It has sub-elements that unequivocally identify the platform as well as the specific sensor that acquired the data.
- Product History : a processing log collecting the historical information dedicated to the maintenance and the traceability of the product. The main feature of this logging system is its capability to store several processing threads regarding all the components that affected the product. As an example, if the product originates from the concatenation of several data objects, all logs of the involved objects will be kept, identifying their precise role of each in the production of the described SAFE product.
For each wrapped or referenced EO dataset, a collection of metadata information may be attached:
- Orbital information : the reference to the trajectory of the platform that acquired the data. This information may locate one or several orbit paths, the corresponding cycle, track, etc.
- Grid Reference : the grid reference is typically used for conveying geographical information attached to data objects. It may, however, be useful in various other cases including the provision of wind vector fields, of temperatures or sun elevation.
- Geolocation information : the information locating the product footprint on the Earth’s surface, either as a series of tie points or by reference to a world reference system of the acquiring platform. The Geolocation information, may also attach additional information to each localized element, including cloud coverage vote notation, meteorological information, etc.
- Quality/Fixity information : information about the quality of an EO dataset. SAFE makes use of techniques (i.e. XPath) that allows the precise location of the corrupted or missing elements up to the bit level.
- Fixity Information : the Information which documents the authentication mechanisms and provides authentication keys to ensure that an object has not been altered in an undocumented manner. An example is a Cyclical Redundancy Check (CRC) code for a file.
- Representation information : any data contained in a SAFE product shall be accompanied with its representation information formally and numerically exploitable. Although the semantic information is partially implemented (i.e. all elements composing the dataset are named but the semantical links to standardized vocabulary is not embedded in the product), the SAFE aims at complying with the OAIS reference model in that area for assuring the maximal theoretical long-term preservation.
Finally, SAFE does not limit the information to the content listed above but supports extensions as far as they preserve the integrity of the mandatory items.
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SAFE Logical Model
As specified in the XFDU model, a SAFE product is a logical tree of “Content Units” [2] forming the so-called “Information Package Map”. Conversely to XFDU, only one map is expected per SAFE product. The root Content Unit has predefined associations to the information applicable to the overall product, i.e. at least the “Acquisition Period”, the “Platform/Sensor Identification” and the “Product History”. The structure of the children Content Units is less constrained and depends mainly of the logical view of the wrapped data. In most cases, one Content Unit matches one EO dataset and its accompanying metadata. Several Content Units may, however, share the same metadata.
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SAFE Physical Model
A SAFE product physically contains the following components:
- Manifest file : an XML document conforming the XFDU Manifest file. It contains the definition of the Information Package Map, the wrapped Metadata Objects (i.e. in general all Metadata Objects are embedded in the SAFE Manifest file), the wrapped Data Objects (i.e. Data Objects are rarely embedded in the SAFE Manifest file) and references to the external files containing the Metadata and Data Objects.
- Binary, ASCII or XML files : the data or metadata object contents. Currently, only this types of files have been identified, i.e. binary matching MIME octetstream definition and XML documents. Each of these files shall be accompanied with one or more XML Schema document controlling its content.
- XML Schema files : the representation information of the data held by a SAFE product. In comparison to XFDU, SAFE does not allow multiple notations for storing the representation information of its objects. This restriction is mainly imposed because SAFE does not only reference a representation information technique but intends to define it. In order to represent the binary information, SAFE also defines specific markups that annotate the XML Schema documents to provide information on the physical structure, i.e. the so-called SDF markups. Thanks to these specific annotations, the contents of the binary files are described up to the bit level with a common technique as for XML documents.
The current definition of SAFE does not support the XFDU “Behaviour Objects”. Studies for their support are already on-going, for example to host translators to/from original format.
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