7.1.2 TelemetryMetaData

    Because Telemetry and Command databases are frequently developed and maintained independently, the XTCE format divides TelemetryMetaData and CommandMetaData into separate, but similar sections. TelemetryMetaData is really nothing more than a grouping for data about Telemetry. TelemetryMetaData has a ParameterTypeSet, a ParameterSet, a ContainerSet, a MessageSet, a StreamSet, and an AlgorithmSet. Following are descriptions of these collection types.

   {key_unique_keyRef,sequenceOrder=3}0..1-ContainerSet

   {sequenceOrder=6} -MessageSet

   0..1 0..1

   -StreamSet{sequenceOrder=5}

   {sequenceOrder=1} 0..1 -ParameterTypeSet

   Figure 7.2 - Telemetry MetaData

   A ParameterTypeSet is an unordered collection of ParameterTypes. ParameterTypes are the MetaData for Parameters; ParameterTypes are instantiated to create Parameters. ParameterType is the description of something that can have a value (a Parameter). Information contained in ParameterType includes the data type, description, alarm limits, engineering units, and string conversion specifications and calibrations. Most Parameters are telemetered parameters (a.k.a measurands) and must also include information about how the Parameter value is encoded for transmission. This information includes size in bits, byte order, data type, and parity checks. All of the encoding information is in the Encoding area.

   A ParameterSet is an unordered collection of Parameters. Parameters are instantiations of ParameterTypes. Parameters are normally a very simple name and reference to a ParameterType. Parameters may also have alias names and may have properties unique to that instantiation. At any point in time (instance) a Parameter has a value; a Parameter is not the value itself. Parameter names are case sensitive, may be any length, cannot contain spaces ‘.’, ‘/’, ‘[‘ or ‘]’ as these are reserved characters. The aliases have no restrictions.

   A ContainerSet is an unordered collection of SequenceContainers. SequenceContainers are defined in the packaging section. The packaging section contains the information required to assemble an uplink from its component parts and disassemble a downlink from its component parts. The packaging section has been created to be extremely generic so that it may be used to define TDM telemetry streams, packetized streams, or any other package format. A SequenceContainer contains (in the EntryList) an ordered list of raw parameters, parameter segments, stream segments, containers, or container segments. SequenceContainers may inherit from other sequence containers. The inheritance aspect of SequenceContainers is useful not only for minimizing the effort required to describe a family of SequenceContainers, but is also a very powerful and expressive means of container identification – the process of distinguishing one container from others (e.g., minorFrame 20 is a type of minorFrame where the minor frame counter equals 8). SequenceContainer inheritance may be arbitrarily deep.

   A SequenceContainer may represent a packet, a frame, a sub-frame, or any other grouping/structure of data items. The simple form of a Sequence element is an ordered set of Parameter References or other Container References.

   XML Telemetric & Command Exchange (XTCE), v1.0

   A MessageSet is an unordered collection of Messages. Messages are an alternative method of uniquely identifying containers within a Service. A message provides a test in the form of MatchCriteria to match to a container. A simple example might be: When minorframeID=21, the message is the 21st minorframe container. The collection of messages to search through will be bound by a Service.

   A StreamSet is an unordered collection of Streams. Spacecraft uplinks and spacecraft downlinks are digital streams of data and there are a number of processing functions that are done on the stream level. The stream section contains the knowledge for how to assemble, disassemble, and process spacecraft uplink and downlink streams.

   An AlgorithmSet is an unordered collection of Algorithms. In spacecraft ground systems, it is necessary to perform some specialized processing to process the telemetry, and preprocess commands. There are a number of predefined algorithms and the algorithm section makes it possible to reference externally defined algorithms for arbitrarily sophisticated data processing.

   NameDescriptionType

   -AliasSet : AliasSetType [0..1]{sequenceOrder=2} -LongDescription [0..1]{sequenceOrder=1}