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their value, as well as zero to many connection properties that specifies the links between process components within the chain.
Figure 9.4. Conceptual model for a connection Link.
The process chain is defined by describing all of the processes in the chain and then describing the appropriate connections between components. The connection property
includes a collection of Link objects that provide references to the source and destination ”ports” within the various component processes. The source is usually an output from a
process component or a data component within a source data object e.g. an observation. The destination data component is typically an input or parameter of a process, or the
output of the ProcessChain itself. The specifics regarding how a linkReference is defined will be described in the encoding Section 11.
8.10 Physical Processes
In addition to describing purely mathematical processes, SensorML describes processes that have some relationship to space and time. These might include transducers detectors
and actuators, sensor systems, samplers, and sensor platforms, for example. For much of the processing of sensor data, it is important that temporal and spatial reference frames be
described and that the relationships between various reference frames be able to be explicitly defined.
All physical processes derive from the abstract PhysicalProcess, which has spatialReferenceFrame and temporalReferenceFrame properties that define any spatial or
temporal Coordinate Reference Systems CRS that might be used for interrelating internal components within the process, as well as reference frames that might allow this
component to be related to other components within a process chain or system. A CRS definition will include a definition of its axes, as well as the location of the CRS origin
and the orientation of the axes relative to object’s physical form.
Figure 9.5. Conceptual model for an ComponentPosition.
A physical process is also modeled as having a position property which relates the object as defined by its spatialReferenceFrame to some external CRS. This position can be
provided using Position or Vector as defined in SWE Common namespace, or through a
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process e.g. ProcessChain, ProcessModel, Component, or System which can be used to compute dynamic position values. Additionally, the boundedBy property within a
physical process defines the extent of the object’s location within some CRS.
Figure 9.6. Conceptual model for an InterfaceDefinition.
Physical processes may also have zero or more interfaces over which commands and data can flow based on particular physical connections and particular protocols. The interface
property will contain an InterfaceDefinition which is based on the Open Systems Interconnection OSI Reference Model network protocol layer model, as modeled in Fig.
9.5. Starting at the lowest-level interface, the InterfaceDefinition can include:
• Mechanical layer not in OSI stack – type of connector used e.g. DB9, DB25, RJ45, USB-B
• Physical layer OSI layer 1 – provides electrical and physical characteristics of the connection including pin layouts, voltages, etc. e.g. RS232, DSL, 802.11g
• DataLink layers OSI layer 2 – provides functional and procedural means of transferring data between network entities and detectingcorrecting errors e.g.
Ethernet, 802.11, Token ring • Network layer OSI layer 3 – provides functional and procedural means of
transferring data from one source to destination via one or more networks while maintaining the Quality of Service e.g. IP, ICMP, IPSec
• Transport layer OSI layer 4 – Provides transparent transfer of data between end users and can control reliability of a given link e.g. TCP, UDP, SPX
• Session layer OSI layer 5 – controls the dialogues sessions between computers by establishing, managing, and terminating connections between the local and
remote applications e.g. NetBios, TCP session establishment • Presentation layer OSI layer 6 – transforms the data to provide a standard
interface for the application layer e.g. SSL, TLS, ASCII, MIDI, MPEG, SWE Common
• Application layer OSI layer 7 – provides a means for the user to access information on the network through an application e.g. HTTP, SMTP, FTP,
XMPP, RTP
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• Service layer not in OSI – type of web service used to access data e.g. SOS, WCS, WFS
Each LayerProperty takes a DataRecord or Category as its value, with the definition attribute used to specify the type of protocol used for example, RS232, USB2, etc.. The
Category or DataRecord components should contain parameters meaningful for this particular protocol e.g.. parity. The presentationLayer can also be specified using
encoding types defined earlier in the SWE Common section.
8.10.1 Component
Any physical process can be modeled as a Component in SensorML if it either cannot be subdivided into smaller subprocesses, or if one chooses to treat it as a single indivisible
process. A Component can be considered as a real-world equivalent of a ProcessModel. A Component can participate as part of a ProcessChain or System.
Component includes all the location and interface properties of any physical process and adds a method property that can describe the basis of physical processing of the
component. In some cases, it may also be possible and desirable to provide within the method definition, a software implementation that can be used for simulation of the
hardware component.
8.10.2 System
System is a physical equivalent of a ProcessChain. A System may include several physical and non-physical processes that all act to provide a certain set of System outputs,
based on the System inputs and parameters. An example might be a weather station that includes several physical sensors as well as some on-board processing for calculation of
wind chill, for example, that all work together to provide a measure of the state of the atmosphere at particular time intervals. Similarly, an airborne remote sensing system may
include, for example, the radiometric scanner itself perhaps modeled as a system, as well as a GPS sensor and Inertial Momentum Unit for reporting location and orientation
of the platform.
Being derived from PhysicalProcess, System inherits all of the properties related to location and physical interface. Like ProcessChain, System provides a list of processes
and the links between these processes, using the components and connections properties, respectively. In addition, System can provide relative positions of its components through
the positions property.
As with Component, System positions can be provided using Position or Vector as defined in SWE Common namespace or through a process e.g. ProcessChain,
Component, or System used to compute dynamic position values. This position information allows derivation of the exact spatial and temporal position of measured
phenomena. Each system component as well as the system itself is attached to a mathematical reference frame making the relative position unambiguous. The position
list is especially important when dealing with remote sensors for which deriving the precise location of the measured phenomenon is fundamental.
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8.11 Process Metadata Group
