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SpaceOps 2004 - Conference 



CCSDS Spacecraft Monitor & Control Service Framework 

Mario Meni \ Michael Schmidt\ Alessandro Ercolani\ Ivan Dankiewicz\ Sam Cooper^ 

Roger Thompson^ Martin Symonds^ 

Amalaye Oyake^ Ashton Vaughs^ Peter Shames^ 

Donald Custis Lokerson* 

Brigitte Behal*, Christine Comiei^ 

Harald Hofmann^ Klaus GnadI® 

Takahiro Yamada^ 

^European Space Agency (ESA)/Europe 

^British National Space Centre (BNSC)/United Kingdom 

^National Aeronautics and Space Administration Jet Propulsion Laboratory (NASA/JPL)/USA 

^National Aeronautics and Space Administration Goddard Space Flight Centre (NASA/GSFC)/USA 

^Centre National d'Etudes Spatiales (CNES)/France 

^Deutsches Zentrum fur Luft- und Raumfahrt e.V. (DLR)/Germany 

^ Japan Aerospace Exploration Agency(JAXA)/Japan 

This CCSDS paper presents a reference architecture and service framework for spacecraft monitoring 
and control. It has been prepared by the Spacecraft Monitoring and Control working group of the 
CCSDS Mission Operations and Information Management Systems (MOIMS) area. 
In this context, Spacecraft Monitoring and Control (SM&C) refers to end-to-end services between on- 
board or remote applications and ground-based functions responsible for mission operations. 
The scope of SM&C includes: 

1) Operational Concept: definition of an operational concept that covers a set of standard 
operations activities related to the monitoring and control of both ground and space segments. 

2) Core Set of Services: definition of an extensible set of services to support the operational 
concept together with its information model and behaviours. This includes (non exhaustively) 
ground systems such as Automatic Command and Control, Data Archiving and Retrieval, Flight 
Dynamics, Mission Planning and Performance Evaluation. 

3) Application-layer information: definition of the standard information set to be exchanged for 
SM&C purposes. 



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Service Approach 

Overview 

Service based architecture is gradually replacing monolithic architecture as the main design principle 
for new applications in both private and distributed systems. It is one of the fundamental design 
principles of network distributed applications where the interface, both operations and data objects, 
must be well defined as the clients are often heterogeneous. 

The architecture is essentially a design that starts with an interface definition and builds the entire 
application based around the internees, internee semantics (state machines of the interface) and 
interface calls (operations allowed and data objects passed). Whilst there are no universally 
recognised standards for service based architecture, the concepts and terminology used are 
nevertheless well developed and consistent, having evolved in line with their wide utilisation. 
Comparison shows that these are analogous to the service model conventions used for the ECSS 
PUS [R1] and SLE [R2]. 

Layered Service Architecture 

All services are not equal. Some services are high level services that provide very specific 

functionality whereas others are more generic utility services that may be used by several service 

consumers. Because of this it is useful to create a layered architecture, where high level services can 

use lower level services if required. 

A key factor of a layered approach is that services are not allowed to invoke operations on a service 

in a higher level. This does not mean that as part of the service it cannot be required that you 

provide call back information so that data can t>e returned asynchronously, what it does mean is that 

a definite dependency hierarchy can be created that will never have circular dependencies. An 

example of this is the OSI reference network layer model. 

The combination of using well defined services and the layered approach means that interoperability 

between disparate systems is far more achievable than when using bespoke systems. It is required 

that at some level the two systems agree on what protocol is used but the advantage of the layered 

service approach is that this can be hidden from the higher level applications in a lower level 

communications service. 



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For example, an application can communicate with a backend system regardless of its platform, 
communication method and location if the application uses a well defined service interface and some 
translation code. The translation code provides the standard interface to the application and converts 
it to the platform specific format required for the back end system. This translation process is known 
as an adapter: 




Figure 1: Adapter functions 

In the above diagram it is only the adapter that needs to be SLE aware for the application to be 
interoperable with an SLE based system, i.e. a spacecraft, it is also only the adapter that needs to 
be changed to support middleware more appropriate for a ground based system, i.e. DCOM, SOAP 
or CORBA: 




Figure 2: Adapter functions 



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The service protocol may be operated over different underlying communication protocols depending 
on the location and characteristics of the communications media used for the link between a service 
consumer and provider. 

Reference Architecture 

Before any work on the operational context can be done it is important that a reference architecture 
be developed. The reference architecture not only shows the physical deployments but also the 
software deployments too. 

To assist in the development and presentation of the reference architecture the CCSDS Reference 
Architecture for Space Data Systems methodology (RASDS) [3] was used. This model presents five 
primary views of the system being modelled, two of which are detailed below, and allows these to be 
combined to present more complex views of the system. 

Functional 

This view shows the separate areas of functionality that are involved in the operation of a spacecraft. 
The connections shown between functions are logical ones; they use underlying communication 
protocols running over actual physical links to transfer data: 




Figure 3: Functional view 



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The characters of the data items that are exchanged over these logical links is an aspect detailed in 
the information views. 

For example, for an advanced spacecraft these automation functions may be present onboard and 
the ground based systems would only acquire status data from these autonomous systems in order 
to monitor and guide their operation, see Figure 4. These guidance systems would use high level 
goals (e.g. "take an observation of Quasar 3C273", or "drive to that rocky outcropping") rather than 
low level commands (e.g. turn on power, open shutter, take 1 min exposure, close shutter, dump 
data, etc) to direct on-board activities: 




Figure 4: Advanced spacecraft functional example 

The above deployment view illustrates that a functional object may be required to be present in more 
than one location, for example Software Management. This splitting is required when the functionality 
is required in more than one location, in the case of Software Management it is required in the 



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ground systems for the production of the software update and also in the onboard systems for the 
applying of the software update. 

Communication 

It is envisaged that a standard low level service is required by the high level services to transfer the 
data objects to the destination service provider, whether that is space or ground based. This 
Common SM&C service hides the underlying communications protocols from the higher level services. 
Data objects that are likely to be required to be transported are: 

- Directive 
Event 

Supported operations of the Common SM&C service on the data objects would be: 

- Send Directive or Event 

The diagram below shows an example of the communication objects being used in a layered way. 
The high level application communicates through the high level services to the high level service 
provider on the spacecraft. The high level sen/ice protocol uses the common services [Common 
SM&C and potentially others] to communicate with its peer and uses appropriate network layers to 
provide this link. Different common service implementations would most likely be required for space- 
ground and ground-ground interfaces if different communications protocols were used: 



Ground systems 




Spacecraft 




Figure 5: High Level Service Concept Communications View 



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The definition of a standard interface and protocol allows the low level communication protocol to be 
hidden from the higher level applications and services. The figure below demonstrates this layer of 
communication protocols in place: 



Ground systems 



ScheiUe Service 



Common SM&C 
Seivice 



CC^SPadcBt 
TM/TC 



SLE 



TCP/IP 



Ground network 




/ CCSDSPacka \ 



Y TM/TC I 




Spacecraft 





Figure 6: Onboard Schedule Execution Service Communications View 

On the ground system node the Schedule Service communication object provides an interface which 
allows the Planning functional object to communicate with the (Onboard) Schedule Execution 
functional object. This interface is independent of the lower level communication protocols required to 
provide the interface. The Schedule Service uses the Common SM&C Service communication object 
to provide the communication link. 

On the spacecraft the (Onboard) Schedule Execution functional object interfaces with other onboard 
functional objects via relevant onboard communication objects (not shown). Again the interface 



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provided by the communication objects are independent of the lower level communication protocols 
required to provide the interface. 

Operational Context 

The operational context was derived from experience with existing system and also by examining 
existing operational service based architectures. By combining the information from both the 
operational context and the reference architecture it was possible to derive a list of high level 
services: 



SM&C 


Core Monitoring and Control service. 


Automation 


Activity automation management. 


Scheduling 


Activity scheduling. 


Interaction 


Operator notification and interaction. 


Planning 


Constraint and resource planning. 


Flight Dynamics 


Orbit detenmination, flight plan generation, 
and manoeuvre generation. 


Time & Location 


Time correlation, tracking, ranging, and 
onboard position determination services. 


Data Product 
Management 


File management and transfer, both ground 
based and onboard. 


Software Management 


Software versioning, patching, and release. 



This list is based on the identification of a core set of operational functions that exist in many 
spacecraft control systems, and the services that these offer. These functions may be distributed 
between organisations and physical entities in many different ways; increasingly, functions normally 
considered ground based are now being deployed on-board spacecraft. The intention is to identify 
service interfaces that allow different distributions of operational functions to be adopted by individual 
organisations or missions. 



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The figure below illustrates the concept for the definition of a set of high level operational services for 
the Monitoring and Control of spacecraft missions: 



Remote Node fSpacecrafi, Gnxjndstsdor, SvrniatDretc} 





SchedtJng Service 



Automaton Service 



SM&C Service 



OpWatOT 

GmmdNodu Notification 



Wiv 



Figure 7: Services Overview 

These high-level services relate to end-to-end operation of spacecraft [and potentially other remote 
systems] and would be independent of underlying transport protocols. Through the use of a layered 
service architecture these same services may be deployed over a protocol stack that use CCSDS 
Packet TM/TC for space-ground connections, while being used over TCP/IP to control an equivalent 
ground-based function. This enables functions to be migrated from ground to space without 
impacting client functions within the ground segment. 

Also legacy systems can be incorporated within the concept through the use of adapter functions 
that present a service fa?ade to client functions. Proxies can also be used for missions where space- 
ground contact is intermittent. 



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Conclusion 

The deployment of standard services and interfaces for the provision of high level services can 
significantly reduce the amount of customisation required for the support of the high level operations 
of a spacecraft. 

The use of adapter components, where one protocol is converted to another, allows hardware 
manufacturers and software developers to provide interoperability simply and for a lower cost. The 
adapters become standard software which allows a specific spacecraft platform to be used with any 
compliant control system as the adapters hide the platform specifics and just expose the standard 
interface. 

Where the corresponding service interface crosses the boundary between operating agencies, 
missions or systems, then the adoption of standard services offers interoperability between 
infrastructures. When the corresponding service interface crosses the boundary between software 
systems, then the adoption of standard services permits the development of "plug and play" 
components from various organisations. This provides the capability of rapid integration into a mission 
specific system using underlying communications objects to be selected that are relevant to the 
environment infrastructure. 



[1] Telemetry and Telecommand Packet Utilisation Standard (PUS) [ECSS-E-70-41A 30 January 
2003] 

[2] Space Link Extension Service Specifications. [Space Link Extension Service Specifications are 
in various stages of development. Current issues of CCSDS documents are maintained at the 
CCSDS Web site: http://www.ccsds.org.] 

[3] Reference Architecture for Space Data Systems (RASDS). CCSDS Architecture Working Group. 
Issue 0.8a, 4 November 2003. 



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