Use of Service-Oriented Architecturein Satellite Interoperability

Funda Karademir', Semih Cetin2'TUSAS Aerospace Industries, Inc., fkarademirgtai.com.tr

Istanbul Yolu 35.km, Akmci, Ankara 06936 Turkey2 Cybersoft Information Technologies, semih.cetingcs.com.trSilikon Blok, No: 18, ODTU Teknokent, Ankara 06531 Turkey

Abstract-This paper investigates the use of SOA promises command, control, and mission data retrieval all that can befor the modular and reusable software design of satellite modeled in a more loosely coupled manner with basic SOAsystems. It is the common belief that SOA is an principles. This is extremely important for the interoperabilityarchitectural style for transactional business applications. of satellite systems, which is typically a heterogeneous domain.Nevertheless, it can also be used to design real-time systems Interoperability is the ability of systems, units or forces tosuch as satellites. Using the inherent capabilities of SOA, provide services to and accept services from other systems,the architectural modeling of both space and ground units, or forces and to use the services so exchanged to enablesegments of a satellite system can exploit loosely coupled them to operate effectively together. SOA-based interoperableand more reusable software building blocks. It may further satellite systems puts loose coupling to the design of satellitefacilitate the separation and composition of non-functional and ground station. An interoperable ground station would beissues such as interoperability and security during the able to command and control a variety of satellites whereas anarchitectural modeling of satellite systems. The paper puts interoperable satellite would be commanded and controlled byspecial emphasis on such non-functional issues while several ground stations.introducing a service-oriented software vision. Previous work on SOA was mainly concentrated on SOA

dynamics, Web Services and grid computing [1]. Actually, the

I. INTRODUCTION previous work mostly depends on coming to an agreement ofwhat SOA is and how SOA can be used for transaction-based

Service-Oriented Architecture (SOA) is a paradigm to use business processing. Nevertheless, other business domains canloosely coupled services to meet the requirements of business also exploit SOA within miscellaneous contexts like scientificprocesses. SOA is software architecture which is an evolution workflow systems in meteorology as well as distributed sensorof component-based abstraction over heterogeneously networks in oceanography [2]. Correspondingly, Requirementsdistributed infrastructures. Hence, SOA aims to provide the and Standards Division at European Cooperation on Spacecomplete system functionality in terms of encapsulated Standardization (ECSS) has been researching a harmonizedbuilding blocks, so-called services (computing units), that are and minimal set of messages (and protocols) needed for remotelocalized independently anywhere in a network. command and control in satellite systems [3].With the promises of component encapsulation and location In this article, a practical way of applying SOA for satellite

independence, SOA can be effectively used in various contexts systems will be introduced and the net effects of using SOA forsuch as satellite systems though SOA has been most commonly satellite interoperability will be elaborated. Additionally, prosreferred to as simple as Web Services so far. Nevertheless, the and cons of SOA in satellite systems domain will be discussed.design of satellite systems typically needs both of the basic At first glance, SOA seems suitable for the implementation ofSOA promises: "independent localization of software remote command and control of satellite systems. Because acomponents" over "heterogeneously distributed mapping between satellite functionalities and remote commandinfrastructures". and control services can be easily achieved. If this is achieved

Satellite systems have essentially two major blocks: "space by means of standardized services and interfaces, satellitesegment" and "ground segment". The space segment behaves systems will be more interoperable. Exploiting SOA is one oflike an agent for a variety of objectives such as earth the groundwork steps to design interoperable satellite systems.observation, communication, navigation, meteorology, etc. The Moreover, the use of SOA in satellite systems triggers variousground segment has complementary functionality of satellite benefits in other applications such as unmanned aerial vehicles,command and control as well as mission data gathering. The which is valuable for achieving interoperability in remotecommunication between ground control station and the satellite command and control applications.in space segment necessitates particular message protocols for

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II. THE SATELLITE DOMAINSatellite systems include two different parts. The first part is

"space segment" having the satellite itself. The second part is"ground segment" enabling the remote command and controlof satellite, mission and flight planning, monitoring, tracking Satelliteand mission operations [3]. 4The satellite includes subsystems for on board data handling, /

payload, attitude orbit determination and control, thermal, cP 0structural, communication, power, propulsion, etc. In a typical \&implementation, all of these subsystems are connected to a databus and on board data handling subsystem controls the rest asthe bus controller. With the use of communication subsystem, Ground Missionthe satellite communicates with the ground segment via the Station High Level Controldata link between the satellite and the ground control station. System Mission Plan SystemData link is typically utilized for telemetry (such as flight and Antenna Antennasubsystems data), mission data downloading from the satelliteto ground segment and telecommand (such as command and Fig. 1. A typical satellite system.control data for flight and subsystems) uploading from theground segment to the satellite [3]. satellite and they can be located separately in different placesA high level description of all ground segment elements is of the world. In such configurations, central command and

provided by [4]. This paper will concentrate on two parts of the control with interoperability and security would be imperative.ground segment. The first one is the ground station system andthe second one is the mission control system. Ground station A. SatelliteServicessystem constitutes the direct interface with the space segment. In order to build a SQA for satellite systems, [3] provides aIt is utilized for space segment control by sending control broad definition and specification base. The set of importantsignals (telecommand packages) to satellite and receiving flight definitions which can directly be applied for building SQA are:and subsystems data (telemetry packages) from the satellite. It * Application Processes: are uniquely identified entitiesalso simulates and monitors the satellite and its subsystems by capable of generating telemetry packets and receivingusing the telemetry data, carries out flight, subsystem and telecommand packets. They are providers of the services.mission planning of the satellite. Mission control system An application process can be implemented in software,comprises all the elements required to control the mission and firmware or hardware.exploit its products. It is used for high-level "mission planning" * Service: is a set of on-board functions offered to a user that(which is then sent to ground station system to be converted to can be controlled and monitored by a ground systemthe related format and uploaded to the satellite) and "mission through a well-defined set of service requests and reports.data downloading" directly from the satellite. * Service Provider: is an on-board application process which

In a satellite system, the typical workflow is as follows: as executes the service activities and which is the destinationfar as the satellite is in the line of sight of the ground segment, of the service requests and the source of the serviceit can connect the satellite via the data link. Ground station reports.system downloads the telemetry packages from the satellite. By * Service User: is the entity that initiates the service requestsusing the satellite and orbital simulators within the ground and receives the service reports.station, the satellite and its subsystems are simulated and * Service Request: is the data exchange between a servicemonitored. The telecommand packages due to the satellite user (the request initiator) and a service provider to initiateneeds are prepared in the ground station. For example, if some the execution of a particular activity.refinements in the satellite orbital position or subsystems areneeded, the related commands to the related subsystems will be

pSrvice report isith ata ang a serviceprepared and added to the telecommand package. Also, low provider (the reportrtinator) and a service user to providelevel mission planning is carried out in ground control station inforatio eitheurrelg. to the exeu tion ofainiltiated by the user (e.g. notification of the completion of(due to high-level mission plan sent by the mission control execution) or correlating to a meaningful event whichsystem before) and the command data related to the mission occurred during the interal execution of a continuousplan is prepared and added to the telecommand package, too.

....

After telecommand data is ready, it is uploaded to the satellite serv service provTder resultsoa sendrig of one or morevia the data link. At the same time, when the satellite is in the board serce proier tsein sene

line of sight, mission control system also connects the satellite tet Asvice user.and downloads the mission specific payload data from the By using these definitions a SQA for satellite systemr can be

satellite.,Fi.1dpit,.yiclstlitytm realized more practically. Fig. 2 shows the use of services in aIn some of the implementations, there may be more than one satellite system. Ground station is the service user which

Ground Station System and Mission Control System of a generates service requests and sends them to the satellite via

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Satellite ' station systems interoperability, satellite to different missionService Providers control systems (download stations) interoperability, mission

Application Application ApplicationApplication control systems to different customers interoperability.Process 1 Process 2 Process 3 _____ Process nProcess 1 Process2Process 3ProcessnCurrently the most common case is the satellite to different

mission control systems interoperability for downloading theService (Service 2 Se I 3 ervice 4 Se i 5Service mmission data from different ground stations and mission control

Service,'," < systems to different customers interoperability (disseminationRUepoArvite \\Se'port of mission data) whereas satellite-to-satellite interoperability

,"round p-------'MissionControlSystemand satellite to different ground station systems interoperability

< 0/Service \ styles are still in development.ServiceUsXRequest ServiceUser Providing a basis for standards on satellite interoperability is

not quite new. Especially, the defense applications have been

Fig.2. Usage of services in satellitesystem. searching satellite interoperability for more than two decades.Fig. So-called MIL-STD-188-146, [10] provides an interoperability

data link. The application process (service provider), which and performance standards for satellite communications inprovides the requested service on the satellite, executes the terms of basic earth terminal functionality, interfaces to earthservice and sends the service report to the ground station. terminal, and satellite communications.The services that are used in the Satellite System have some Satellite interoperability is an emerging issue both in defense

common properties. Soundly, the services should be coherent and commercial arena as indicated in [11] such that there arewhich means capabilities to be provided by a service should be several factors currently forcing the issue:closely related and their scope should be unambiguously . Rising Volumes: the massive satellite system rollouts arespecified. A service should cover all the activities for managing scheduled currently and service providers have beguninter-related state information and all activities using that state placing sizeable orders for open-standard terminals to beinformation. A service should have minimum and well-defined deployed commercially in Africa, the Middle East, theinteractions with other services. Their implementation should Americas, Asia, North America and Europe. More thanbe independent. Particularly, [3] gives a minimal set of services 100,000 DOCSIS-based units have now been ordered, andneeded for command and control of the satellite, defines and thousands of DVB-RCS earth stations are also slated forstandardizes them. These services are: delivery.

* Telecommand verification service . Hybrid Verticals: the industrial systems and services are* Device command distribution service increasingly being seized upon for applications served by* Housekeeping and diagnostic data reporting service hybrid solutions, including satellite-based Wi-Fi, GSM,* Parameter statistics reporting service cable, and fibre with broader impacts for interoperability.

*Event reporting service (One global satellite operator reported double-digit*Memory management service percentage growth in revenues from the GSM sector in

Function management service 2003... just from Africa!).*.Time management service Market Dynamics: as open standard satellite systems suchOn-boardopernag ensshedulingservice as DVB-RCS - and would-be de facto standards such as*On-board moperationsrscheduingservice DOCSIS - gain in worldwide prominence, would be end*LOnoargedaam itoringservice users and government policy makers have begunPLarge data transfer service stipulating interoperable satellite systems in international

* Packet forwarding control service tenders.* On-board storage and retrieval service At present, industry is assessing the immediate implications* Test service for the satellite communications business and is evaluating* On-board operations procedure service which technical approach most effectively supports theEvent action service continued growth of the satellite communications business. The

B. Satellite Interoperability industry also must examine the role of proprietary systems inB.Satellite Interoperability is becoming prominent asakey

the next generations of satellite networks. Over the next fiveSatellite interoperability is becon ing prominent as a key years, these market segments are likely to grow more slowly

consideration for theiinternationalindustry. There are several than "interactive services" and an increasing proportion offactors forcing this issue such as placing the download stations satellite broadcast and communications equipment [11]. The(mission control center of the satellite system) all around the major impact of addressing new services in satellite domainworld and need of hybrid solutions for efficient applications. In more than anything else will trigger the development of basisorder to practically realize cost-effective systems, current and for providing richer, more secure and interoperable services.future systems should work together. Military and commercialattempts try to build open and interoperable satellite systems. III. SQA AND QUALITY OF SERVICES

Satellite interoperability can be achieved in different ways: The military communications at this century will criticallysatellite to satellite interoperability, satellite to different ground depend on satellite systems for the global reach. The two major

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networking technologies underlying the Global Information SOA can be regarded as an architectural style that enablesInfrastructure (GII) will be asynchronous transfer mode (ATM) the creation of applications that are built by combining looselyand Internet (IP) [5]. Chitre also addresses in [5] that seamless coupled and interoperable services, which is based on a formaloperation of ATM and Internet over the hybrid (satellite and definition independent of platform and programming language.terrestrial) GII will be essential for the unhampered access, The interface definition hides the implementation of thedissemination and exchange of information in addition to the language-specific service. Applications running on differentspecific issues related to high-quality services over satellite platforms can also consume services running on the other aslinks and networks. Today, satellite interoperability can be services, which facilitates reuse.explored in a wider perspective independent of protocols and a Today's ubiquitous computing integrates computation intohigher-level abstraction and SOA seems to be the best fit. the environment, rather than having computers as distinct

objects. Considering the notion of ubiquitous computing, theA. Service-OrientedArchitecture larger question is the paradigm for communications in and

Service-Oriented Architecture, SQA in shorthand form, is a between all of these different devices that no longer look likebest practice, standards-based software architecture pattern for traditional computers. The basic principles behind SOA are thethe design of usual request/response applications with loosely most applicable here. If you consider all of these new devicescoupled, reusable components presenting their public interfaces as a collection of services and abstracted data, then the level ofand bindings in a manner that allows other software systems to service use between them will be the next logical step [12].interact with them over standard protocols [1]. Considering this SOA Meta-Model given in Fig. 3, one canSome of the key reasons why so much interest has recently simply deal with each device as a system until itself with a

been generated around SOA include [6]: physical data structure, abstracted data, services, composite* It allows the close alignment of IT resources with business services, and perhaps some processes as well. From there the

function. services existing in the device are available to other devices,* It offers the flexibility to create dynamic, cost-effective computer systems, or most importantly to an orchestration or

systems via optimal use ofbuy versus build decisions. process layer where the interaction with all of these devices* It enables more rapid delivery of composite applications and computer systems can be defined in terms of businessusing a unified, process-oriented view. solutions [12].

* It allows flexible requirements management, incrementaldevelopment and legacy application integration. B. Quality ofServices and Security in SQA

* It allows applications to be integrated in heterogeneous With the widespread proliferation of SOA, quality of serviceenvironments. (QoS) will become a significant factor in distinguishing the

The use of standards for interface definition that allow success of service providers. QoS determines the serviceheterogeneous boundaries to be spanned and the loose coupling usability and utility, both of which influence the popularity ofof services exposed over multiple channels are key technical the service. QoS covers a whole range oftechniques that matchdrivers for the architectural patterns holding business agility. the needs of service requestors with those of the serviceIndeed, SOA differs from traditional integration architecture in provider's based on the network resources available. By QoS,its authoritative emphasis on loose coupling between software we refer to non-functional properties of services. The majorcomponents, and in its use of published interfaces. At design requirements for supporting QoS in services are as follows [8]:time, a service is an encapsulated business software component * Availability: is the quality aspect of whether a service isthat is rendered as a pair of separately defined elements: present or ready for immediate use. Availability representsservice interface and service implementation [7]. the probability that a service is available.

* Accessibility: is the quality aspect that represents thedegree it is capable of serving a service request. It may beexpressed as a probability measure denoting the successrate or chance of a successful service instantiation at apoint in time.

* Integrity: is the quality aspect ofhow the service maintainsthe correctness of the interaction in respect to the source.Proper execution of service transactions will provide the

lllllll tiData AlgbstractionB | i i correctness of interaction where a transaction refers to a

measured in terms of throuhput and latency Higher

performance of a service.s Sourcl5e Linlthicum Groutp* Reliability: is thequalityaspectofaservice thatrepresents

the degree of being capable of maintaining the service andFig. 3. The SA Meta-Model.service quality. The number of failures per month or year

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represents a measure of reliability of a service.* Regulatory: is the quality aspect of a service conforming to

the rules, the law, compliance with standards, and theestablished service level agreement. Strict adherence to SpaceSegmentcorrect versions of standards by service providers is Satellite 1 Satellite 2necessary for proper invocation of services by requestors. . __ __Earth Observation Meteorology

Interoperability Layer 1* Security: is the quality aspect of a service for providing -

confidentiality and non-repudiation by authenticating the Gron ron Gronparties involved, encrypting messages, and providing System System Systemaccess control. Security has added importance because 1 2 3

service invocation may occur over the public networks. Ground SegmentCo ntrolI Co ntrolI Co ntrolISystem System System

IV. ACHIEVING SATELLITE INTEROPERABILITY WITH SOA 1 2 3

Within the scope of this paper, we concentrated on two Interoperability Layer 2

interoperability layers in satellite systems. The first one is Customer Segment Customer 1 Customer)2 ustomer03 ustomer4between space segment and ground segment and the second . -

one is between ground segment and customer segment, as Fig. 4. Interoperability layers in a satellite system.shown in Fig. 4. Within the context of Interoperability Layer 1,communication of different ground segments with differentsatellites can be accomplished, which means satellites can be

s acontrolled from different ground stations all over the world and Discovery Agenciesdata on the satellites can be downloaded to different groundstations. Similarly in the context of Interoperability Layer 2, Querycustomers can reach different ground segments and get the Publishexploited data quickly from wherever they want.

In order to build these interoperable layers, basic SOA modelgiven in Fig 5. has been used [9]. In this model there are three Service Requestor Interact Service Providerkinds of actors: "service providers", "service requesters" and"discovery agencies". Service providers expose some software Fig. 5. Basic SOA model.functionalities as services to its clients. In order to allowrequesters to access this service, the provider has to publish a standardized protocols; interface specifications are exchangedservice description. at run-time and, thus, clients can replace services at run-time.

Since service provider and service requester usually do not This might be advantageous if a new service provides a betterknow each other in advance, the service descriptions are alternative to the former one concerning functionality or QoS.published via specialized discovery agencies. They categorize Or, it might become necessary for self-healing purposes if athe service descriptions and deliver them in response to queries service is no more reachable because ofnetwork problems.issued by service requesters. As soon as the service requester At interoperability layer 1, service requestor is the groundhas retrieved a service description meeting its service segment and service provider is the space segment. Groundrequirements, it can use it to interact with the service. Such a segment handles command and control issues as servicenature of SOA facilitates even the Service Level Agreements requests and the space segment respond to the service provider.(SLA) specific for satellite interoperability at layer 1 and 2. There should be discovery agencies that publish and categorizeHowever, SLAs at layer 1 and 2 of Fig. 4 may slightly differ the satellites and their capabilities to the ground stations.

from each other since the standards and protocols used at the For an ideal scenario, consider space segments and groundfirst layer are more complicated than the ones used in the segments that are designed according to standardized protocolssecond layer. The second layer is more application-oriented, and interfaces. Since all of them are obeying these protocolshence such SLAs have already been in the current research area and interfaces, any ofthe ground stations can be used to controlof SOA. Various video formats, image formats, associated QoS any of the satellites as far as it acquires the service descriptionsshould also be investigated in other domains like broadcasting of the satellite from the discovery agency. Discovery agenciesand digital content management. Nevertheless, interoperability deliver the services provided by distinct satellites to the groundat layer 1 is harder since managing the ubiquity at layer 1 is segments so that without the knowledge of all implementationmuch more complicated since it dramatically affects the QoS details, a ground segment will be capable of commanding andrequirements such as security. Having secure communications controlling a satellite.in command and control of satellite systems is more imperative At interoperability layer 2, service requestor is the customerthan assuring the security of satellite message content. segment and service provider is the ground segment. CustomerSQA is typically highly dynamic and flexible: components segment requests exploited payload data from the ground

and services are only loosely coupled and communicate due to segments and the ground segment, which has the appropriate

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data, responds this request. Discovery agencies here publish the This interoperability can be classified at two layers: the oneservice data catalogue of ground segments all over the world. between satellites and ground segments and the one betweenCustomers look at these service data catalogues and select the ground segments and the actual customer. Each layer has itsappropriate service data and therefore the ground station. own promises and difficulties to achieve the interoperabilityAs a scenario for the interoperability at layer 2, consider the and employing SOA principles can help realize the promises

communication between the customers and ground segments. and tackle with the difficulties.The customers all around the world are connected to discovery Applying the SOA principles for satellite interoperability hasagencies. From the catalogues of the ground segments, which some indirect advantages as well. Legacy migration is one ofare published by the discovery agencies, they select the service them. Like every system used for a long time to co-exist withthey need to fetch the relevant data. After that, the customers new versions, even satellite systems might be consideredwill be connected to the satellite by using the standardized legacy as well. In order to manage older and newer versions ofservice interfaces and got the data they need. This data can be satellites even from the same ground segments, SOA can helpin various contexts such as image data for earth observation in abstracting the satellite functionalities independent of theirsystems, weather forecast for meteorology systems, etc. legacy hardwired protocols and technologies. On the contraryWith SOA, standardized service interfaces between space, of its promises, SOA may introduce some difficulties as well. It

ground and customer segments can be formed, and therefore requires managing the Enterprise Service Bus, self-healinginteroperability between these layers can easily be achieved. mechanisms, Service Level Agreements and the security ofAs the services get more common, the QoS factors that are asynchronous message passing.discussed in the previous section will become more advanced REFERENCESso that issues like security, availability, reliability, integrity, [1] T. Er, Service-Oriented Architecture. Prentice Hall, 2004.performance, etc. are achieved easily. [2] E. Jeager, I. Altintas, J. Shang, B. Ludaescher, D. Penington, and W.

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