For many years, the International Space Station (ISS) has served as the gateway to multinational cooperation, research and development in space. Support for space missions, structural expansions and maintenance of the station in a hostile environment for humans necessitated the use of robotics to aid humans from the beginning. The Mobile Servicing System (MSS) is one of several robotic systems aboard the ISS that was developed by the Canadian Space Agency (CSA) with the partnership of the National Aeronautics and Space Administration (NASA). The three components of the system serve as the arms & hands of the station and are used to move payload, assist astronauts in space, and even perform precision repairs. The system can be operated within the station or remotely and was designed to become part of a larger and expanding Systems-of-Systems (SoS). Made in Earth but operated in space, all components were subject to rigorous planning and testing not seen before in the area of robotics at the time of its inception. The purpose of this paper is to analyzes and discuss the architecture and SoS integration challenges of the MSS. 

1 Introduction

1.1 ISS overview

When complete, the ISS will be the largest and most complicated spacecraft ever assembled [1]. The spacecraft is designed to support human life, host laboratory equipment to conduct research and serve as a test bed for new technologies in space. Apart from scientific studies, astronauts aboard the ISS will also participate in the construction, maintenance and expansion of the spacecraft. The development of the ISS was possible thanks to the participation of multiple International Partners (IPs), mainly NASA, Russian Space Agency (RSA), CSA, Japan Aerospace Exploration Agency (JAXA), and European Space Agency (ESA). With NASA leading a sixteen-country international team through the ISS system development, module production, visiting vehicle fleet scheduling and integration, on-orbit construction, and the long-term station operation [2].

1.2 ISS Robots

Several robotic systems have been developed by different IPs that accomplish different task to aid ISS crewmembers in their everyday activities. These include the Mobile Servicing System (MSS), the European Robotics Arm (ERA), and the Japanese Experiment Module Remote Manipulator System (JEMRMS). Table 1 summarizes the primary robot systems operating on the ISS.

Table 1. Summary of robots aboard the ISS [1]

Together these robotic systems form part of a Federation of systems aboard the ISS. Each is designed to perform specific tasks and is independent from the other, but built with the same objective in mind. They are developed by different agencies; therefore have different chains of command. Figure 1 illustrates these three systems with reference to the ISS as a whole.

Figure 1. Images showing the ISS, MSS, ERA and JEMRMS.

While each system is worth exploring individually, this paper will focus on the MSS as it’s the most complicated of the three and was conceived since the beginning of the ISS. Additionally, some of the topics discussed in this paper share similarities among the other two systems.

1.3 Why the MSS?

Space missions have been an expensive operation for NASA. Although multiple IPs share the costs of the ISS program and multiple redesigns have been done to reduce expenses, the program keeps facing escalating costs. An early discovery on the MIR by the Soviets was the large amount of time spent on station maintenance compared to research. The ISS has been no different. In the five-year time period since initial occupation (2000-2005), astronauts spent over 4000+ hours on ISS preventive and corrective maintenance—not counting how long it took to assemble the station [2]. Repairs and maintenance are an unavoidable reality in earth as well as space. The difference being that a human is far more expensive to maintain in space than on earth. While experiments are underway to test how long a human can last in space, currently each astronaut can spend a limited amount of time aboard the ISS. The overhead associated with training, transporting, adapting and nourishing an astronaut in space is high enough to classify an astronaut as an expensive employee. The goal of an astronaut aboard the ISS is to make progress either by conducting research or solving complex problems aboard the ISS. Menial (but important) tasks such as maintenance and repairs should be left to robotic aids.

The reasons why robots are used in space are very similar to those that make robots used on Earth. In fact it is the relative importance of the reasons that changes. In order of importance, Space robots are adopted for their [3]:

1. Safety: some tasks are too dangerous (e.g. because of the hostile environment) for astronauts
2. Performance: the given tasks are too difficult or impossible (e.g. because of large masses involved, high precision and repeatability required, long duration) for astronauts
3. Cost: astronauts in space require a very expensive life support infrastructure and eventually they must return to Earth, robots just need power and they can be disposed of after they have attained their goal.

The increased autonomous operation of the MSS will result in reduced involvement of the crew and ground personnel in the operation and maintenance of the system. Reduced EVA time and crew involvement in operating the MSS will result in increased availability of on-orbit-crew-hours to other station users. The reduced EVA will result in an attendant reduction in the risk to astronauts [8].

Therefore, the main reasons why the ISS needs to employ the use of robotic systems like the MSS are:

• Expand and repair the ISS
• Transport components and payload around the ISS
• Assist and at the same time minimize EVA (extra vehicular activity or spacewalks)
• Provide overall visibility of the ISS for health monitoring (e.g. damage caused by micro-meteors)
• Assist with On-Orbit Servicing (OOS) of visiting satellites and shuttles
• Serve as a test bed for future robots

2 System & SoS Analysis

2.1 SoS Definition

In [4], the authors list several attributes that classify the ISS as a Virtual SoS primarily due to its decentralized and independent authority. While NASA plays the key integrator role, each agency develops its own systems under their own government guidance and interests. Any agency can stall the entire program if there is a disagreement or discontinue their membership at any given time. On the other hand, it could be argued that the ISS also shows characteristics of a Acknowledged SoS since even small unintended change in the ecosystem can have fatal consequences. For example, almost every module that gets launched into space has to be intercepted and installed by the MSS and requires agreements with the owner of the MSS. Though there is a central manager in an Acknowledged SoS, each agency still retains independence and ownership over its own module. 

The MSS is a fundamental part of the ISS robotic suite and a complex system by itself. The success of the ISS program relies on the participation of all IPs as well as the MSS. It’s worth exploring whether the system can be further decomposed into separate systems. In “System of Systems Integration: Key Considerations and Challenges” [5], the authors list the qualifications needed to the criteria for a particular system to qualify as a SoS. The MSS can be said to have:

1. Operational Independence of the Elements: The MSS is composed of the SSRMS, SPDM and MBS. Each element was built for a specific purpose and has the capability of working together or independently. Other than power, each element is responsible for its own functionality and survivability.
2. Managerial Independence of the Elements: Each element can be independently controlled within the ISS or tele-operated from the ground station.
3. Evolutionary Development: The MSS along with the ISS evolves with every new module that gets integrated. Changes in budget, government policy and addition of new IPs have changed the evolutionary path of the SoS.
4. Emergent Behavior: With each new element that gets added, hidden behaviors can be exposed and cascade to other systems. Additionally, space environment morphs’ the system as a whole, any component can exert unexpected forces to the entire system. Most of the elements cannot be fully tested on earth, and once a component is integrated on space the true behavior is exposed.
5. Geographic Distribution: Within the ISS, the MSS elements can be positioned in separate areas to fulfill different tasks. The ground stations is distributed across agencies, the US and Canada.

The MSS mostly resembles the characteristics of a “Directed SoS” due to its controlled nature. The use of its elements are carefully coordinated and come with months of planning from ground control.

2.2 SoSI Ontology

Stakeholders are conformed from several countries and organizations and have different needs and goals to fulfill. The clash of cultures and methodologies can result fatal in space if not orchestrated correctly. Given that integration is primarily a communications problem, the development and adherence to a systems integration (SI) ontology can inform and guide system design and integration, ensure consistent communication among stakeholders, and enforce consistent physical and logical interfaces within the system [6].

2.3.1 Stakeholders & Influences

During the planning phases of the ISS, NASA partnered with CSA to develop a robotic suite for the construction and maintenance of the station. Prior to the MSS, CSA had developed a robotic arm for the Space Shuttle program. The MSS built upon the fundamentals of the prior program as it would again be utilizing NASA’s framework. Therefore NASA played an important role as a stakeholder of the MSS. Figure 2 shows a high level representation on how the two agencies collaborate together.

Figure 2. CSA OPS Position within ISS Real-Time Support Organization [7]

The design of the system and robots employed was entrusted to CSA. The MSS was then designed and built by a team of Canadian companies led by Spar Aerospace under contract to the CSA [8]. The addition of other IPs to the ISS program also influenced the design of the MSS, which caused its stakeholder base to expand. In part because the MSS would be assembling and handling payloads from global sources (e.g. contractors). Not to forget that the primary motivation for Canada to participate in the ISS program was to boost technological advancement in its home sectors. Figure 3 captures the entities which influenced CSA in the design, construction, maintenance and financing of the system.

Figure 3. CSA’s main partners and stakeholders [19].

2.3.2 Governance & Requirements

The MSS was developed and managed by the CSA. While local decisions were subject to the CSA and Canadian government, the broader scope and mission operation was lead by NASA. Space Agencies usually rely on NASA as the middleman to communicate with other agencies. The robotic components of the MSS would be handling modules and components from other IPs. This would require coordination with the MSS to receive, unpack and install new payload. Any participant and contributing country in the ISS would be granted access and utilization of ISS facilities and resources. Figure 4 shows the geographic distribution of the IPs.

Figure 4. NASA and International Partner Operational Scope [2]

The governing body of the ISS is composed of multiple agencies with NASA as the orchestrator. Each agency negotiated and signed detailed agency-specific Memoranda of Understanding (MOUs) that defined partner contributions, payments for support, and operational responsibilities [2]. Within these agreements, there is also a clause that describes the role and usage of the MSS. The ISS Program was set up to operate using a board and panel structure, each of which functioned on consensus. In cases where consensus could not be reached, the NASA representative had the right to make a decision for the board; however, this right was rarely used in practice [2]. Due to this governing structure, any friction or disagreement from either the parties could result in setting the program at a standstill.

Coordinating multiple agencies required multilateral agreements but most importantly trust from all agencies. Table 2 lists some of the core documents that establish agreements with operation and cooperation of the ISS that are also relevant to the MSS.

Table 2. ISS IP agreement core documents.

The MSS interacts with multiple ISS segments and components during assembly and maintenance routines. Therefore the MSS must follow the same ISS standards that agencies use for the construction of their segments, modules and other payloads. Likewise, any agency that will rely on MSS services to integrate their segment, module or payload into the ISS must adhere to MSS guidelines and specifications. Table 3 lists the some of the core documents used when MSS interaction is needed.

Table 3. MSS core specification documents.

An essential requirement placed on the design of the MSS is to provide technological transparency and the necessary “hooks” and scars to enable the introduction of new technology phased in over a period of time [8]. In other words, the SoS must be kept open and be flexible enough to integrate new systems to its ecosystem or robots.

2.3.3 Integration, Verification and Validation

Integration challenges arise within the MSS organization (e.g. working with contractors) and externally with the ISS program (e.g. operations planning). Program verification requirements stipulate that no first-time verification of ISS hardware/software can take place on orbit. These same requirements also state that software must be integrated with the hardware on the ground for verification purposes [9]. The MSS is subject to the same requirement when integrating itself to the ISS. The ISS program has been through budget turbulence and uncertain times which have required redesigning unproduced components. Therefore the MSS program requires integration methodology that is flexible for unforeseen changes. Figure 5 demonstrates the integration and validation process used when new components and elements are gradually incorporated to the SoS.

Figure 5. a) Integration and Verification process flow for the MSS components and system (Hardware and Software). b) Software integration & verification flow for phased deliveries increasing levels of functionality. [9]

During the initial days, a vast amount of the information inside organizations on the International Space Station Program (ISSP) took the form of unstructured content such as e-mail messages, spreadsheets, textual documents, and even voice-mail messages [12]. This created many problems when exchanging information across agencies. By emulating similar tools in the private sector, the Integrated Mission Planning and Robotic Analysis System (IMPRAS) was proposed as an integrated collaborative platform to support the MSS [12]. In addition, the On-orbit Integration, Test and Maintenance Information (OITMIS) was put in place to develop “intelligent” data base management techniques to allow for the easy retrieval and display of procedural information necessary to affect such operations as integration, test, verification and maintenance [8].

Verification and validation spans across a wide range of domains as well. The ATD laboratories Facilities provide the infrastructure necessary to perform the advanced technology activities and to carry out proof-of-principle demonstrations [8]. As each development phase completes and new concepts are introduced, there are multiple tests carried out to prove these concepts. As listed in [8], below are some of the high level tests performed:

The MSS is part of what is defined as the United States On-Orbit Segment (USOS) of the ISS. Different parts of the MSS also include hardware elements that were provided by the U.S. through NASA [7]. Therefore, CSA not only had to test internal compatibility that they directly managed and developed, but also test them across other agencies.

2.3.4 Tailoring and Reuse

The first robotics system used for the construction of the ISS was the Shuttle Remote Manipulator System (SRMS) onboard the Space Shuttle. The SRMS was permanently anchored to the space shuttle and returned to earth multiple times along with the space shuttle. This allowed for adjustments, enhancement and design changes in the comfort of earth.

The SSRMS evolved from the SRMS, and among its several differences, the SSRMS was designed to stay on the ISS until its end of life. This meant that the system had to be robust enough to survive in space with no option for reentry repairs. ORU (Orbit Replaceable Units) were added to the design to easily perform repairs to the SRMS and SPMS. Additionally, the ORU interfaces will enable the ORUs to be replaced by other station robotic devices [8].

The SSRMS and SPMS arms, except for their size and payload capacity, have considerable functional similarity and the concepts developed for SSRMS will be directly applicable to SPDM and vice versa [8]. By keeping similar design models, testing and risk can be minimized and operations maximized.

3 Architecture

This section attempts to illustrate a high-level view of the subsystems and functionality of the MSS.

3.1 Operational Context

Figure 6. Robotic Work Station [14]

The MSS can be operated within the ISS or remotely from the ground station. Initially, robotic tasks were controlled via the Robotic Work Station (RWS) by crew aboard the ISS, and where eventually moved to the ground station. This capability allows station crews to dedicate more time to payload science while ground controllers take on some of the more mundane arm maneuvering chores [13].  

The SSRMS and SPDM can be relocated to any part of the ISS that has a Power & Data Grapple Fixture (PDGF). These bases are located throughout the ISS (and MBS) and provide the SSRMS and SPDM with structural support, power and data. Due to its ability to move from one PDFG to another, the SSRMS is usually analogous to the movement of an inchworm. On the other hand, the SPDM requires the assistance of the SSRMS to relocate. Once latched, it provides the MSS with the [dexterous] capability required to perform tasks similar to those performed by a space suited EVA astronaut [8]. The MBS provides a platform along with a toolbox set to move SSRMS and SPDM across the ISS through tracks. Figure 7 illustrates the subsystems and components of the MSS:

Figure 7. Left Side, the three combined robotic elements of the MSS. Right Side, SPDM and SSRMS latched to separate PDGFs on the ISS. [14][15]

The SSRMS, SPDM and MBS can interconnect to form a single robot capable of moving, reaching and grabbing payloads in unison. Elements are equipped with a state of the art camera and lighting system that provides a multi perspective. Using these visual inputs, operators can control and precisely position the elements strategically to perform a task.

Each of the SPDM arms is equipped with a Tool Changeout Mechanism (TCM) to interface between the arm and the tools it uses for servicing. Any payloads, component or object that has the potential to interact with any component of the MSS for installation, repair or installations needs to have a compatible interface.

3.2 Use Case Analysis

In conjunction with [17], a typical MSS Operational Scenario is described below:

1. Initialization of Equipment
   1. RWS
   2. MBS
   3. SSRMS
   4. SPDM
2. Configure for the insertions of a payload into the MBS POA (Payload/ORU Accommodation)
   1. Configure Overlays for the tasks
      1. Grapple Fixture Target Overlay
      2. Point of Resolution Rate

• Point of Resolution Position

1. Force/Moment Feedback

1. Configure POA for accepting payload
2. Configure SSRMS to manual mode
3. Insert Payload into POA
4. Complete Latching End Effector operations of the POA

• Suspend/Standby/Turn off RWS
  1. Move elements to Keep Alive Mode

3.3 System, Subsystem and Functional Description

The MSS is a highly distributed system that is integrated into the power, data, and video infrastructures of the US on-orbit segment (USOS) of the ISS [13]. On the RWS, the Display and Control Display (DCP) is the main gateway for inputs and outputs used by the operator. The RWS connects with the Command and Control (C&C) computer which then connect with the robotic elements. At the heart of each element, is the Arm Computer Unit (ACU) which manages all high level functionality of the element.

The MSS draws power from the ISS to operate. There are two separate electrical strings that power and control each element; known as the Prime and Redundant strings. The MSS architecture allows different elements to be operated on different strings thereby increasing the operational flexibility of the system [13]. Even at times when elements are not in use, the arms keep drawing power to maintain a lock position as well as thermal protection. Below figure illustrates an overview of the MSS architecture.

Figure 8. Overview of MSS Architecture [13][8]

The more autonomous the system is, the more it will fulfill its role as a robotic system. The MSS and its elements are manually operated but are designed to eventually become self sufficient. The following Software components are the core characteristics that will enable the system to become independent and provide greater robotic value:

Control System. The SSRMS and SPDM can be controlled in both Human-in-the-Loop mode and Automatic mode [8]. In the first mode, an operator has complete control of all joints and enables the system to be teleoperated. In the second mode, the operator has several automated modes that position the arm with the aid of special algorithms. The Automatic Trajectory mode with the provision of suitable command language and stored program control will enable the operation of the MSS as a robotic system [8].

Collision Prediction, Avoidance. The process of assembly can be viewed as a series of controlled collisions [8]. Through the Collision Prediction System (CSP), the element has a virtual awareness of the payload being handled and the surrounding environment through the use of models (e.g. CAD) and sensors. Special algorithms for path planning in the Collision Avoidance System (CAS) run in parallel with the CSP.

Fault Detection and Diagnostic System. This involves bringing back onboard ground operations that involve the inspection and replacement of faulty ORUs and dealing with them autonomously. Also, make use of Machine Learning to collect a knowledge base of all faults encountered in the system.

Robotic Vision. Much of the captured video data is processed and interpreted by humans. The ultimate objective of machine vision research is to develop a vision system with capabilities comparable to that of the human vision [8].

Programs such as the CSA Strategic TEchnologies in Automation and Robotics (STEAR), develops evolutionary technologies to make the MSS more autonomous in the above areas. Despite the advancements in these areas, human intervention is still needed to some degree to operate the MSS.

3.4 Human System Integration

ISS crews are trained to perform the major assembly tasks that are scheduled to occur during their increment and are also trained in generic robotics skills as well [1]. The MSS is a complicated system to understand and operate, so all personal that operates any of its systems requires going through specialized training in CSA facilities. Some of the challenges encountered were low retention of skills by the time the crew arrived at the ISS, and the difference between simulation and actual operation. This resulted in significant increases in the time required to train astronauts on the ground and maintain their skills while on-orbit using a robotics proficiency maintenance program [18]. In [1], several human centric characteristics are listed to facilitate human operation and assimilation with the MSS:

Figure 9. Image showing the composite video of one of the SPDM’s end-effector views with the alignment overlay. [18]

• Control Interfaces. Easy and flexible to use but with enough security built in to avoid catastrophes if the operator makes a mistake.
• Graphical user Interface Commonality. Should clearly notify the operator of the status and activity being performed as well as follow ISS standards so that it’s understood by all multinational crewmembers.
• Alignment Cues. Provide accurate perception where elements will be engaging. Example shown in Figure 9.
• Spatial Awareness. Establish a point of reference that distinguishes ceiling from floor.

4 Future trends and Conclusions

The MSS has opened the roadmap for future space projects as it has proven the reliability and cost effectiveness of employing robots in space. On one side, space robots are adopting a more human aspect, such as NASA’s Robonaut, for precision tasks inside the ISS. On the other, robotic arms are being adopted across other vehicles such as satellites to self-repair and extend their lifetime. It is important that future technologies consider interoperability to extract the most value out of space missions. Interoperability between the systems will increase on-orbit robotic operational efficiency, decrease operator training, decrease robotic proficiency training, and increase mission success [16]. For example, the SSRMS and SPDM elements are incompatible with the PDGFs on the legacy Russian segment; therefore these areas cannot be accessed and require their own specialized robotic systems (such as the ERA) which adds more expenses. On the other hand, the Japanese segment can host the SSRMS and SPDM due to compatible PDGFs.

This paper discussed the architecture and integration challenges of the MSS from a SoS perspective. It also provided a brief perspective on where the MSS stands in reference to the ISS and other robotic systems that inhabit the station. The goal was to create a compilation of different sources & lessons learned and structure them with an emphasis in Systems Engineering. By presenting some of the topics that led to the development and successful integration of the several elements through a period of time, future robotic systems can reutilize the architecture and integration techniques.

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