Minimize ISS Servicing: MSS

ISS Servicing: MSS (Mobile Servicing System)

The MSS is a robotic system used for space station assembly and maintenance: moving equipment and supplies around the station, supporting astronauts working in space, and servicing instruments and other payloads attached to the space station. The MSS includes facilities on Earth for mission support and astronaut training. The MSS plays a key role in the construction of the ISS and general Station operations. It allows astronauts and cosmonauts to work from inside the Station, thus reducing the number of spacewalks. The MSS Operations Complex in Longueuil, Quebec, is the ground base for the system. 1) 2)

The SPDM (Special Purpose Dextrous Manipulator) or Dextre, along with the SSRMS (Space Station Remote Manipulator System), also known as Canadarm2, and the MBS (Mobile Base System) forms the Mobile Servicing System, provided by CSA (Canadian Space Agency), representing Canada’s major contribution to the ISS. 3)


Figure 1: Overview of the MSS with its components installed on the ISS (image credit: CSA) 4)

The MSS is composed of three separate components:

1) Canadarm2, also known by its technical name, SSRMS (Space Station Remote Manipulator System). The SSRMS as part of Canada's contribution to the ISS. The next generation Canadarm2 is a bigger, better, smarter version of the Space Shuttle's robotic arm. It is 17.6 m long when fully extended and has seven motorized joints. This arm is capable of handling large payloads and assisting with docking the space shuttle. The Space Station Remote Manipulator System, or SSRMS, is self-relocatable with a Latching End Effector, so it can be attached to complementary ports spread throughout the station's exterior surfaces. 5)

The SSRMS is comprised of long composite booms made from PEEK/IM7 (PolyEtherEtherKetone / Carbon fiber) composite, which were manufactured by FRE Composites Inc., Saint-André, Québec, Canada. Canadarm2 is similar to the Canadarm used on the Space Shuttle, but Canadarm2 is larger, incorporates many advanced features, and includes the ability to self-relocate.

• Canadarm2 was flown to the ISS on STS-100 flight of Endeavour (April 19 - May 1, 2001). It has been attached to the Destiny module and used as “construction crane” of ISS, lifting payloads and performing maintenance work. Canadarm2 has been used to move hundreds of tons of supplies and equipment and even astronauts, supporting about 100 spacewalks to the end of 2012.

2) SPDM (Special Purpose Dextrous Manipulator) or simply called Dextre, has a dual-arm design that can remove and replace smaller components on the Station’s exterior, where precise handling is required. It will be equipped with lights, video equipment, and a tool platform, as well as four tool holders.

Dextre was designed and developed at MD Robotics of Brampton, Ontario, a subsidiary of MDA (MacDonald Dettwiler Associates), Canada. The two-armed Dextre is an advanced robotic system consisting of a central body with a grapple fixture at one end and a latching end-effector at the other. Each of its two arms is identical, has seven joints, and is equipped with a tool change-out mechanism at the end. The mechanism can remotely change-out orbit-replaceable units and can also pick up and use special tools stored on the body of Dextre.

Dextre can be carried by two elements, the MBS (Mobile Base System) and by Canadarm2, or move independently. Dextre adds robotic tools and a range of fine manipulation abilities to construction and maintenance operations on the Space Station, which is now (2013) much larger than in 2001 when Canadarm2 was installed. Dextre has a mass of ~1660 kg and dimensions of: 3.67 m (height), 2.37 m (width), and an arm length of ~3.5 m. It can lift payloads of up to 600 kg to an incremental positioning accuracy of 2 mm. The average operating power is 1.4 kW.

• Dextre is a two-armed robot, or tele-manipulator which is part of the Mobile Servicing System on the ISS, and extends the function of this system to replace some activities otherwise requiring spacewalks. It was launched March 11, 2008 on mission STS-123.

3) MBS (Mobile Base System). The MBS provides a movable work platform and storage facility for astronauts during spacewalks. With four grapple fixtures, it can serve as a base for both the Canadarm2 and the Special Purpose Dexterous Manipulator (SPDM) simultaneously. Since it is mounted on the U.S.-provided Mobile Transporter (MT), the MBS can move key elements to their required worksites by moving along a track system mounted on the ISS truss.

• With the installation of Canada's Mobile Base System (MBS) in June 2002 (STS-111), Canadarm2/SSRMS is able to slide along the US-provided main truss of the station (lateral mobility), providing an additional functional capability with four Power Data Grapple Fixtures.


Figure 2: The MBS is moved by the Canadarm2 for installation on the ISS (image credit: NASA) 6)

Legend to Figure 2: On June 10, 2002, astronauts Peggy A. Whitson, Expedition Five flight engineer, and Carl E. Walz, Expedition Four flight engineer, attached the MBS to the Mobile Transporter on the S0 (S-zero) Truss. The MBS allows the station’s robotic arm (Canadarm2) to travel the length of the station to perform construction tasks.




MBS (Mobile Base System)

Arm length

17.6 m

3.5 m linear stroke

5.7 m x 4.5 m x 2.9 m


~1800 kg

1662 kg

1450 kg

Mass handling/transportation capacity

116,000 kg

600 kg

20,900 kg

DoF (Degrees of Freedom)




Peak power (operational)

2 kW

2 kW

825 W

Average power (idle)

435 W

600 W

365 W

Applied tip load range

0-1 kN

0-111 N


Stopping distance

0.6 m

0.15 m


Table 1: MSS (Mobile Servicing System) subsystems


Figure 3: The complete MSS (Mobile Serving System) of ISS (image credit: NASA, CSA) 7)


Figure 4: Canadarm2 and Dextre post capture and berthing of JAXA’s HTV-2 in late January 2011 (image credit: NASA)



Advanced robotic capabilities of Dextre:

In addition to improving its utility to ISS by expanding and adapting Dextre to new devices and payloads that exceed its original design intent, Dextre has enabled the ISS to act as a test bed to evaluate a new generation of space-based instruments and robotic tools, along with operational techniques and the control strategies necessary for future on-orbit robotics and servicing missions.

Dextre’s primary role is as the ISS Handyman, replacing failed components, designed as ORUs (On-orbit Replaceable Units), unloading cargo and now supporting technology demonstrations and furthering the use of the ISS as a technology development platform.

Services provided by the MSS and Dextre to external payloads include power, video and data transfer functions that allow commanding of the attached payload, and robotic manipulation including the accurate position of sensors and the construction of complex external systems.

Examples of recent operations (Ref. 3):

Since deployment, Dextre has consistently proven its capabilities to perform the baseline functions of ISS maintenance and logistics, with so much success that Dextre now performs all external cargo handling from visiting vehicles, a function nominally performed by with astronauts via EVAs (Extra Vehicular Activities) during the assembly phase of the ISS.

The ISS program confidence in the overall performance and reliability of the MSS is such that designs for new vehicles providing external cargo services such as SpaceX’s Dragon vehicle (Figure 2) do not even include support EVA access to the external cargo hold at all, driving increased dependency on Dextre and many more hours of EVR (Extra Vehicular Robotics) activities.

Since all dexterous operations are ground-led, i.e. command and control is tele-robotically performed by robotics officers, this significantly reduces the on-orbit crew overhead for such tasks and allows them to concentrate on the main goal of the ISS which is to act as an on-orbit laboratory. The ground operators are MSS specialists who perform their duties from consoles located at JSC (Johnson Space Center) and at CSA facilities near Montreal, Canada with engineering support teams at CSA and MDA (Brampton, Canada).

On top of its baseline functions, from 2012 through the start of 2013, Dextre supported several technology and satellite servicing technique demonstrations on board the ISS, continuing to stretch its capabilities beyond its originally conceived design.

RRM (Robotic Refueling Mission) payload, a joint NASA-CSA project, it was delivered to the ISS on flight STS-135 in July 2011. RRM is an experiment designed to demonstrate and test the tools, technologies and techniques needed to robotically refuel satellites in space, even satellites not designed to be serviced. RRM is an external ISS experiment that will use Dextre, a space robot of CSA, to demonstrate and test the tools, technologies and techniques engineers on Earth would need to robotically refuel satellites in space-even satellites not designed to be serviced.

On September 2, 2011, the Space Station's Canadarm2 and Dextre transferred RRM to its permanent location on the ExPRESS (Expedite the Processing of Experiments to the Space Station) Logistics Carrier-4 (Figure 5). 8)


Figure 5: Illustration of the NASA designed RRM module (center) and the Dextre robot (left), image credit: NASA 9)

• The first RRM experiment was conducted very successfully on March 7-9, 2012, marking important milestones in satellite-servicing technology and the use of the space station robotic capabilities. During the three-day RRM Gas Fittings Removal task, Dextre performed the most intricate task ever attempted by a space robot: cutting two separate "lock wires" of 0.5 mm in diameter using the RRM WCT (Wire Cutter Tool). Deftly maneuvered by ground-based mission operators and Dextre, the WCT smoothly slid its hook under the individual wires and severed them with only a few mm of clearance. This wire-cutting activity is a prerequisite to removing and servicing various satellite parts during any future in-orbit missions.

Tests to prepare for on-orbit RRM operations were conducted at Goddard and at MDA in Brampton, Ontario (formerly SPAR Aerospace, the prime contractor for the Canadarm as well as the CSA's Mobile Servicing System on board the station).

RRM Fuel Transfer Demo (Session 3): Over 6 full days of robotic activity in January 2013, Dextre successfully completed the third session of operations for RRM (Robotic Refueling Mission) of NASA/GSFC. The session built on the work of previous RRM on-orbit demos, in which Dextre manipulated RRM tools to demonstrate servicing techniques for unprepared satellites (i.e. satellites with features and interfaces not originally designed for robotic servicing).

The session focussed on demonstrating the ability to robotically access a satellite’s FDV (Fill & Drain Valve) and transfer liquid propellant – a capability necessary to enable the robotic refuelling of satellites in space. This session began by preparing the FDV assembly, so that the ENT (EVR Nozzle Tool) could access the Actuation Nut. This setup consisted of cutting safety wires and removing and stowing protective caps installed over the Actuation Nut, as shown in Figure 6. 10)


Figure 6: Sequence to access bare actuation nut of typical spacecraft FDV (image credit: MDA)

The ENT is attached to a spring-loaded pressurized hose which tethers it to the RRM module, as shown in Figure 7. This hose facilitates the transfer of ethanol, but (to make the operation more challenging) exerts a variable pull force of up to 40-50 N on Dextre’s Arm once the ENT is unstowed from its tool bay.

Dextre maneuvered and aligned the ENT, outfitted with a Quick Disconnect Connector installed on its tip, towards the now bare FDV, threaded the Quick Disconnect onto the FDV and opened the actuation nut using its torquing mechanism.


Figure 7: Dextre unstows ENT (image credit: NASA, MDA)

Once the nut was open, commands were sent by GSFC via Marshall Space Flight Center to the RRM to start the fluid transfer process through the tool (Figure 8). After this was completed successfully, the lines were purged with Nitrogen and the Actuation nut was closed. The ENT then released the valve, leaving the Quick Disconnect behind for future use.


Figure 8: ENT installed on FDV – fluid transfer in progress (image credit: NASA, MDA)

This session completed the primary objectives of the RRM demo mission, which was to demonstrate successful fluid transfer via remotely controlled robotics.

RRM Servicing Techniques (Session 4) in May 2013: Whereas the third RRM session focussed on the tasks required to access and service satellite fuelling and coolant systems, a fourth and final session demonstrated several “simple” skills required to access the inner workings of satellites. These tasks included removing SMA caps with a modified socket wrench, unfastening captive screws with a Phillips screw driver and cutting and folding a taped panel of MLI (Multi-Layer Insulation), Figure 9.


Figure 9: Satellite servicing technique tasks for the Fourth session of RRM Phase 1 (image credit: NASA, MDA)

An attachment was used with RRM’s MFT (Multi-Function Tool) to manipulate sets of caged and captive screws, as shown in Figure 10. This activity provided a challenge as Dextre had to demonstrate a delicate touch to align and acquire the screw using a “space screwdriver” at the end of 20 m of robot in a constantly changing thermal environment. Despite the challenges, two screws were successfully acquired and manipulated during the demo.


Figure 10: Manipulating captive fasteners (image credit: NASA, MDA)

The final task of this session was cutting through a taped MLI panel. One of the challenges of tape cutting encountered during the SPDM GT (Ground Test) was maintaining very low push forces while maneuvering in contact with the worksite. In no task were these challenges more evident than while using the RRM WCT (Wire-Cutting Tool) blade to carefully slice through a tape seam while limiting the force of the blade tip against the panel. This was the most challenging task executed in both the GT Lab and on-orbit.

To cut the MLI panel, the tip of the WCT blade must be carefully inserted to pierce between the two MLI layers and then be translated horizontally to cut the seam (Figure 11). This was repeated for 2 other sides of the panel creating a “flap” that could be opened.


Figure 11: WCT tool tip cutting the MLI seam (image credit: NASA, MDA)

Constant lateral tension of the blade on the tape needed to be maintained to prevent “bunching” of the MLI and force build-up on the blade (which would have resulted in uncoordinated blade motion). Near the end of the first cut, some MLI bunching occurred, resulting in at least two small pieces of MLI detaching from the panel. Debris is always a potential concern for the ISS and its external hardware. Once debris was generated, several visual and system checks were required to verify that the debris had not lodged itself on Dextre or RRM.

The remaining two sides were cut without generating further debris and Dextre used the WCT to pull back the MLI flap and secure it in its open position, as shown in Figure 12.


Figure 12: WCT opening the MLI flap (image credit: NASA, MDA)

Given the nature of unprepared satellites, the ability to transfer fuel is not useful on its own unless the servicing system can also remove a panel, cut through a tape, or lift a blanket to access the necessary satellite interfaces. That is why the transferable techniques demonstrated in the Fourth session are vital for the future of satellite servicing.

The overall project success of this current phase of RRM (referred to as Phase I of the RRM project) could only be achieved through a close coordination and strong partnership between programmatic, engineering and operations teams at CSA and MDA, NASA/GSFC, and NASA/JSC.

The creation of a coordinating body by JSC and CSA to manage the use of the MSS for technical demonstration projects on the ISS also enabled the first time coordination between GSFC and MDA on an ISS robotics project. The specialized operations teams which prepared for and lead the execution of these most complex and lengthy robotics operations ever executed on-orbit were directly supported by the work from MDA and NASA/GSFC engineers working in close coordination over the last three years (Ref. 3).



Future planned operations of the MSS:

• JAXA's JEM/Kibo (Japanese Experiment Module) was installed on the ISS in July 2009, using Canadarm2 (STS-127 assembly flight 2J/A of Endeavour). The JEM is the largest laboratory module on the ISS and also supports its own robotics manipulator, dedicated to payloads and experiments on the JAXA exposed facility attached to the JEM.

JEM/Kibo also hosts an airlock that allows the transfer of small payloads, ORUs (On-orbit Replaceable Units) and experiments from the internal pressurized section of the ISS to the external environment and back. The JOTI (JEM ORU Transfer Interface), shown in Figure 13, was launched in August 2013 aboard HTV4; it provides a robotically compatible interface to the JEM airlock slide table for these small to medium-sized ORUs and other payloads including scientific experiments, currently stored inside the ISS.

Once outside, Dextre will be able to remove the ORUs and/or payload and transfer them to their installation location, resulting in a fully robotic solution for this hardware outside the ISS. Similarly, Dextre will bring back failed ORUs and spent experiments to the JOTI for internal transfer to the ISS (Ref. 3).


Figure 13: Planned ORU configuration for JOTI (image credit: MDA)

• RRM Phase 2: Building on the success of the Phase I demo, Dextre’s analysis and operations team is currently gearing-up to support NASA/GSFC’s RRM Phase 2 demonstration commencing on-orbit in early 2014. The first delivery of hardware for Phase 2 was launched as internal cargo on HTV4 in August 2013 and will be transferred outside via JOTI, where SPDM (Dextre) will install it on the RRM worksite. The remainder of the hardware, including a new inspection tool and additional MFT adapters, are planned for launch in mid-2014.

RRM Phase 2 tasks include:

- Coolant valve installation

- Installing a plug into an open vent port and providing an air-tight seal

- Installing/removing electrical connectors

- Inspections of tubes and difficult to access spaces with new inspection tool.

In addition to performing analysis and specialized reconfiguration file delivery for the Phase 2 operations, MDA will also be performing an evaluation of the flight-equivalent hardware in the SPDM GT Lab (Oct 2013), similar to the evaluation performed for Phase 1.

The new tasks in phase 2 will present many of the same challenges that were overcome by Canadarm2 and Dextre, namely fine arm positioning and controlling worksite forces and moments with small tools and interfaces.

One of the outputs of these tasks will be increased confidence in the ability for dextrous robots with specialized tools and control systems to handle a broad array of servicing activities for unprepared or lightly prepared satellites.

• OpTIIX (Optical Testbed and Integration on ISS eXperiment): Payload Construction. JPL's OpTIIX, currently in development, presents a new type of task for Dextre. As opposed to solely transferring an assembled payload or a replacing a single ORU, this experiment is a modular telescope that would be assembled by Dextre and installed externally on the ISS (Figure 11).


Figure 14: JPL’s OpTIIX modular components and assembled configuration (image credit: NASA, MDA) 11)

1) “Mobile Servicing System (MSS),” NASA, URL:

2) “MSS Operation Complex,” CSA, URL:

3) Lyndsey Poynter, Richard Rembala, P. Andrew Keenan, Andrew Ogilvie, “The Advancement of Robotic Servicing Capabilities through Dextre Utilization and Technology Demonstration on the International Space Station,” Proceedings of the 64th International Astronautical Congress (IAC 2013), Beijing, China, Sept. 23-27, 2013, paper: IAC-13-B3.4-B6.5.6

4) “Mobile Servicing System,” CSA, URL:

5) “Canadarm2 and the Mobile Servicing System,” NASA, URL:

6) “STS-111 Shuttle mission imagery,” NASA, June 10, 2002, URL:

7) “ISS Elements: Mobile Servicing System (MSS),” URL:

8) Ken Kremer, “Revolutionary Robotic Refueling Experiment Opens New Research Avenues at Space Station,” Universe Today, July 16, 2011, URL:

9) “Robotic Refueling Mission : Image Gallery,” NASA, URL:

10) “RRM (Robotic Refueling Mission),” NASA, URL:

11) Kenneth G. Carpenter, Shar Etemad, Bernard D. Seery, Harley Thronson, Gary M. Burdick, Dan Coulter, Renaud Goullioud, Joseph J. Green, Fengchuan Liu, Kim Ess, Marc Postman, William Sparks, “OpTIIX: An ISS-based Testbed Paving the Road map toward a Next Generation, Large Aperture UV/Optical Space Telescope,” UV Astronomy: HST and Beyond Conference, Kauai, Hawaii, June 18-21, 2012, URL:

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.