I didn't realize that either end of the robotic arm could act as both a "shoulder" and a "wrist", and that it could plug it's "wrist" into a second power-data-grapple fixture and then unplug it's "shoulder", effectively walking from one fixture to antoher.

How may fixtures are there, and how many different walk-off maneuvers can the Canadarm-2 do? For example, if there were three fixtures A, B, and C, the answer could be as many as six (A->B, B->A, A->C, C->A, C->B, B->C) but it could be less if A and C are too far from each other to go directly.

Are there other animations of the arms's various walk-off maneuvers that I could look at besides the one in this cool video? (Walk-off after 01:00, but it's a cool video so watch the whole thing!)

Video's caption:

Uploaded on Aug 24, 2010 This NASA video features astronaut Dottie Metcalf-Lindenburger giving an overview of the International Space Station's Robotic Arm. I received the raw video of Dottie on camera (recorded on ISS during STS-131) and was asked by the NASA client to polish it up. I produced and edited the resulting product.

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    $\begingroup$ I'll answer when I get home tomorrow, but to walk off, the arm must grapple a PDGF (Power and Data Grapple Fixture), and there are quite a few of those; one was even added to the Russian segment. $\endgroup$ Feb 19, 2017 at 1:05
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    $\begingroup$ It's not so much a robotic arm, as a robotic caterpillar! $\endgroup$
    – SF.
    Feb 20, 2017 at 10:32

1 Answer 1


I'll have to let you count them because there are some ambiguities that arise from the use of the Mobile Transporter (MT)...but here are the possible walk-offs.

These are the fixed base-capable Power and Data Grapple Fixtures (PDGFs) on the ISS

  • US Laboratory Nadir-Port Side
  • Node 2 Nadir Side
  • FGB Forward-Port Side

This picture shows the Space Station Remote Manipulator System (SSRMS) aka the "big arm" based on the Lab (this is an old ISS config).

photograph of SSRMS based on the Lab with a shuttle orbiter in the background

This picture shows the SSRMS based on Node 2.

photograph of the central truss part of the ISS showing the SSRMS based on Node 2

This is a picture of the PDGF on the FGB.

photograph of the PDGF that is mounted on the FGB

(note there are many other PDGFs on the ISS that were used for assembly, but they are not base-capable)

And the MT has 4 base-capable PDGFs on it. The MT can move to and site itself at any of eight worksites along the truss. This diagram shows the worksites. 9 and 10 are not used - they are outboard of the alpha gimbals and it's too much trouble to use them.

schematic drawing showing the worksites along the truss where the MT can be parked and their numbers

This picture shows the MT with its four PDGFs. (in the real world it's covered with white multi-layer insulation). Technically the top part with the PDGFs is called the MBS (Mobile Based Servicer) but I just call the whole thing the MT. Saves a letter.

schematic drawing of the MT with the MBS mounted on it

Here is a picture of the SSRMS based on the MT.

photograph showing the MT with the SSRMS based on it

So, among these PDGFs, the allowable walkoffs are

Walkoffs to/from Lab PDGF

  1. Lab <-> Worksite 3, MT PDGF 1, 3, 4
  2. Lab <-> Worksite 4, Any MT PDGF
  3. Lab <-> Worksite 5, Any MT PDGF
  4. Lab <-> Worksite 6, Any MT PDGF
  5. Lab <-> Worksite 7, MT PDGF 1, 2
  6. Lab <-> Node 2 PDGF
  7. Lab <-> FGB PDGF

Walkoffs to/from Node 2 PDGF

  1. Node 2 <-> Worksite 3, Any MT PDGF
  2. Node 2 <-> Worksite 4, Any MT PDGF
  3. Node 2 <-> Worksite 5, Any MT PDGF
  4. Node 2 <-> Worksite 6, Any MT PDGF

The FGB can only walk to/from the Lab, as previously stated. It cannot reach any PDGFs on the MT at any site.

Additionally, the SSRMS can walkoff from any MT PDGF to any other. So, do you count this times 8 because it can happen at any worksite? I'll let you decide.

Source: sadly, personal notes. All photos and schematics credit NASA.

  • $\begingroup$ This looks really helpful! I see what you mean about counting. I'll think about this today and try to imagine what's going on. This answer could really benefit from a diagram or animation. I'm sure the information is here, but for most of us who aren't so familliar with the ISS layout, this could be a learning experience. Oh, is the Mobile Transporter (MT) the same as the "Mobile Base System that moves back and forth on the truss" in the video at 00:55? Is SSRMS the "robot arm"? $\endgroup$
    – uhoh
    Feb 19, 2017 at 23:57
  • $\begingroup$ Revised, let me know if I didn't cover something. $\endgroup$ Feb 20, 2017 at 0:18
  • $\begingroup$ Perfect. This gives a view of the system from different perspectives - systems, CAD, and visual - which gives some insight into how the whole thing has been planned and works as a single system. I had no idea all this was going on! I imagined there was a very long arm somehow sticking out of the center of the ISS, but knew that wouldn't work. But I had no idea there was an arm walking around outside the ISS and occasionally 'riding the rails'. $\endgroup$
    – uhoh
    Feb 20, 2017 at 1:07
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    $\begingroup$ In fact, when motion starts on the MT, a old fashioned train whistle is blown in Mission Control :) $\endgroup$ Feb 20, 2017 at 1:19

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