Mobile Base System - Backgrounder

In April 2001 Canada launched the first component of the Mobile Servicing System (MSS) – Canadarm2. A larger and more sophisticated version of its predecessor the Shuttle Arm, Canadarm2 has seven joints and can flip hand-over-hand along the length of the International Space Station (ISS). Canadarm2 was built to operate with two other components, a work platform called the Mobile Base System (MBS) and a smaller dual-arm robot designed to accomplish more precise tasks, the Special Purpose Dexterous Manipulator (SPDM). These three elements are essential to the assembly and maintenance of the Station and represent the MSS, Canada's contribution to the ISS.

As the assembly of the ISS continues and the Station grows larger, the MBS supports the need to provide greater mobility to Canadarm2 and access to the length of the Station.

In May 2002, as part of NASA Shuttle Mission STS-111, the MBS will be launched and installed on the Space Station. The MBS was designed and built by MD Robotics, the main contractor of Canadarm2, and a team of Canadian subcontractors.

The MBS will be mounted on a US-provided Mobile Transporter (MT) that will slide on tracks along the length of the Station. Attached to the MT, the MBS will provide greater mobility to Canadarm2, and allow the transport of payloads across the Station using a fixture called the Payload/Orbital Replacement Unit Accommodation or POA. The platform's POA will mainly be used to carry large structural elements and payloads of up to 20,900 kg, such as trusses, along the length of the Station. Another attachment point called the MBS Common Attach System (MCAS) will be used to carry pallets containing lighter payloads like work tools and scientific experiments.

Astronauts will also use the MBS as a platform from which to perform spacewalks. The MBS can be used by the astronauts as a storage facility where they can keep various work tools and also as a means of transportation from one end of the Station to another. Canadarm2 is designed so that repairs can be made in orbit throughout its lifetime and the MBS may serve as a maintenance platform that would hold both ends of Canadarm2 if the replacement of any of its joints were warranted.

The MBS has four anchor points, referred to as Power Data Grapple Fixtures (PDGF), which can serve as a base for operation of Canadarm2 and the SPDM robot. Through these anchor points, the MBS provides power and data to the robots as well as to the payloads that they may be supporting. These anchor points also transfer computer commands and video signals to and from the Robotic Work Station, a console located inside the Station, from which the astronauts operate the entire MSS.

The MBS is also equipped with one removable Camera Light Pan & Tilt Assembly (CLPA). The colour camera is located on a mast behind the POA. This location provides not only a general view of the top surface of the MBS, but also unobstructed views of all four of the anchor points. Initially, the camera will be installed on the underside of the MBS Common Attach System frame in order to provide suitable views of the mating of the MBS to the MT during STS-111. After the successful mating of the MBS to the MT, the camera will be relocated to its final position during a spacewalk.

The MBS is a strong and resistant aluminium structure with a life expectancy of at least 15 years. Like all elements of the ISS, the MBS is built, with a series of separate and interchangeable modules called Orbital Replacement Units (ORU). In case of problems, the separate and interchangeable ORUs can be replaced during spacewalks or remotely, once the SPDM has been installed on the Space Station. The MBS is also equipped with two main computer units.

About the MT

Much like the railroad system that carried supplies from the East, beyond the vast plains and mountains to Canada's western shore, a new highly sophisticated railway system will be used to do much the same work on the Space Station. The MT, provided by the United States, will be an integral part of the first railway system located on the ISS.

The MT, an 886-kg structure, will travel along the rails of the Integrated Truss Structure (ITS) and, together with the MBS, will provide the work platform for Canadarm2, astronauts and eventually the SPDM. Strong and powerful, the high-strength aluminium transporter allows the MBS to move to 10 pre-designated Space Station worksites and helps deploy segments of the ISS with its payload capacity of 20,954 kg. The MT will lock itself to the rails to move the massive payloads.

The MT is controlled by complex software. Twenty different motors are energized in sequence to run the transporter from one point to another, latch it down to the integrated truss segment for construction and plug itself into the power source, the Umbilical Mechanism Assembly port.

About the Integrated Truss Structure

The MT will slide on a rail installed on a major truss spanning the length of the Station, which, when completed, will extend a distance of over 108.5 metres, slightly longer than a Canadian football field. Often referred to as the backbone of the Station, the U.S.-provided truss structure is made of eleven pre-integrated truss segments identified by their location, starboard or port, (SÆ, S1, P1, P3/P4, S3/S4, P5, S5, P6, S6). Delivery and assembly of the rail will be completed in four years, and over the course of eight space shuttle missions. Canadarm2 will continue to be essential to the assembly of this major truss.

Wires and cables will snake through the Truss to carry power and information to the Station's farthest reaches. The MSS components will move along the tracks attached to the Truss. The Truss will also house batteries, radiators, antennas and gyros.