 The Mobile Base System (MBS) is a platform (mounted on the Mobile Transporter) for the robotic arms Canadarm2 and Dextre carrying them 108 metres down rails between the S3 and P3 truss.Beyond the rails Canadarm2 can step over the alpha rotary joint and relocate to grapple fixtures on the S6 and P6 truss.During STS-120 Astronaut Scott Parazynski rode the Orbiter Boom Sensor to repair a tear in the 4B solar array.