Expedition Four Flight Engineer Carl Walz sent commands from a laptop computer to the Mobile Transporter to move off of its launch position on the forward face of the new S-Zero (S0) truss, and at 7:22 a.m., the flatcar began its slow trek to an initial worksite 17 feet down a rail which spans the entire 44 feet of the girder.
It took only a half hour to traverse the distance, but sensitive software in the transporter prevented an automatic latching of the railcar to the worksite. Ground controllers accomplished the latching through a methodical series of commands.
Engineers believe that the subtle effects of weightlessness are causing the railcar to "lift" off its tracks by a microscopic distance, thus interfering with magnetic sensors that tell the transporter its position relative to each worksite. The effect is that the sensors are losing contact with magnetic positioning strips on the truss rail, preventing an automatic latching of the transporter. Manual commanding of the latching is working however, and the system is said to be in excellent working order.
The Mobile Transporter software controls about 20 motors, directing it to travel from one point to another, latch itself down to the truss, and plug itself into a power source. The transporter must latch with about three tons of force to insure a stable platform for the eventual mounting of the Canadarm2 robotic arm. On the next shuttle assembly flight to the ISS in June, a platform called the Mobile Base System will be mounted to the transporter upon which Canadarm 2 will eventually be attached so it can travel the length of a football field to support future assembly of station components.
Late today, the transporter traveled to a second worksite where manual latching commands were again required, then inched back to the first worksite, where the railcar was parked at 5:40 p.m. Central time and manually latched in place for a final time to await the arrival of the Mobile Base System component on the STS-111 mission. In all, the transporter traveled 72 feet from worksite to worksite at a glacial pace of about one inch per second.
Engineers believe a minor software modification may restore the transporter's ability to automatically latch itself to any worksite. All other transporter systems functioned perfectly throughout its initial test.
Atlantis Commander Mike Bloomfield, Pilot Steve Frick, Mission Specialists Ellen Ochoa, Rex Walheim, Lee Morin, Jerry Ross and Steve Smith, and Expedition Four Commander Yury Onufrienko and Flight Engineers Walz and Dan Bursch spent the day monitoring the transporter tests and continued the transfer of equipment and supplies from Atlantis to the station.
In addition, about 100 pounds of oxygen and 30 pounds of nitrogen have been transferred from Atlantis to the tanks on the Quest Airlock to support future spacewalk activity.
The fourth and final spacewalk of the flight will be conducted on Tuesday by Ross and Morin beginning around 9:30 a.m. Central time. At the start of the planned 6 ½ hour excursion, Ross and Morin will pivot a 14-foot ladder away from the S-Zero truss for attachment to Quest to act as a pathway for future spacewalkers. They will also install external lights on the Unity module, test microswitches on the sides of the S-Zero truss which will be used to confirm the attachment of future truss segments, troubleshoot a balky bolt on a cable cutting system on the Mobile Transporter and tie down a portion of insulation on one of four navigational antennas on the S-Zero.
The ten Shuttle and Station crew members are scheduled to begin an eight-hour sleep period at 7:44 p.m. Central time and will be awakened shortly before 4 a.m. Tuesday to prepare for the final spacewalk of the mission.