After flying 60 million miles over 20 missions, the space shuttle Atlantis has arrived at the Boeing Orbiter Major Modification Facility in Palmdale for a $70-million make-over. Atlantis will undergo a comprehensive inspection and more than 100 modifications designed to cut maintenance costs, improve shuttle safety and increase its ability to support construction of the International Space Station, officials said. About 350 engineers and technicians--many of whom built Atlantis--began overhauling the shuttle in November and will complete work around July. It is scheduled to return to the Kennedy Space Center in August. Atlantis will also get a satellite-based navigation system as part of its face lift. Eventually, the satellite guidance system will replace several ground navigation sites at the Kennedy Space Center and other transatlantic landing sites. Additionally, crews will scour every inch of the shuttle, using borescopes, X-rays and ultrasound technology to search for possible fatigue, corrosion, broken rivets or welds on the 12 1/2-year-old ship.