Orbital ATK, a Dulles-based aerospace manufacturer, is suing the Defense Advanced Research Projects Agency over plans to award a Canadian firm a $15 million contract to build a fleet of space-faring robots capable of repairing government and commercial satellites. Orbital argues the federal program, called the Robotic Servicing of Geosynchronous Satellites, would unfairly compete with its own privately funded effort, a system called the Mission Extension Vehicle 1, backed by at least $200 million from investors.