 After major modifications and upgrades, including installation of a two-stage turbocharger in place of its original single-stage unit, a larger fuel tank and additional intercooling capacity, the ALTUS II returned to flight status in the summer of 1998.The goal of its development test flights was to reach one of the major ERAST Level 2 performance milestones, to fly a gasoline-fueled, piston-engine remotely piloted aircraft for several hours at an altitude at or near 60,000 feet.On March 5, 1999, The ALTUS II maintained flight at or above 55,000 feet for three hours, reaching a maximum density altitude of 57,300 feet during the mission.