at the request of the air force, the caa's technical development and evaluation center did an analysis of the radar sightings. their conclusion was that "a temperature inversion had been indicated in almost every instance when the unidentified radar targets or visual objects had been reported." project blue book would eventually label the unknown washington radar blips as false images caused by temperature inversion, and the visual sightings as misidentified meteors, stars, and city lights. in later years two prominent ufo skeptics, dr. donald menzel, an astronomer at harvard university, and philip klass, a senior editor for "aviation week" magazine, would also argue in favor of the temperature inversion/mirage hypothesis. in 2002 klass told a reporter that "radar technology in 1952 wasn't sophisticated enough to filter out many ordinary objects, such as flocks of birds, weather balloons, or temperature inversions." the reporter added that "ufo proponents argue that even then seasoned controllers could differentiate between spurious targets and solid, metallic objects. klass disagrees. it may be that 'we had two dumb controllers at national airport on those nights'...[klass] added that the introduction of digital filters in the 1970s led to a steep decline in ufo sightings on radar."