On a training mission with the Nimitz carrier group in November 2004 about 100 miles off the coast of Southern California, technicians on the USS Princeton began to notice anomalous "ghost tracks" and "clutter" on the ship's radars.
Concerned the Princeton's brand new AN/SPY-1B radar system was acting up, the crew recalibrated the high tech gear to clear the errors, but the ostensibly "false" radar tracks only amplified and became stronger, the first sign something rather strange was happening.
"Once we finished all the recalibration and brought it back up, the tracks were actually sharper and clearer," former Petty Officer 3rd Class and Princeton radar technician Gary Voorhis told Popular Mechanics. "Sometimes they'd be at an altitude of 80,000 or 60,000 feet. Other times they'd be around 30,000 feet, going like 100 knots."
Their radar cross sections didn't match any known aircraft; they were 100 percent red ... no IFF [Identification Friend or Foe].
Comment: David Paulides, author of the Missing 411 series, poses some interesting questions about the case: