The US Navy is developing a tiny submarine to use as a target in teaching surface warriors to spot submarines. Known as the Autonomous Mobile Periscope System, it is used to train radar operators to spot periscopes on radar and to test other technologies that might be able to spot a periscope. The small submarine is 25 feet long, three feet in diameter, and weighs 3,000 pounds. It has a fixed "periscope" (no optics or observer are involved) and can travel at depths where the periscope is exposed, then duck down to hide it. It can travel at 5 knots (about the fastest a submarine travels with the periscope exposed as going faster would create an easily spotted "feather" of spray). Submarines, since WWII, have tried to limit periscope exposures to six or seven seconds, and the system is programmed to pop up for that long and then go back down.--Stephen V Cole