It's a regrettably common scene: hostages at gunpoint, police shouting through bullhorns, holding off for fear of harming those inside. Now Huntsville, AL-based Time Domain has a device that could give the cops a better sense of when to move in: an instrument that can see through walls. Much like a radar set, the system sends out a signal that bounces off objects, so distances can be calculated. But while radar sends out a continuous wave, Time Domain's instrument sends out millions of pulses per second, allowing it to screen out still objects and precisely locate moving objects. The device, which the company expects to sell by year's end, will be responsive enough to detect even the minute motions of a person attempting to stand still. An even more sensitive version is being developed for use in finding people during earthquake rescue.