It's one of the classic movie plots: the bad guys—foiled in their attempt to grab piles of cash or some priceless artifact and make a speedy getaway—have taken hostages. The police hatch a plan to covertly enter the building and capture the criminals, and the hero almost always chooses just the right air duct that will let him spy on the captors before he springs into action. But in real life, where such heroic gambits are often deemed too risky, researchers have been working on radar that can "see" through walls, so police can know where hostages are congregated or soldiers can tell where the enemy is lying in wait. Two devices that meet demanding criteria are on the market, and one has been adapted for use by the U.S. military in Iraq.

Some conventional radar can penetrate walls, but it cannot distinguish objects just ahead, it emits far too much power to be safe for operators, and it requires equipment about the size of a lab bench. Advances in digital signal processors and microwave integrated circuits have made it possible to fit a complete microwave system in a box the size of two encyclopedia volumes. Now, through-the-wall radar devices that are lightweight, portable, and able to focus up to 20 or 30 meters ahead are available to municipalities and law enforcement agencies. Two such devices are RadarVision, built by Time Domain Corp., of Huntsville, Ala., and the Prism 100, from Cambridge Consultants Ltd., in Cambridge, England. Both rely on ultrawideband, a fairly new technology known mainly as a promising high-speed, low-power radio communications transmission technique.

A change in software can turn an ultrawideband radio, whose pulses of RF energy normally carry data, into an ultrawideband radar. Though these new portable radars are based on each firm's own flavor of ultrawideband technology, they are quite similar. Both devices can detect the presence of inanimate objects through the wall, but only motion (in the form of a moving blob of color on their built-in color screens) is shown to the user. The devices are so sensitive that even if someone on the other side of the wall is sitting still, the machines can detect the rise and fall of the person's chest with each breath.

The radars transmit millions of very short pulses. What they see through a wall is related to the timing of the return pulses. RadarVision generates 10 million 300- to 500-picosecond-long pulses every second—each one at well below 100 microwatts. Its receiver knows to within a few picoseconds when any one of the pulses will return and will switch on only for a brief sampling window, after which it shuts off again. This feature greatly improves the signal-to-noise ratio of the return signal and reduces the radar's power consumption.

Either device can run for a couple of hours on a single battery charge. Each also has the added benefit of making it difficult for the bad guys to know they are being monitored, because signal detection devices can't distinguish the devices' low-power transmissions from background noise.

On return, the pulses are picked up by a linear array of antennas. The time of arrival for each return pulse is measured at each antenna, providing an accurate determination of where the moving object is with respect to the machine's field of view. The radar systems look for changes in the range and angle at which successive pulses strike an object on the other side of the wall.

If, say, Pulse 1 comes back revealing that there is an object at range x and angle y, a difference in range or angle for Pulse 2 is registered as movement. An onscreen representation of that is shown to the user. Whenever there is no difference between the latest pulse return and the one preceding it, which is the case for pulses that bounce off inanimate objects, the system disregards those objects and omits them from the display.

What the user sees is a plain view of what lies on the other side of the wall, but seen onscreen from above [see illustration, "Looking Over"]. An optional mode shows the space on the other side of the wall the way it would appear from the side. This option allows an experienced operator to distinguish between tall and short objects, such as an adult and a small child or pet.

To get around spectrum interference rules and to make the radar even more immune to detection, the pulses, which are spread across frequencies ranging from 1 to 5 gigahertz, are pseudorandomly dithered in time. Dithering requires a time code that determines the position of the pulse within a time window. This ensures that the signal is like noise: it is evenly distributed in the frequency domain and thus presents only a tiny amount of energy in any frequency band.

For Soldier Vision, a version of RadarVision commissioned by the U.S. Army for overseas deployment, there is a boost mode that ups the transmit power of the pulses, making movement easier to detect. Prism 200, scheduled for release in early 2006, operates at higher power as well.

Meanwhile, other devices said to be better suited to scanning disaster sites are being put through their paces. These include Radar Flashlight, developed at the Georgia Institute of Technology, in Atlanta, which relies on Doppler shifts in return pulses to detect motion.