Radar, an indispensable tool for troops who need to see through darkness and detect enemies through smoke, dust, fog and rain, has trouble functioning effectively where U.S. troops increasingly find themselves operating: in cities.

Buildings, utility poles, trees, vehicles, garbage cans and other urban fixtures can reflect radar signals, scrambling them into an unintelligible mess, said Edwin Chong, an electrical and computer engineering professor at Colorado State University.

Chong and a team of engineers have completed research that could lead to radar systems that can unscramble the urban radar echo chamber. “The goal of our project was to develop radar waveforms and receiving methodologies for doing detection and tracking in urban environments,” Chong said.

Based on his team’s research, Chong proposes to build new radars that would be 10 times more effective in cities than those now in use. The new radars would work by using multiple small radar transmitters that are each capable of emitting multiple waveforms. Returning radar echoes would be analyzed and sorted by computers to produce a clear picture of the target and its surroundings.

Traditional radar uses a transmitter to emit a burst of high-frequency radio waves, which bounce off objects and are detected by a receiver. The returning waves reveal that something is out there, and the time it takes them to be received tells how far away the object is. The problem in cities is that radar waves bounce back not just from the intended target, but from whatever else is nearby, Chong said. And radar waves bouncing off the target may then bounce off other objects before returning to the receiver. The result is a cacophony of radar echoes that make it nearly impossible to clearly see or accurately judge distance or movement of the target, he said.

With $1.6 million from the U.S. Defense Advanced Research Projects Agency (DARPA), Chong’s team tackled the problem in three ways. First, they decided that multiple small radar transmitters would work better than a single large one. “If you put up a dozen radars on a city block and collect the information from them and process it, you might get a very accurate picture,” Chong said. Viewing the same scene from multiple vantage points provides a sharper image.

Next, they designed radar transmitters that could each emit a number of different waveforms. Different waveforms provide distinctive echoes when they bounce back from different substances. That way, it is possible to tell whether the returning echo is bouncing back from stone, metal, wood or other material, Chong said. He likened the waveforms to colored lights. When a red light is shined on a target, it reveals the red elements. A green light shows what’s green, and so on.

Finally, the Colorado State team compiled “a library of waveforms” for the radar transmitters and algorithms to interpret the echoes. Computer algorithms also determine which waveforms the radar transmitters will emit and when they will emit them, Chong said. Radar returns from all of the receivers are transmitted to a single computer to be processed. One benefit from that is that the radar transmitters and receivers are relatively simple and cheap.

“You can put them on vehicles; you can stick them on walls with self-adhesive pads. They cost very little, so you don’t care if the wall gets blown up,” Chong said.

An area to be monitored could be liberally sprinkled with small radar transmitters and receivers. Planted along a city block, the radars could be kept on continuously for general surveillance, then switched automatically into tracking mode when the system senses that something important has happened, Chong said.

For the DARPA project, the system was designed for use as a fixed device on the ground. But Chong said he is “very interested” in the radar’s potential for use in small UAVs. “On a UAV, they could have extra flexibility,” he said. In response to an incident, “you could tell a bunch of flying UAVs to all go chase that truck.”

The Colorado State team’s new radar methods were tested by the U.S. Air Force Research Laboratory in Rome, N.Y., and a final report has been submitted to DARPA, Chong said.

The research was part of a multiproject push that DARPA began in 2005 to develop radars that will work better in urban areas. With U.S. troops operating in Baghdad and other Iraqi cities, DARPA’s Special Programs Office declared urban areas “a new frontier for radar.”