Cell Towers Can Double As Cheap Radar Systems

Passive radio signals sent by cellular stations could detect small boats and improve security at ports and harbors

Illustration: Fraunhofer FKIE
Visualization of the Passive Coherent Location system for detecting ships in a port
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How do you see ships without a pricey radar system? The question has troubled seaports around the world as they work to improve security. Without radar installations, it can be hard for port employees to detect small ships like those employed by pirates or by the terrorists who attacked the USS Cole in 2000. A team of researchers in Germany can now offer security teams a new option, though: putting existing cellular towers to work as quick and dirty radar systems.

Developed at the Fraunhofer Institute for Communications, Information Processing and Ergonomics, the new security system employs a technology known as Passive Coherent Location (PCL), which harnesses the radio signals sent out by cell towers to pinpoint the location of ships entering a harbor. (PCL) works in much the same way as radar—sending signals that bounce off of objects and reading the signals that return to determine the objects’ locations.

Radar uses strong, directed waves that make it easy to find objects. In contrast, PCL uses the much weaker signals that are being bounced off of objects by cell towers. These bounced waves help a PCL system build a dynamic map of a port and traffic moving through it by looking at where cell signals come into contact with objects in the water. 

While this technique takes advantage of waves that are already being produced by cell towers and doesn’t require the installation of a new radar system, it also means the signals are more difficult to accurately interpret. “One challenge is that our sensor system tends to pick up the strong signals from the cell towers themselves,” Fraunhofer project manager Reda Zemmari said in a statement.  “The signal echoes reflected off the boats on the water are considerably weaker.” To make PCL useful, the Fraunhofer team had to write new algorithms that distinguish the echoes created by objects from the mélange of signal noise.

In other words, filtering out the strong signals emanating directly from cell towers lets the PCL system concentrate on finding the weaker signals that represent boats in the water. Improvements to the sensitivity of the new system have even allowed it to track craft as they move across the water. In tests of the PCL system, researchers were able to identify small speedboats from as far away as 4 kilometers. And all the equipment for operating a PCL system can fit in a trailer, making it feasible to install in remote locations and on a budget.

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