Researchers at the Dresden University of Technology have developed a surprisingly reliable tracking method that can remotely identify individual GSM phones based on unique characteristics in their radio signals, even if the phone's identification codes or SIM cards have been swapped. In lab tests using "real-world conditions," the researchers were able to identify 13 different mobile phones 97.62 percent of the time. That's an impressive initial result, and the team behind the work says that it's just the first step in this type of remote mobile forensics.

"It cannot be detected."

The method relies on the "inaccuracies in the manufacturing process" that create variations in a phone's electronics. As researcher Jakob Hasse tells New Scientist, "the radio hardware in a cellphone consists of a collection of components like power amplifiers, oscillators, and signal mixers that can all introduce radio signal inaccuracies." Hasse says that the method "works completely passively and just listens to the ongoing transmissions of a mobile phone — it cannot be detected."

While the tests were performed on 2G phones, Hasse says that every phone has similar defects and that 3G and 4G phones are also vulnerable to identification. New Scientist reports that the method is technically demanding, but that it could help forensic investigators and law enforcement to track devices without relying on SIM identification and other characteristics that can be changed or spoofed.