It's common for criminals in the know to regularly swap SIM cards in their cell phones or even change the handset's identification code to elude law enforcement. That may no longer be an effective method as the researchers at the Dresden University of Technology have discovered a way to track certain phones based on unique hardware characteristics.

Under real-world conditions in the lab, researchers said they were able to identify 13 different GSM phones based on unique qualities found in their radio signals. In fact, the team said they were able to correctly identify the phones 97.62 percent of the time - an impressively accurate metric.

We're told that the method relies on inaccuracies during the manufacturing process. These subtle differences in a phone's hardware are related to a collection of components including power amplifiers, oscillators and signal mixers which can introduce radio signal inaccuracies according to researcher Jakob Hasse.

What's more, the method works passively as it just listens to the ongoing transmissions from a phone. The bad news for the bad guys is that it can't be detected and the signals can't be altered after the fact.

It's worth pointing out that the tests were conducted using 2G phones. That's irrelevant really because Hasse said similar defects are also present on 3G and 4G handsets. It's a technically demanding process but it could be used to help law enforcement officials and forensic investigators alike track devices.