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. 

via Cell Towers Can Double As Cheap Radar Systems – IEEE Spectrum.

Using low level cell tower signals as radar — cool application of existing infrastructure!