Radio scanners monitor specific radio frequencies, allowing users to listen in on police, fire and other emergency services. The simplest radio scanners tune to multiple frequencies, shifting between them until locating a broadcast. More advanced systems follow radio trunking systems that switch frequencies or even decode and receive digital communications. In some cases, users can follow emergency radio traffic from major cities using online scanners.
Few emergency services broadcast over the open airwaves, so hobbyists wanting to listen in often need trunking or digital scanners. A trunking system allows multiple agencies to share a small number of open frequencies, automatically switching communications to free channels as needed. Digital radios convert audio into a binary signal and transmit it over the airwaves, resulting in a stream of data unintelligible to the human ear. Modern scanners, when correctly programmed, can decode communications using these systems, allowing users to eavesdrop.
The legality of radio scanners varies from jurisdiction to jurisdiction. In many areas, listening to a police or emergency scanner is legal as long as the scanner does not attempt to break into encrypted traffic or monitor cell phone frequencies. Some municipalities restrict the use of a police scanner in an automobile, and using a scanner in furtherance of a crime may bring additional charges.