A law enforcement scanner, also called a scanner or police scanner, is a radio receiver designed to let people listen to the radio transmissions from various types of law enforcement agencies and emergency services. Some types of scanners are pre-programmed with the frequencies and channels commonly used by law enforcement and emergency services, notes RadioShack.com.
The scanners come in multiple forms including handheld, mobile and desktop. There are also applications available that can be downloaded to phones that give users live feeds. Most scanners are set up to pick up channels that have activity, so when the scanner picks up activity within the given mile radius it stops on that channel. The radio stays on this channel until the transmission stops and automatically moves onto the next channel that has activity, notes RadioShack.com.
While the scanners can change stations on their own, most of them have manual controls in addition. That means users can change between the channels as they want instead of being stuck on one until that channel stops broadcasting. Users of these scanners can listen to transmissions from police, ambulances, firefighters and even government agencies and private companies. Users of scanners cannot intentionally try to pick up telephone conversations or encrypted messages; this may happen accidentally on occasion, but anyone caught listening in may face fines and other legal actions.