Radio stations are required by the FCC to announce their legal call sign and community of license every hour to identify where the station is broadcasting from so the FCC can determine if the signal is legal or not.

1. Radio stations have to announce their identification every hour. They must announce a legal call sign
followed by the stations community of licence.
Some examples of identification include:
http://www.youtube.com/watch?v=aFAMDAgpEcg
http://www.youtube.com/watch?v=UfhjARL9_Mk&feature=related
It is important for radio stations in the United States to identify themselves every hour so that the
FCC can regulate where the signal is being broadcast from so that they can determine if its legal or
not.
Here are some other examples of some idents for various products and television channels.