Wiktionary
sign on
n. (context broadcasting English) The time of day when a radio or television station begins broadcasting, usually after being off the air for several hours. vb. 1 To join something, after signing. 2 To commit oneself, as to a project, a goal, on organization, a cause. 3 (context broadcasting English) To begin broadcasting a radio or television signal, usually at the beginning of a broadcasting day and after being off the air for several hours. 4 (context idiomatic English) To log on; to start using a computer, radio, etc., or to start talking. 5 (context UK intransitive English) To receive unemployment benefits.