Transponder

Transponder codes identify the USS Phoenix and two Cardassian ships.

A transponder is a radio system that, upon receiving a designated signal, sends a radio signal of its own. The transponder signal identifies the source and may also send other information such as location, heading, local environmental data, etc.

In Star Trek

Star Trek vessels rely heavily on transponder signals to locate and identify other starships. In TNG "The Wounded", for example, Cardassian ships could not locate the Federation starship USS Phoenix -- which was destroying cargo ships in their territory -- until Captain Picard gave them the Phoenix's transponder code.