A temporal transponder is a device that can be used to emit tachyons.
In 1996, Henry Starling used a temporal transponder to lead the crew of the USS Voyager to believe that he was having the 29th century timeship Aeon transported to a remote location for launch, when he was in fact preparing to launch from the Chronowerx Industries building in Los Angeles. (VOY: "Future's End, Part II")