transponders

n. device that automatically transmits when it receives a signal

In telecommunication, the term transponder (short-for Transmitter-responder and sometimes abbreviated to XPDR, XPNDR or TPDR) has the following meanings:An automatic device that receives, amplifies, and retransmits a signal on a different frequency (see also broadcast translator). An automatic device that transmits a predetermined message in response to a predefined received signal. A receiver-transmitter that will generate a reply signal upon proper electronic interrogation.
See more at Wikipedia.org...

Noun
1. electrical device designed to receive a specific signal and automatically transmit a specific reply
(hypernym) electrical device

RADARFARI. TRANSPONDITORI