transponder
noun [ C ] electronics specialized uk/trænˈspɒn.dər/ us/trænˈspɑːn.dɚ/
（发射机）应答器，转发器
an electronic device that gives out a radio signal when it receives a similar signal telling it to
Aircraft are required by law to carry transponders, so that they can be identified. 按照法律规定，飞机必须携带应答器以便识别。