Repeater

It is an automatic amateur radio stations the receive on a frequency and re-transmit the same signal on a different frequency. Repeaters are usually located on mountain tops, towers, skyscrapers, etc. because their main purpose is to provide a wide area coverage to stations operating in VHF, UHF and up.
They are also useful when operating with hand-held, low power radios or when transmitting from a car. Transponders installed on satellites can be considered a kind of repeater.