Zombie satellites are craft in orbit that stop functioning for many different reasons that can come back to minimal life for short or longer periods of time. LES1 launched in 1965 was lost and considered dead. It was found because some sort of short circuit was able to wake the beacon when it gets enough sun in it's tumbling orbit. Since it's not enough energy to fully function added to the abnormal orbit, it sends back a weak moaning signal. Sorry, this was my first ELI5. Edit: a word.

Doppler shift on a beacon transmitter.