ADS-B OUT and ADS-B IN are two basic concepts in ADS-B. The first thing we need to understand is that both ADS-B OUT and ADS-B IN refer to the ADS-B functions that aircrafts have.
ADS-B OUT, is to send out aircraft’s own ADS-B information, such as its own position, speed, flight ID… As a metaphor, ADS-B OUT can be the mouth of the aircraft, keeps shouting out its own location and other information.
ADS-B IN, is to receive ADS-B information from other aircrafts nearby, means the position, speed, flight ID… from other aircrafts nearby. As a metaphor, ADS-B IN can be the ears of the aircraft, keeps listening to the ADS-B information shouted out by other aircrafts nearby.
For example, aircraft A has the ADS-B OUT function, so aircraft A would constantly send out its own ADS-B information. Aircraft B has the ADS-B IN function, so aircraft B would constantly receive the ADS-B information sent by aircraft A. In this way, aircraft B can know the information of aircraft A. Let’s say, aircraft B can “see” aircraft A.
Well, we may say one more word. For ATC or ground-to-air surveillance, ADS-B OUT is enough. ADS-B OUT and ADS-B IN together are for air-to-air surveillance applications.