Typically, air traffic controllers use a system referred to as secondary radar, which sends a signal to a plane’s transponder and then receives a reply that identifies the flight and its coordinates.
Secondary radar is limited by line of sight, meaning that if a plane traveled behind a mountain the signal would not be received, so it is affected by the curvature of the planet. The signals transmitted by radar are also dispersed by the Earth’s atmosphere, so they get weaker the farther a plane travels. And, eventually, the signal may be lost all together.
Another form of tracking is primary radar. This technology hasn’t evolved much since its creation in the 1930s and tracks objects by sending out a signal, which then reflects off an object, and then bounces back.
Through primary tracking, every object is trackable whether it wants to be or not. However, this technology has the same limitations because of line of sight and atmospheric factors.