Live air traffic radar works by picking up on satellite transmissions of each aircraft's location. Transponders located on each aircraft transmit signals to the radar's receiver that then compiles the location data and makes it available for public consumption.
The technology that analysts use to collect and compile flight data is called automatic dependent surveillance-broadcast. Most aircraft contain an ADS-B transmitter that sends the aircraft's location to a satellite. The satellite's receiver collects the ADS-B-transmitted location information and feeds the data to the servers for public distribution.
Most commercial aircraft have ADS-B technology, and most non-commercial aircraft do not have the technology. The closer an aircraft is to a receiver, the lower the aircraft can fly and still transmit its location to the receiver. If the aircraft is far away from the receiver, it must fly high in the air in order to transmit its location to a receiver.
The Federal Aviation Administration also provides information that radar technologists compile to provide public flight location information. Because of FAA regulations, flight location data provided in this manner is slightly out-of-date. Analysts often use estimations based on a flight's known flight pattern to guess at a flight's location for up to one hour after a known transmission. This period of estimation is shorter if the aircraft has not published a known flight plan. Some aircraft or reporting agencies occasionally block specific location information for security or privacy reasons.