Abstract: An automatic dependent surveillance (ADS) system for tracking aircraft includes a GPS navigation receiver for determining the position of an aircraft, a Mode S transponder for transmitting the position of the aircraft, and a Mode S receiving station for receiving the aircraft's position and for determining the position of the transmitting aircraft relative to other aircraft and the ground. The Mode S position information is broadcast as a squitter on a random time basis having a present average transmission rate, or in response to an interrogation signal from a Mode S receiving station. The Mode S receiver can be another aircraft, an en route ground station, and/or a terminal ground station. The en route ground station can include a multi-sector receiving antenna with each sector connected to a separate Mode S receiver. The terminal ground station can include an omni-directional antenna connected to a Mode S receiver.