Abstract: A portable detection system, including a receiver tuned to the frequency of signals transmitted by an aircraft transponder, will detect the presence of an aircraft in proximity to the system. The system has signal processing circuitry to transform the received signals to indicate the altitude of the detected aircraft. A display or alarm may alert the user to the presence of an aircraft. The system is useful to determine whether an aircraft may be engaged in a speed detection and ticketing operation or the closeness of other aircraft. The aircraft detection system may be used in conjunction with a radar detection system to provide a means by which various methods of traffic control can be detected.