The transponder data recorder: Implementation and first results

The secondary surveillance radar (SSR), that is an evolution of the military identification friend-or-foe systems, is widely used by air traffic control service providers to localize and identify co-operating aircraft equipped with a standard transponder [1]. The ground SSR installation transmits interrogations, from a rotating, narrow azimuthal beam antenna. The airborne transponders, once they have received an interrogation, transmit at a 1,090-MHz carrier a reply signal containing the requested data, i.e., identity (mode A reply) or fight level (mode C reply). Azimuth and range of the aircraft are measured by the interrogator, based on the delay of the reply and on the antenna pointing angle. The current SSR standard is based on the use of selective interrogations and replies and is called mode S: to reduce the interferences, the mode S protocol is based on a message format that includes the unique address of the aircraft. The airborne segment of the SSR is composed of the transponder and a pair of antennas on top and on the bottom of the fuselage. As the aircraft antennas are omnidirectional, many ground stations can receive the replies. This allowed the development of multilateration (MLAT) systems for the aircraft localization based on 1,090-MHz signals [2]. A typical MLAT system is composed of a distributed network of 1,090-MHz sensors, an interconnecting facility, and a central processor for the fusion of the sensor data. The data fusion relies on the estimation of the signals arrival time at the different stations and on hyperbolic localization. To identify the emitters, the processing is done with mode A replies or mode S replies that contain the emitter identity. A transponder with mode S capability can also transmit a particular downlink format message, called squitter, containing the aircraft's unique address and other information. The squitter signals are not elicited by the SSR interrogation, but they are spontaneously emitted at pseudoperiodical intervals. They are the basis of the automatic dependent surveillance-broadcast (ADS-B) concept [3]: the ADS-B OUT function periodically transmits information (identity, position, state, etc.) about the aircraft, and the ADS-B IN function receives the messages from nearby traffic. On the airborne side, the ADS-B OUT broadcasts data with onboard equipment using the squitter signals. The airborne ADS-B IN equipment provides to the pilot the traffic scenario, receiving the messages from ground and nearby aircraft ADS-B OUT. Moreover, the ADS-B IN can receive other ground services: the traffic information services-broadcast (TIS-B) and the fight information services-broadcast (FIS-B) [3]. Airport vehicles can be equipped with an ADS-B OUT, a simplified, nonflyable device transmitting 1,090-MHz squitter signals containing the identity and the position. As a matter of fact, there are various users of the 1,090-MHz channel: 1) SSR transponder replies (modes A, C, S); 2) ADS-B OUT messages; 3) MLAT systems with interrogation capability; and 4) TIS-B.