As briefly mentioned in the previous chapter, this Master’s research project has been developed within the pale of collaborative research and development project AVIO-505. Therefore, its main objective is given by the definition of the AVIO-505 project, which aims at demonstrating through a proof-of-concept prototype the feasibility of Software Defined Avionics Radio (SDAR). This prototype must perform a functionality of each of the three main functions required by aviation, i.e. Communications, Navigation, and Surveillance. For the communications systems, a wideband radio system was selected in addition to satellite communications equipment. As regards the surveillance systems, the nineties-developed system transponder mode-S (TMS) and the NextGen keystone system named Automatic Dependent Surveillance Broadcast (ADS-B) were selected. Finally, for the last function, which is navigation, the selected system is the distance measuring equipment (DME). This project will integrate these three avionic systems into a single piece of hardware; contributing in this way to the simplification of the intricate avionics systems interconnection that exists nowadays. The following sections are devoted to briefly describing the particularities of each of the selected systems:

Distance Measuring Equipment 

The Distance Measuring Equipment is an avionics navigation equipment that measures the geometrical distance between the aircraft and a ground station (Collinson R. P., 2013). As the location of the ground station antenna is known, this information can be used in combination with other in order to determine the aircraft position. From one DME measurement, the aircraft can be located within a sphere around the ground station. Combined with another DME measurement, or VOR, the uncertainty can be reduced to a circumference (half a circumference when the VOR and DME stations are collocated, which is usually the case) normal to the ground plane. Thus, only the altitude is needed in order to determine the aircraft position. The airborne DME equipment operates very similarly to radar equipment. A pair of short pulses is omnidirectionally transmitted and received by the ground station. After introducing a known deterministic delay, the ground station retransmits this pair of pulses at a different frequency that determines the full-duplex channel. The ground station acts, thus, as a transponder. Finally, the reply from the ground is received at the aircraft and the measured is between the interrogation and the reply is measured, from which the distance or slant range is determined. As it can be seen, the DME is similar to a secondary radar, with two differences, the radar is not on the ground but airborne, since the antenna used is not a scanning one but omnidirectional, no angular information can be extracted from the measurement. The interrogation is transmitted at frequencies within the range 1025MHz to 1150MHz, while the replies can be received from 960 MHz to 1215 MHz. As it can be seen, both frequency overlay and proper channel frequency planning must be done in order to prevent self-interference. Another source of intra-system interference is produced by multiple aircraft interrogating the same ground station, which can only operate in one frequency channel. The medium access is done by collision using low duty-cycles and using a randomly generated interrogation pattern. Therefore, only replies following the same random pattern unique for every aircraft are identified as own. The use of different transmission schemes called mode X and mode Y doubles the number of channels. (LASSENA, 2013) (Spitzer, 2014) (Collinson R. , Introduction to Avionics Systems, 2011).

Automatic Dependent Surveillance – Broadcast 

The Automatic Dependent Surveillance – Broadcast is the keystone of the NextGen and SESAR programs at the USA and the European Union, respectively, both programs being harmonized by ICAO. ADS-B is the modernization of the Air Traffic Management (ATM) Surveillance system currently based on the ATCRBS and the Transponder Mode-S briefly described below (Collinson R. P., 2013). When both are installed in an aircraft, the TMS and ADS-B equipment must be part of a single piece of hardware equipment (RTCA Special Committee 170, 1992). Through ADS-B, all the air traffic participants (not only aircraft, but also ground vehicles, UAS, parachutes, obstacles, etc.) broadcast their 3D position, latitude, longitude and height to the rest of participants. The system is therefore “dependent” on an additional navigation system providing it with this 3D position. Usually, the navigation system used is GPS, which dramatically diminishes the uncertainty of the aircraft position, from hundreds of meters achieved by secondary radar-based traffic control to few meters provided by the GPS system, especially in those areas where an augmentation system such as WAAS (Wide Area Augmentation System) or EGNOS (European Geostationary Navigation Overlay Service) has been approved for its use in aviation. This will enable separation limits reduction between aircraft in safe conditions, which directly translates to an increased capacity (in flights per hour) of the air transportation system. The ADSB system operates at frequency 1090 MHz, (the same as TMS) which is why the systems need to be synchronized when collocated. Traffic control can therefore be automated, although such automation is not part of ADS-B. Currently, computers on the ground integrate all the information and provide controllers with timely warnings of potential problems. (LASSENA, 2013) In the USA, the ADS-B is also enabled to operate at the Universal Access Transceiver (UAT) frequency 978 MHz.

Transponder Mode-S 

The Mode-S (S for Select) of the secondary surveillance radar (SSR) was designed as an evolutionary addition to the Air Traffic Control Radar Beacon System (ATCRBS) to provide the enhanced surveillance and communication capability required for Air Traffic Control (ATC) automation; the spec was delivered to FAA in 1975 (Freeman, 1995) (Collinson R. P., 2013). However, the promised automation has not been yet achieved, which is one of the main motivations of ADS-B. A transponder Mode S performs all the functions of Mode A (squawk code only) and C (squawk, pressure and altitude) transponders and has data link capability. This capability is the one exploited by ADS-B for its operation, although the use of software defined avionic radios would have enabled much more flexible (and long term) solutions. Some of the TMS specific advantages over Modes A and C are :

• Since aircraft are selective interrogated, as opposed to Modes A and C, the channel is less saturated which increases the capacity of the system. However, it is expected that modern ADS-B can produce an increase in the 1090 MHz channel saturation again, going back to the times of the modes A and C transponders;
• The accuracy of TMS is improved when compared to Modes A and C, thanks to the use of monopulse techniques and increased bandwidth. However, ADS-B (when based on GPS) overpasses by far this improvement and makes TMS obsolete as a source of traffic positioning information;
• High degree of data integrity in ground-to-air, air-to-ground and air-to-air data link, as CRC codes are used with a probability of an undetected bit error lower than 10-7. (RTCA Special Committee 209, 2011);
• In TCAS equipped aircraft, the TCAS transmits coordination/interrogations to the other aircraft via the Mode S link in order to ensure the selection of complementary Resolution Advisories. (LASSENA, 2013).