ceiving this interrogation, the aircraft returns the appropriate reply. Mode S is in fact a digital data link that lets the interrogator choose among a multitude of possible replies in addition to determining the aircraft’s identity and altitude. The interrogations are in what is called uplink format (UF) and are transmitted on the 1.03-GHz frequency, while the replies are in downlink format (DF) and use 1.09 GHz. Figure 1 shows the operating principle.
Automatic Dependent Surveillance – Broadcast (ADS-B)
Automatic Dependent Surveillance – Broadcast (ADS-B) is a system that is part of the U.S. Federal Aviation Administration (FAA) NextGen initiative to improve radar surveillance and will become mandatory as of January 2020. ADS-B is used mainly by aircraft to periodically report their positions, which they determine by means of Global Navigation Satellite Systems (GNSS). ADS‑B will eliminate the need for ground stations such as Air Traffic Control Radar Beacon Systems (ATCRBS) and thus allow new commercial routes to be opened up . Figure 2 shows how ADS-B works.
Distance Measuring Equipment (DME)
Distance Measuring Equipment (DME) is a system that measures the distance between a ground beacon and an aircraft. The aircraft transmits an interrogation composed of pulses spaced a specific distance apart. With a predetermined delay after receiving the interrogation, the ground station transmits a reply on a different frequency. The on-board DME equipment receives this reply and, by calculating the time between the interrogation and the reply, determines the distance (the “slant range”). Figure 3 shows how DME works.
Software-defined radio systems
Software-defined radio systems are systems that can be used to transmit and/or receive radio signals. But in contrast to the functions of dedicated radio equipment, those of software-defined radio systems are programmed in software written by the user. Thus the same radio system can be used to perform multiple tasks, Moreover, it can be reprogrammed at any time. For this project, two software-defined radio systems from Ettus Research were used: the USRP E110 and the USRP N210. Table 1 shows the features of these two systems.
Project phases and design
The design of the Mode S transponder and the ADS-B were separated into several phases. This strategy has several advantages. First of all, it simplifies the development process by allowing us to develop and test only one part of the system at a time, which also makes the debugging phase easier. Second, it makes the system far more portable, because if there is a change in platform, only a few steps have to be redone. Third, it simplifies optimization by making it easy to isolate those processes that are demanding too many resources. Lastly, this architecture meets an industry need: the latest avionic architectures are highly modular and provide greater flexibility by allowing the processes to be separated into different pieces of equipment and by sharing resources with several technologies.
We were thus able to integrate the DME and ADS-B/Out functions into the USRP E110, along with an interface that let us test the system with a flight simulator. However, we developed the systems on the radio’s Cortex-A8 processor, which has such limited resources that we could not get the DME and the ADS-B to work in parallel. To overcome this problem, we will have to begin developing on a field-programmable gate array (FPGA), which is far faster and offers more resources. Also, the software that we develop in future will run on new software-defined radio systems from NUTAQ.
All of these projects mesh with the development plans of several major players in the aviation industry, such as Bombardier and Marinvent.
William Coady has completed a Bachelor, and a Master degree in Electrical Engineering at ÉTS, specializing in embedded systems for aerospace. He explore various fields, such as software-defined radio development at ÉTS LASSENA laboratory.
Program : Electrical Engineering
Research laboratories : LASSENA – Laboratory of Space Technologies, Embedde