A flexible VHF-band aeronautical datalink receiver based on software defined radio

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(13) https://oatao.univ-toulouse.fr/19661. http://doi.org/10.1109/MAES.2018.170131.

(14) . Chamaillard, Baptiste and Lastera, Maxime and Roque, Damien A flexible VHF-band aeronautical datalink receiver based on software defined radio. (2018) IEEE Aerospace and Electronic Systems Magazine, vol. 33 (n° 1). pp. 58-61. ISSN 0885-8985. .

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(24) 1. A Flexible VHF-band Aeronautical Datalink Receiver Based on Software Defined Radio Baptiste Chamaillard, Maxime Lastera, and Damien Roque, Member, IEEE (Student Project Highlight) Abstract—In this paper, we show how software defined radio can be used to decode aeronautical datalink communication such as VHF-band aircraft communication addressing and reporting system (ACARS). An academic analysis of the physical and medium access layers enables the specification of a complete discrete-time oriented receiver. The latter receiver is implemented with the help of GNU Radio and released as a free software. A flexible software architecture makes the project suitable for various applications such as teaching, integration testing, safety and security assessment. Index Terms—Aeronautical operational control, Datalink, ACARS, Software defined radio, GNU Radio.. I. M OTIVATION : IN - DEPTH ANALYSIS OF ACARS AND SOFTWARE DEFINED RADIO TECHNIQUES. The primary objective of the project was to acquaint ourselves with software defined radio (SDR) techniques to put into action some of the concepts covered during digital communications courses. SDR offers an easy way to design and implement flexible radiofrequency (RF) transceiver architectures using mainly software developments [1]. Thus, it allows to get promptly familiar with classic issues to overcome when designing receivers (RF electronics impairments, asynchronously-sampled signals...) without getting restricted by material constraints traditionally brought by full-hardware implementations (i.e., cost of hardware components to be soldered together, need of various development and testing instruments...). On the other hand, the aerospace-oriented nature of our graduate program innately oriented the project towards the study of an aeronautical datalink namely the aircraft communication addressing and reporting system (ACARS) over VHF link. In particular, the physical and access layers have been studied to decode VHF ACARS frames: it includes the design of an optimal minimum-shift keying (MSK) modulation receiver in presence of synchronization impairments along with a flexible frame parser. Besides many attractive educational features experienced during the project, SDR techniques may also enhance aeronautical datalinks engineering (e.g., prototyping new waveforms, performing integration testing, assessing security and safety). B. Chamaillard and M. Lastera are students from the Institut Supérieur de l’Aéronautique et de l’Espace (ISAE-SUPAERO), Université de Toulouse, 31055 Toulouse, FRANCE. E-mail: baptiste.chamaillard@student.isaesupaero.fr. D. Roque was the advisor of the project described in this paper. He is with the Institut Supérieur de l’Aéronautique et de l’Espace (ISAE-SUPAERO), Université de Toulouse, 31055 Toulouse, FRANCE.. II. C ONTEXT: ARCHITECTURE OF AN ACARS TRANSCEIVER AND RELATED SDR PROJECTS ACARS is a popular datalink transmission system deployed since the late eighties and used to send digital messages between airliners and ground stations. It relies on two main providers (i.e., ARINC and SITA) that offer an almost worlwide coverage. It allows aircrafts to be linked up with the ground via HF, VHF or satellite airbands. In this context, our project is bounded to the design of an ACARS receiver in VHF band. VHF ACARS transceivers are usually made of two main hardware modules (Fig. 1a): (i) an ACARS modem enabling (bidirectional) conversion between text messages and analog signals in the voice band (ii) a conventional analog voice transceiver to put/retrieve the signal on/from the desired airband channel. A key advantage of such an hardware architecture was to reuse an already qualified and integrated airband voice transceiver. In this work, hardware implementation in Fig. 1a is replaced by an SDR receiver linked to a generic computer (Fig. 1b). Signal processing functions are thus mainly implemented in the discrete-time domain through high-level programming languages. Due to the generic nature of the SDR receiver (i.e., large frequency range and wideband sampling), other waveforms may be deployed through software developments and potentially executed simultaneously. rRF (t). Analog voice receiver (ampl. demod.). m(t) ˜. ACARS hardware decoder (sampling, MSK demod., synch.). ˆ d[i]. (a) Traditional implementation with two hardware building blocks. rRF (t). SDR receiver (freq. translation, sampling). r[k]. ACARS software decoder (ampl. demod, MSK demod, synch....). ˆ d[i]. (b) SDR implementation involving mainly software components. Fig. 1. Flowgraphs of two ACARS receivers. Hardware (gray) and software (white) components are emphasized.. There are many SDR VHF ACARS receivers that have been developed over the years. Some of them are subject to a proprietary licence but others are open source projects. For instance, one can mention the GNU Radio open source projects ”gr-acars” and ”gr-acars2” developed by J.-M Friedt and A. Neuenschwander, respectively [2], [3]. For ”gr-acars”, even if the receiver is capable of decoding on-the-air ACARS frames, it seems to miss some of them and the code deposit appears to be rather difficult to take in hand. As for ”gr-acars2” although properly coded and documented the project seems unfinished for now. The ”acarsd” software is a free software.

(25) 2. available since 2004 [4]. It offers a lot of interesting features with a complete graphical user interface (GUI). However, it does not allow the community to get access to its source code or to make contributions.. A/D B LNA. rRF (t). f0. r[k]. f0 ± B/2. A. Theoretical study of a VHF ACARS system The VHF ACARS physical layer is pretty basic. At the transmitter level, a MSK modulator turns the binary (differentially encoded) data into an analog signal. The MSK modulation is a continuous-phase modulation with orthogonal elementary signals. Here, the signaling set is composed of two sine waves at respectively 1200 Hz and 2400 Hz of duration T = 1/1200 s. Then, a double sideband amplitude modulator (with carrier) is used to center the former baseband around a carrier frequency f0 ∈ [129.125; 136.9] MHz. The architecture of the SDR ACARS receiver developed during the project is presented in Fig. 2. We consider a received RF signal rRF (t) centered around f0 which is roughly the transmitted signal plus an additive white Gaussian noise. After a frequency down-conversion and an analog-to-digital conversion, a complex envelope r[k] is obtained (Fig. 2a). From here, further processing steps are managed by software means within GNU Radio (Fig. 2b). As a first step, the amplitude demodulation of the signal is done to get the digital signal m[k] ˜ (envelope detector followed by an high-pass filter). Then, a synchronization step detects the incoming ACARS frame thanks to a preamble (pre-key) found at the beginning of every message. Lastly, a MSK demodulation is performed ˆ [6, Chapter based on a filter bank to yield the received bits d[i] 6.3].. j. π 2. III. C ONTRIBUTION : AN EXTENSIBLE RECEIVER DEVELOPED IN P YTHON OVER GNU R ADIO The aim of our contribution was twofold: (i) establish a discrete-time equivalent model of the VHF ACARS optimal receiver (ii) develop an extensible prototype released as a free software. We chose to implement our SDR ACARS receiver with the help of GNU Radio [5]. This open source development toolkit provides signal processing functions intended for (pseudo) real-time execution with SDR applications in mind. Its support of various low-cost SDR transceivers and its extensibility through Python/C++ modules makes it particularly suited for our project.. 1/Ts. A/D B. 1/Ts. (a) Simplified flowgraph of the SDR hardware module. r[k]. m[k] ˜. |·|. Frame synch.. MSK demod.. ˆ d[i]. →0. Frame parser and GUI. (b) Simplified flowgraph of the proposed digital ACARS receiver Fig. 2. Detailed flowgraphs of the proposed ACARS receiver. We distinguish reused hardware equipment (gray), reused and developed software blocks (plain and thick, respectively).. with a cut-off frequency of 5 kHz discards the out-of-band noise. A Polyphase Arbitrary Resampler block is used to yield an integer number T /Ts of samples-per-symbol. Then, a non-coherent amplitude demodulation is done by the ComplexToMag and DC Blocker blocks. At the end, PyQT Text Output blocks display two consoles: (i) raw received messages and (ii) interpreted and enriched messages (Fig. 4). Along with legacy GNU Radio components described so far, we developed the two following blocks in Python. 1) ACARSdecoder block: An oversampled baseband signal should be provided at its input with an integer number of samples per symbol. The role of this block is to perform ACARS frames decoding: frame and symbol synchronization, parity check control, bits to character conversion. The parameters are the sampling frequency and a threshold used to detect the beginning of the ACARS frames. The output is an asynchronous character stream. 2) ACARSparser block: Asynchronous messages from ACARSdecoder should be provided at the input of this block. Its purpose is to make ACARS frames human readable. The parameters are the request mode and the international air transport association (IATA) API Key. If the request mode is set to ”True” then the parser will perform some Web requests to get extra information about the detected airplanes. The second parameter, if set, queries the IATA database. The architecture of the parser is based on the ARINC 618/620 standards [7], [8]. C. Integration within the GNU Radio community. B. GNU Radio architecture of the proposed ACARS receiver Fig. 3 shows the ACARS receiver flowgraph as implemented with the help of GNU Radio Companion. In the following, we provide a blockwise description with a particular emphasis on ACARSdecoder and ACARSparser which have been developed from scratch during this project. The UHD USRP Source block serves as an input from/to the SDR receiver’s driver (here, a National Instrument USRP 2920). It could be replaced by any other supported driver. A center frequency f0 = 131.215 MHz and a sampling frequency of 500 kHz are configured. The Low Pass Filter block. Our ACARS receiver has been named “gr-supacars”. The source code is available at https://github.com/mmmaaaxxx/ SUPACARS.git. Note that ACARSparser block may be easily extended to account for new labels. Contributors are welcome. ACKNOWLEDGEMENTS The authors would like to thank Dr Michel Villemur for his valuable help to understand ACARS messages. This work has been supported by RALF, the software defined radio platform of the University of Toulouse (see http://ralf.isae.fr)..

(26) 3. Fig. 3. GNU Radio Companion (GRC) view of the proposed receiver.. Maxime Lastera was born in Toulouse, France, in July 1984. He received the Ph.D. degree from the University of Toulouse in 2012 and the Engineer degree in computer science from the Institut des Sciences et Techniques de l’Ingénieur d’Angers (ISTIA) in 2009. From 2009 to 2012, he worked at the Laboratoire d’Analyse et d’Architecture des Systèmes (LAAS), Toulouse, France. From 2013 to 2016, he worked at Altran Research. From 2016 to 2017 he is working towards the advanced master Télécommunications et Réseaux pour l’Aéronautique et l’espace (TERA) at the Institut Supérieur de l’Aéronautique et de l’Espace (ISAE-SUPAERO), University of Toulouse, France.. Fig. 4. Graphical user interface of the proposed receiver: raw and parsed messages are displayed on the left and right columns, respectively.. IV. B IOGRAPHIES. Damien Roque was born in Chambery, France, in July 1986. He received the Ph.D. degree from the University of Grenoble in 2012 and the Engineer ´ degree in telecommunications from the Ecole Nationale Supérieure des Télécommunications (ENST) de Bretagne in 2009. From 2009 to 2013, he worked at the Grenoble Images Paroles Signal Automatique (GIPSA-lab), Grenoble, France. Since 2013, he is an associate professor at the Institut Supérieur de l’Aéronautique et de l’Espace (ISAE-SUPAERO), University of Toulouse, France. His research interests are in the area of interference mitigation techniques for high throughput satellites, faster-than-Nyquist signaling, multicarrier modulations.. R EFERENCES. Baptiste Chamaillard was born in Grenoble, France, in May 1993. He received the Engineer degree in telecommunications from the ´ Ecole Nationale Supérieure de l’Energie, l’Eau et l’Environnement (ENSE3) in 2016. Concerned by the aerospace telecommunication challenges, he is working towards the advanced master Télécommunications et Réseaux pour l’Aéronautique et l’espace (TERA) at the Institut Supérieur de l’Aéronautique et de l’Espace (ISAE-SUPAERO) in 2017. During his studies, he had the chance to gain ´ work experience in a research lab at the Commissariat a` l’Energie Atomique et aux e´ nergies alternatives (CEA) and in a R&D department of one of the main Europe’s satellite manufacturers Thales Alenia Space.. [1] C. Laufer and E. Hoffman, The Hobbyist’s Guide to the RTL-SDR: Really Cheap Software Defined Radio. CreateSpace Independent Publishing Platform, 2015. [2] J. Friedt, “gr-acars project website.” [Online]. Available: https: //sourceforge.net/projects/gr-acars/ [3] A. Neuenschwander, “gr-acars project website.” [Online]. Available: https://github.com/antoinet/gr-acars2 [4] KjM, “acarsd project website.” [Online]. Available: http://www.acarsd.org [5] E. Blossom, “GNU radio: Tools for exploring the radio frequency spectrum,” Linux J., vol. 2004, no. 122, pp. 4–, June 2004. [Online]. Available: http://dl.acm.org/citation.cfm?id=993247.993251 [6] J. Barry, E. Lee, and D. Messerschmitt, Digital Communication, S. US, Ed. Springer US, 2004. [7] ARINC, “Arinc 618 - air/ground character-oriented protocol specification,” Jun 2013. [8] ——, “Arinc 620 - datalink ground system standard and interface specification,” Jan 2014..

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