Aerospace and Electronic Systems Magazine January 2018 - 58

Student Research Highlights:

DOI. No. 10.1109/MAES.2018.170131

A Flexible VHF-Band Aeronautical Datalink Receiver
Based on Software Defined Radio
Baptiste Chamaillard, Maxime Lastera, Damien Roque, Université de Toulouse, France

MOTIVATION: 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 very high frequency (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, and assessing security and safety).

CONTEXT: ARCHITECTURE OF AN ACARS TRANSCEIVER
AND RELATED SDR PROJECTS

and ground stations. It relies on two main providers (i.e., Aeronautical Radio, Incorporated (ARINC) and Société Internationale de
Télécommunications Aéronautiques (SITA)) that offer an almost
worldwide 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 (Figure 1a): (i) an ACARS modem enabling
(bidirectional) conversion between text messages and analog signals in the voice band and (ii) a conventional analog voice transceiver to put/retrieve the signal on/from the desired airband channel. A key advantage of such a hardware architecture was to reuse
an already qualified and integrated airband voice transceiver.
In this work, hardware implementation in Figure 1a is replaced
by an SDR receiver linked to a generic computer (Figure 1b). Signal
processing functions are thus mainly implemented in the discretetime 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.
There are many SDR VHF ACARS receivers that have been
developed over the years. Some of them are subject to a proprietary license 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

ACARS is a popular datalink transmission system deployed since
the late eighties and used to send digital messages between airliners
Authors' addresses: B. Chamaillard and M. Lastera, The
Institut Supérieur de l'Aéronautique et de l'Espace (ISAESUPAERO), Université de Toulouse, 31055 Toulouse, France; D.
Roque, The Institut Supérieur de l'Aéronautique et de l'Espace
(ISAE-SUPAERO), Université de Toulouse, DEOS, 10, avenue
Edouard Belin BP 54032 31055 Toulouse, France, E-mail:
(damien.roque@isae-supaero.fr).
Manuscript received July 9, 2017, revised September 9, 2017,
and ready for publication November 14, 2017.
Review handled by W. Dou.
0885/8985/17/$26.00 © 2018 IEEE
58

Figure 1.

Flowgraphs of two ACARS receivers. Hardware (gray) and software
(white) components are emphasized.

IEEE A&E SYSTEMS MAGAZINE

JANUARY 2018

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