Aerospace and Electronic Systems Magazine January 2018 - 59

software 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.

CONTRIBUTION: AN EXTENSIBLE RECEIVER DEVELOPED
IN PYTHON OVER GNU RADIO
The aim of our contribution was twofold: (i) establish a discretetime equivalent model of the VHF ACARS optimal receiver and
(ii) develop an extensible prototype released as 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.

THEORETICAL STUDY OF A VHF ACARS SYSTEM
The VHF ACARS physical layer is pretty basic. At the transmitter
level, an MSK modulator turns the binary (differentially encoded)
data into an analog signal. The MSK modulation is a continuous-

Figure 2.

Detailed flowgraphs of the proposed ACARS receiver. We distinguish
reused hardware equipment (gray), reused and developed software
blocks (plain and thick, respectively).

JANUARY 2018

phase modulation with orthogonal elementary signals. Here, the
signaling set is composed of two sine waves at respectively 1,200
Hz and 2,400 Hz of duration T = 1/1,200 s. Then, a double sideband
amplitude modulator (with carrier) is used to center the former baseband signal around a carrier frequency f0 ∈ [129.125;136.9] MHz.
The architecture of the SDR ACARS receiver developed during the project is presented in Figure 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 (Figure 2a). From here, further processing steps are managed by software means within GNU Radio (Figure 2b). As a first step, the amplitude demodulation of the signal is
done to get the digital signal m [ k ] (envelope detector followed by a
high-pass filter). Then, a synchronization step detects the incoming
ACARS frame thanks to a preamble (prekey) found at the beginning of every message. Lastly, an MSK demodulation is performed
based on a filter bank to yield the received bits dˆ [i ] [6, chapter 6.3].

GNU RADIO ARCHITECTURE OF THE PROPOSED ACARS
RECEIVER
Figure 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 were 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 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 noncoherent 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 (Figure 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

IEEE A&E SYSTEMS MAGAZINE

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine January 2018