Signal Processing - September 2017 - 28

it to output counterfeit seamless valid PVT (through attacks
called meaconing and/or spoofing, which refers to the transmission of counterfeit GNSS-like signals, with the intent not
to disrupt the system operations, but rather to produce false
information in the victim receiver). Conceptual illustrations for
these two categories of attacks are shown in Figures 1 and 2.
A further example can be found in recent mobile gaming
applications that are making smartphone location spoofing
viral. A search for "GPS spoofing"-related videos on YouTube yielded around 2,000 results in June 2016 and more
than 70,000 results half a year later [6]. Beyond mass market
applications, civil GNSS vulnerabilities can be exploited with
hazardous consequences when GNSS is used for critical infrastructures, as power or communication networks [7].

GNSS
Constellation

Genuine
Signals

Jammer

Victim
Receiver

Overview of attacks against civil GNSSs

FIGURE 1. An illustrative example of a jamming attack.

GNSS
Constellation

Genuine
Signals

False
Signals

Victim
Receiver

Meaconing/
Spoofing Device

FIGURE 2. An illustrative example of a meaconing and/or spoofing attack.
28

GNSS positioning threats have been of intense interest to
the research community over the last decade, motivating the
increasing awareness on the GNSS vulnerabilities and the need
for suitable countermeasures (e.g., see [3]-[10]). A remarkable
number of solutions have been proposed in recent literature.
Special interest has been devoted to possible modifications to
civil GNSS signal structures to accommodate cryptographic
countermeasures [11]-[14].
In this context, the significance of this article is based on
the following contributions:
■ an up-to-date review of the civil authentication solutions
proposed for different GNSSs (starting with GPS and
Galileo) at both the data symbol and spreading code chip
level
■ an analysis of the feasibility and performance of civil
GNSS signal structure-based authentication approaches,
with a particular focus on spreading code authentication,
considered as a particularly relevant feature for the next
generation of GNSS signals
■ a practical case study with a high-level performance
assessment of a spreading code authentication solution, tailored to the evolution of the Galileo E1 Open Service (OS)
signal but generally applicable to other GNSS signals and
authentication approaches.
The article will also provide up-to-date information on
the steps that the Galileo program is undertaking toward the
introduction of an actual authentication provided through its
OS signals. Indeed, the Galileo program is making an effort to
gradually add authentication services to its first- and secondgeneration signals.

Intentional attacks to GNSS-based devices can target the
receiver (i.e., nonsignal attacks) by tampering with the information within the receiver or reported by it to end users. Several types of software attacks belonging to this category are
already known (e.g., data injection via the navigation message
or augmentation data [15]). On the other hand, the so-called
signal attacks work at the signal-in-space (SIS) level, directly
corrupting genuine radio-frequency (RF) signals [3], [4], [16].
Within signal attacks, jamming refers to the deliberate inband emission of electromagnetic radiations that reduces the
signal-to-noise level and induces the disruption of the receiver
functionalities (i.e., a denial of service). The term spoofing refers
to the transmission of counterfeit GNSS-like signals, with the
intent not to disrupt the system operations but rather to produce
false information in the victim receiver. A simpler type of attack,
referred to as meaconing, foresees the reception and rebroadcasting of a delayed signal, composed of the sum of the SIS from
different satellites. Moreover, modified or combined versions of
jamming, spoofing, and/or meaconing attacks are mentioned in
recent scientific literature (e.g., smart jamming [17]).
Spoofing-like attacks are generally more threatening than
jamming, simply because the receiver might not be able to
alert the user about the falseness of the provided information.
Though numerous countermeasures against jamming and

IEEE SIGNAL PROCESSING MAGAZINE

September 2017

Table of Contents for the Digital Edition of Signal Processing - September 2017