Signal Processing - September 2017 - 34

techniques is still necessary to mitigate meaconing-type attacks
[10]. In this sense, a proper partition between system and receiver
contribution to the robustness against spoofing is a key aspect to
be considered in the design of new generations of civil GNSS signals and receivers.

Table 3 presents the aforementioned NMA, SCA, NME, and
SCE cryptographic defenses and summarizes their features, technical feasibility, and implementation impact, at both the receiver/
system. Notice that some references appear several times, as they
provide more than one implementation.

Feasibility and impact of civil authentication solutions

Case study: Authentication of Galileo E1 OS signals

When considering possible standalone authentication solutions, it
is important to not just focus on the benefits of future participant
users (i.e., those receivers capable to process the authentication
bits or chips), but also take into account the possible impact on
the existing satellites, ground segment, and other receivers (i.e.,
nonparticipant users).
Key aspects, such as the implementation complexity, cost, and
backward compatibility, have to be carefully assessed to ensure
the feasibility both at the system and user levels. As an example, focusing on the NMA and SCA families of authentication
approaches, their possible impacts on the civil GNSS receivers
can be summarized as follows:
■ NMA: limited complexity increase and negligible (or minimal) degradation of conventional performance indicators for
an existing receiver [13], and moderate complexity increase
in receivers when combined with antireplay defenses.
■ SCA: moderate additional complexity for the a posteriori verification of SCA snapshots and possible (limited) performance degradation for nonparticipant users [11].

A simple case study is presented in this section, providing a
performance analysis for Galileo E1 but generally applicable
to several SCA approaches. In detail, the following analysis is
limited to a practical example, particularized for the evolution
of the Galileo E1 OS signals, while a complete signal design is
considered out of the scope of this article.
This section analyzes the impact of replacing some portions of
the known spreading code sequences (E1-B or E1-C for Galileo)
with bursts of unpredictable chips for authentication purposes.
Under this assumption the following analysis covers two aspects:
1) the possible degradation of the performance for nonparticipant users
2) the evaluation of the performance of participant users in relation to specific parameters of the authentication technique.
Nonparticipant users are those without the knowledge of
the SCA bursts chips, and they will just use the baseline code
sequences for the received signal despreading. Depending on
whether or not they know the SCA burst timing, two options
have to be considered: nonparticipant receivers that discard the

Table 3. Civil GNSS signal cryptographic defenses.
Family

Technique

Feasibility

Impact

NMA

Digital
signatures

ECDSA, for GPS CNAV (L2C, L5) and Galileo [33]
ECDSA, for GPS CNAV (L2C, L5) [12]
ECDSA, with TESLA support, for GPS (L2C, L5) [37]
LDPC/RSA, for QZSS (L1/CA and L1 SAIF) [34]

H

L

One-way crypto chains

TESLA, for GPS CNAV (L2C, L5) and Galileo [33]
TESLA, for eLoran [41]
TESLA, for SBAS/ground-based augmentation system (GBAS) [42]
TESLA, single chain for all satellites [43]
TESLA, for Galileo E1 OS [14]
TESLA with DS support, for Galileo E1 OS [13]
TESLA for Galileo E6-B [31]
EMSS + DS [24], [44]

H

L

SCA

SSSC [11]
Hidden markers [19]
SAS (time-division/time hopping) [27]

M/H

M

NME

Generic NME [3]
Supersonic codes (SCE+NME+CSK) [32]

L/M

H

Standalone

Galileo E1 OS + E6 CS [14]
Galileo E6 CS [31]
Supersonic Codes (SCE+NME+CSK) [32]

M/H

H

Assisted

GPS P(Y) codes [25]
Generic remote authentication service [27]
Galileo E1 OS + E6 CS [28]
Galileo OS authentication based on PRS codes [29], [30]

H

M

SCE
(civil only)

H: high - M: medium - L: low.

34

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September 2017

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