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Wang et al. Satell Navig (2021) 2:4 Page 2 of 9

is available, it is difcult for attackers to predict the SCA
Phase hopping
chips correctly. Te disadvantage is that the output of the modulation unit
correlator will be greatly attenuated as the proportion of
the SCA chips in the code sequence increases, resulting Input signal Phase Output signal
Tout(t)
shifter
Tin(t)
in the failure of acquisition and tracking, for receivers
do not participate in identity authentication (Pozzobon c(t)
2011). When the proportion of the inserted chips is Phase
hopping
Clock
small, the signal is vulnerable to multiple access interfer- reference sequence
ence. Te adjustment of time, position and scale of chip generator
insertion is not fexible. Fig. 1 Phase hopping modulation unit
In view of the above problems, this paper proposes a
binary phase hopping based SCA technique. Te pro-
posed technique avoids non-cooperative parties to obtain
information, and improves the signal confdentiality per- Phase hopping
formance. Phase hopping modulation can be in multi-ary, demodulation unit
and the proposed technique uses binary phase hopping. Input signal Phase Output signal
By adding pseudo-random phase hop into the civil signal, Rin(t) compensator Rout(t)
and correlating demodulation results with pseudo-ran- c(t)
dom code in the receiver, we can achieve identity authen-
Phase
tication. Tis technique can reduce the performance loss Synchronous hopping
of the original signal and the impact on the receivers, system sequence
which do not participate in authentication. Besides, it generator
has good compatibility with the existing receiver archi- Fig. 2 Phase hopping demodulation unit
tecture. Tis technique also has stronger anti-multiple
access interference ability and higher authentication suc-
cess rate. Moreover, it is more fexible because the trans- by a phase shifter, which makes the phase of the input
mitter can adjust the ratio of authentication component signal change with the hopping sequence. Tese two hop-
fexibly, and the receiver can also choose a fexible receiv- ping procedures are complementary, which is essential
ing mode. Tis SCA method provides a good technical for the signal synchronization.
solution for the design of modern GNSS signals.
Phase hopping modulation unit
Phase hopping modulation Te phase hopping modulation unit is shown in Fig. 1.
Phase hopping modulation is a new anti-interception Te phase hopping sequence generator generates N-
method for improving the security and reliability of a sys- ary pseudo-random sequence c(k) , which is used as the
tem. Its aim is to improve the security performance of a phase hopping sequence, and the corresponding phase
wireless communication system without increasing the ofset is
system bandwidth.
Phase hopping modulation is suitable for a variety of ϕ(k) = 2π c(k) (1)
signals, such as baseband signal, Radio Frequency (RF) N
signal, and carrier. Tis modulation can also be regarded Te output signal T out (t) is
as a secondary modulation after the basic modulation,
including Phase Shift Keying (PSK) modulation, Quad- T out (t) = T in (t)e jϕ(t) (2)
rature Amplitude Modulation (QAM), etc. Te phase jϕ(t)
hopping sequence generator generates a phase hop- where e is phase shift factor. Te relationship between
ping sequence to control the phase shifter, so the initial t and k is
phase of the input signal changes with the hopping of the k = ⌊t/T c ⌋ (3)
phase hopping sequence. Ten the output signal can be
processed according to diferent requirements and trans- where T c is the chip width of the phase hopping sequence.
mitted by the antenna. For the demodulation unit in the
receiver, the same phase sequence generator generates Phase hopping demodulation unit
the phase hopping sequence and controls the phase com- Te phase hopping demodulation unit is shown in Fig. 2.
pensator to compensate the signal phase so as to achieve Under the control of a synchronous system, the
demodulation. Te phase compensator is implemented phase hopping sequence generator generates the same