Aerospace and Electronic Systems Magazine August 2017 - 70

An Approach to Detect GNSS Spoofing

Figure 3.

Figure 4.

shows a clean data set spectrum as benchmark in a 25 MHz
bandwidth. The GPS L1 C/A signal main beam occupying about
2 MHz of bandwidth is observable in the figure. The centre of
the main GPS signal spectrum is located at 12.5 MHz. Figure
2(b) shows the GNSS signal spectrum in the presence of the CW
interference. Only a small sector of the spectrum is affected by
the interference. Figure 2(c) shows the signal affected by the
chirp interference. This type of interference is affecting almost
12 MHz of the signal spectrum. Figure 2(d) shows the signal
spectrum in the presence of the wideband noise. The entire receiver bandwidth is affected by this jammer. Figure 2(e) and
Figure 2(f) show the signal spectrum affected by spoofing and
multipath, respectively. In the spoofing case, the energy of the
signal is increased due to the presence of several higher power
spoofing signals.
The extra bump in the power spectral density in the spoofing
case is the spoofing image frequency in the up-conversion process
of DS3. This image frequency does not affect the spoofing simulation scenario or detection performance.
As can be seen in Figure 2, power spectral density analyses can
be used to detect and classify the interfering signals. This metric is
probably more practical for high power jamming signal detection
and classification compared with the matched power spoofing and
multipath interference.
Figure 3 shows the mean value of SPCA metric outputs of
various interference signals normalized to that of the clean data
set. As discussed, SPCA can detect a cyclo-stationary signal in the
band, which in this case consists of chirp and spoofing signals. The
SPCA metric values in the case of CW and noise are lower than
the clean data set. This is due to the fact that the presence of these
jammers has increased the noise power and consequently affects
the total GNSS signal power-to-noise ratios. The SPCA metric values in the case of multipath is slightly higher than that of the clean

data set due to the existence of multipath; however it is still below
the detection threshold (details of detection threshold calculation
is provided in [7]). The SPCA metric outputs in the presence of
the spoofing attack are much higher than those of the clean data
set. The SPCA metric outputs in the case of chirp jammer also
exceeds the detection threshold. Hence, if the SPCA metric detects
a signal above the threshold, it can be due to both spoofing or chirp
interference sources. This is in agreement with the decision logic
provided in Table 2.
Figure 4 shows the mean values of IF sample variances for
different data sets. These values are normalized to the variance
of the clean data set. IF sample variance values of CW, chirp,
and wideband noise jammers are all the same (since JNR for
these three cases were the same) and about six times that of the
clean data set. The multipath sample variance is not affected by
multipath propagation and it is at the same level as the clean data
set. The spoofing signal variance metric values are enhanced due
to the existence of additional signal power in the band. This is
the main differentiation factor between spoofing and multipath
signals as provided in Table 2. The IF samples variance values
as a function of time for clean and spoofing data are also shown
in Figure 1.
Figure 5 shows C/N0 metric outputs for CW, chirp, and
wideband noise jammers in the case of PRN 3. The CW jammer
causes up to 20 dB attenuation on C/N0 values. The variation in
C/N0 values in the case of the CW jammer is due to the variable
jammer frequency during the attack. The CW jammer distortion
on GNSS signals depends on the relative jammer and Doppler
frequencies of each PRN [20]. The CW jammers are most effective when they overlap spectral lines of GNSS signals. Chirp
and wideband noise jammers also decreased the C/N0 values by
about 8 dB. Although the mean C/N0 values in the case of noise
and chirp jammers are almost the same, the chirp jammer affects

Normalized IF sample variances.

SPCA test statistics.

IEEE A&E SYSTEMS MAGAZINE

AUGUST 2017

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine August 2017