Aerospace and Electronic Systems Magazine August 2017 - 71

Broumandan, Siddakatte, and Lachapelle

Figure 6.

SQM values of GPS PRN 3 in the presence of jamming signals.

Figure 5.

C/N0 values of GPS PRN 3 in the presence of jamming signals.

the carrier tracking performance more than that of the noise jammer and causes fluctuations in signal tracking and consequently
in C/N0 values. Comparing the three jammers' distortion for the
given jamming scenario, chirp and noise jammers' effects on C/
N0 values are a function of the received jammer power. The chirp
jammers cause more fluctuations on C/N0 values compared with
the noise and CW jammers. The CW jammer's effect on the C/
N0 metric is a function of received signal power and CW carrier
frequency and therefore it causes a slow variation on C/N0 values in the given example.
Figure 6 shows the absolute values of the SQM metric outputs for jamming signals. The green plots are SQM detection
thresholds that are three times the theoretical values of the SQM
metric standard deviations provided in (6). These thresholds satisfy the probability of false alarm of 0.003 in the presence of
a clean data set. The SQM metric variance is a function of C/
N0 and that is why it is varying during the test. The C/N0 was
calculated based on the computation of the narrow to wide band
power ratio [16] and smoothed over 1 s. CW and wideband noise
jammers do not significantly affect the SQM metric outputs.
However, SQM metric outputs in the case of chirp interference
are significantly affected and in some cases pass the detection
threshold. Among jamming signals considered herein, the chirp
interference has the highest effect on the SQM metric. This is
due to the fact that chirp interference disturbs carrier tracking
performance and causes fluctuations in the code and carrier
tracking loops.
The standard deviation of the SQM metric outputs is higher
than that of the other jamming sources. The reason for this is the
structure of the SQM metric used as it is a single difference metric that is sensitive to code tracking errors, which can be induced
AUGUST 2017

Figure 7.

PRN 3 C/N0 values and SQM metric for multipath1 scenario.

by multipath or other distortions affecting tracking performance.
As discussed before, chirp jammer affects tracking performance
and the SQM metric variance is increased. To avoid false spoofing
detection utilizing SQM metrics, the detection threshold should be
adjusted in the presence of chirp interference.
Figure 7 shows SQM outputs for short- and long-range multipath corresponding to the Multipath1 and Multipath2 scenarios
described in Table 3.
During the test scenario, the distance between LOS and multipath signals was gradually increased by 60 cm. The SQM monitoring correlators were located at 0.2 chips from the prompt correlator. Considering the SQM results of Figure 7(a), the SQM metric
can only detect the multipath signal when it is out of phase with
respect to the LOS signal at t = 25 s and t = 45 s for short multipath

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