IEEE - Aerospace and Electronic Systems - March 2021 - 37

Feng et al.

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PRN codes, the amplitude term of the steering vector has
been estimated. This method has a low computational complexity and can be employed as a simple additional block
before a conventional GPS receiver. Nevertheless, this
method may not work well when the spoofing power is low
as the phase term of the steering vector cannot be accurately
estimated in such a case. Besides, the assumption of
unchanged gains for different incoming signal directions may
not be well satisfied in practice, and the integration for amplitude estimation may not accumulate coherently based on the
periodicity of PRN when the navigation data bit changes,
resulting in inaccurate amplitude estimation of the steering
vector. On the contrary, the postcorrelation spatial processing
methods, such as the one proposed in [18], can avoid these
problems as they can work effectively in both low and high
spoofing power cases and no antenna gain assumption or
PRN periodicity has been used to estimate the steering vector. However, compared to the precorrelation method, the
computational load of the postcorrelation methods is much
increased and some modifications of the acquisition/tracking
procedure are normally needed for a GPS receiver.
In this article, we propose an alternative spatial processing method to detect and suppress spoofing signals, where
we also assume a single spoofing emitter since keeping synchronization of multiple spoofing sources, which is beyond
the capability of a basic spoofing attack. First, the codeless
decoding approach [21] squares the received signal to get rid
of the binary phase-shift keying (BPSK) of GPS L1 C/A (or
L2C although only the former will be tackled here) PRN
codes and navigation messages, and the Doppler frequencies
of the squared authentic and spoofing signals of different
antennas (to be simplified, two antennas are used in this article) are estimated by fast Fourier transform. Second, all the
frequency peaks identified above the noise floor are extracted
to measure their phase differences between two antennas.
The phase differences are used to detect the spoofing phenomenon and to classify the frequency peaks into two groups
to distinguish the authentic and spoofing signals for spoofing
MARCH 2021

suppression purpose. At last, with the help of the method
proposed in [17] or a stochastic gradient descent (SGD)based iterative least squares (LS) algorithm [22], which is
also applicable to strong spoofing and jamming suppression,
the steering vector of spoofing signals is formed and the
orthogonal-projection-based beamforming technique [17] is
used to suppress spoofing signals and restore authentic signals. The proposed method makes a tradeoff between existing precorrelation and postcorrelation methods: It works
well in low and high spoofing power cases; the gains of the
uncalibrated antennas are assumed to be different for different incoming signal directions; the periodicity of PRN codes
is not necessary to be used; and its performance can be
improved in low-SNR situations by increasing the integration time. It is computationally efficient and can be achieved
by a compact, low-cost microcontroller before the GPS
receiver without running the full acquisition procedure.
The outline of the article is as follows: In " GPS Spoofing Demonstration " section, we demonstrate the ability to
spoof GPS using commercial, off-the-shelf (COTS) SDR
hardware fitted with a sufficiently stable quartz oscillator
to properly mimic medium-term (0.01-1 000 s) stability of
atomic clocks. In " GPS Spoofing Detection " section, we
detect spoofing by using a codeless decoding approach to
analyze the phase of the signals collected by a two-antenna
array. In " GPS Spoofing Suppression " section, we demonstrate the suppression of the spoofing signal to allow for
recovering the authentic signals by steering a null toward
the spoofing source. Finally, in " Experiment Results " section, we show various experiment results to illustrate GPS
spoofing suppression performance of the presented methods and their extension to jamming cancellation.

GPS SPOOFING DEMONSTRATION
This section focuses on demonstrating the ability to spoof a
GPS receiver using commonly available hardware. The

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