Aerospace and Electronic Systems Magazine August 2017 - 65

despreading metrics towards achieving the correct spoofing detection. This approach focuses on the differences between spoofing and
other interfering sources that can be used for discrimination.

INTERFERENCE SIGNALS

MULTIPATH

Interference signals considered herein include continuous wave
(CW) jammer, chirp jammer, wideband noise, multipath, and
spoofing. The signal model and description of these signals are
now provided.

NARROWBAND CONTINUOUS WAVE INTERFERENCE
This category of narrowband interference refers to a single sinusoidal tone within a GNSS frequency band. This type of interference
can be represented as
I CW ( t ) = A cos ( 2π f cwt + ϕ0 )

CHIRP INTERFERENCE
This category of interference consists of a sinusoidal waveform
whose frequency repeatedly sweeps across its bandwidth. Its mathematical representation is

(

)

(2)

where fchirp(t) is the instantaneous frequency of chirp signal. This
type of interference is the most common signal transmitted by low
cost personal privacy device (PPD) jammers. The frequency span
is commonly between 7 MHz to 60 MHz and the sweep time is
on the order of tens of μs [17], [18]. This type of interference can
be considered as a wideband interference since it sweeps a large
amount of the frequency band several times during a coherent integration period of a typical receiver.

WIDEBAND NOISE JAMMERS
The source of this type of interference transmits wideband noise
across the entire frequency band of the target GNSS system. The
AUGUST 2017

This type of interference occurs when a receiver antenna receives
signal via two or more paths either through reflection or diffraction. As the path distance travelled by non-line-of-sight (NLOS)
signals is larger than that of the line-of-sight (LOS) part, multipath components are always delayed relative to the LOS signal. A
composite signal formed from LOS and NLOS is received by the
user and the measurement generated is erroneous [19]. The NLOS
signals can either sum up constructively or destructively, causing
amplification or attenuation of the composite signal [23]. The multipath effect on the correlator outputs is very similar to that of a
matched power intermediate spoofing attack [1].

(1)

where A is the amplitude, fcw is the interference frequency, and t
is the time. φ0 is the initial phase of the interference signal. For
CW interference it is assumed that the signal frequency is time
invariant.

I chirp ( t ) = A cos 2π f chirp ( t ) t + ϕ0

wideband noise jamming signals cannot be discarded via temporal/
spectral mitigation approaches since the power content is divided
across all frequency components.

SPOOFING
Since the structure of civilian GNSS signals is public, a jammer
can generate a waveform with a structure similar to that of the
authentic signals. This type of interference could be very destructive since it can pass through the target receiver's correlator and
destructively affect its post correlation operations. The structural
interference signals may range from a randomly generated PRN
signal to more sophisticated meaconing and spoofing signals. For
the case of spoofing signals, the interference source generates multiple consistent PRN signals that lead to a fake navigation solution
[1]. Unlike other types of previously discussed jamming signals,
spoofing and meaconing signals do not deny the positioning capability of a receiver but they induce a fake position to the target receiver without the latter's knowledge. The spoofing signal scenario
and its effect on the correlation outputs depends on the spoofingauthentic signals' relative Doppler, delay, and amplitude values. In
the following, these metrics are investigated.

Relative Doppler
Based on the discussions provided in [24], [25], a spoofing attack
on tracking receivers in terms of their relative Doppler frequencies can be generally divided into two categories, namely locked
Doppler and consistent Doppler. In the locked Doppler mode, a
receiver based spoofer tries to align the Doppler frequency of the
fake signal with that of the authentic GNSS signal while their rela-

IEEE A&E SYSTEMS MAGAZINE

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