Interference, Multipath, and Scintillation - Understanding GPS: Principles and Applications - page 244

6.2.1 Types and Sources of RF Interference
Table 6.1 summarizes various types and potential sources of RF interference. Inter-
ference is normally classified as either wideband or narrowband , depending on
whether its bandwidth is large or small relative to the bandwidth of the desired
GNSS signal. Note that what might be considered wideband interference to the GPS
L1 C/A code or L2C might be narrowband to P(Y) code, M code, or L5. The ulti-
mate limit in narrowband interference is a signal consisting of a single tone, referred
to as a continuous wave (CW). 1 The RF interference may be unintentional or inten-
tional (jamming). There is a certain level of interference among signals of the same
type, where signals from different satellites within one system interfere with one
another's reception. Such interference is referred to as self-interference or
intrasystem interference . Interference between two satellite navigation systems such
as between GPS and GALILEO signals is referred to as intersystem interference .If
pseudolites are used, operation at close range to these ground transmitters will
almost certainly result in interference to the satellite signals, although the effects of
such interference can be reduced through use of burst (pulse) techniques by the
pseudolites to reduce the duty cycle. In fact, an efficient wideband jamming tech-
nique uses a waveform based on the same modulation, at the same carrier frequency,
to form matched spectrum interference. If the intent of the intentional transmission
is to not just disrupt GNSS operation, but rather to produce a false position within
the victim receiver through the broadcast of false GNSS signals, the transmission is
referred to as spoofing . As a benign example of spoofing, when a GPS receiver is
connected to a GPS satellite signal simulator for testing, that receiver under test is
being spoofed.

6.2.1.1 Jamming and Spoofing
Intentional jamming and spoofing must be anticipated in the design of military
receivers. Hence, all classes of in-band jammers, including multiple access jammers
(i.e., jammers from a strategic array of multiple locations), must be considered in the
design. Smart spoofers track the location of the target GNSS receiver and use this
information along with a quasi-real-time GNSS signal generator to create strong
GNSS signals that initially match the actual weaker signals in TOA until loop cap-
ture is assured, then lead the target receiver astray. The smart spoofer must be able
to synthesize (duplicate) the target PRN code. Repeat-back spoofers utilize an array
of steered very high gain antennas to track all satellites in view, then rebroadcast an
amplified version toward the target receiver. The end effect on the target receiver
navigation solution (if captured by these signals) is a location and velocity of the
repeat-back spoofer antenna array phase center with a time bias solution that
includes the common mode range between the spoofer and the victim receiver. The
major weakness of both spoofing techniques is that all of the spoofing signals arrive
from the same direction (unless spatial diversity is also used), so that directional
null-steering antenna techniques can be used to defeat spoofers. The encrypted AS Y
code is used to replace the public P code for military applications to minimize the

1. In the literature, this term is sometimes defined differently to mean continuously transmitting, as opposed to

pulsed.