16 research outputs found
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Development and Demonstration of a TDOA-Based GNSS Interference Signal Localization System
Background theory, a reference design, and demonstration
results are given for a Global Navigation Satellite
System (GNSS) interference localization system comprising a
distributed radio-frequency sensor network that simultaneously
locates multiple interference sources by measuring their signals’
time difference of arrival (TDOA) between pairs of nodes in
the network. The end-to-end solution offered here draws from
previous work in single-emitter group delay estimation, very long
baseline interferometry, subspace-based estimation, radar, and
passive geolocation. Synchronization and automatic localization
of sensor nodes is achieved through a tightly-coupled receiver
architecture that enables phase-coherent and synchronous sampling
of the interference signals and so-called reference signals
which carry timing and positioning information. Signal and crosscorrelation
models are developed and implemented in a simulator.
Multiple-emitter subspace-based TDOA estimation techniques
are developed as well as emitter identification and localization
algorithms. Simulator performance is compared to the CramérRao
lower bound for single-emitter TDOA precision. Results are
given for a test exercise in which the system accurately locates
emitters broadcasting in the amateur radio band in Austin, TX.Aerospace Engineering and Engineering Mechanic
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A Testbed for Developing and Evaluating GNSS Signal Authentication Techniques
An experimental testbed has been created for developing
and evaluating Global Navigation Satellite System (GNSS)
signal authentication techniques. The testbed advances the state
of the art in GNSS signal authentication by subjecting candidate
techniques to the strongest publicly-acknowledged GNSS spoofing
attacks. The testbed consists of a real-time phase-coherent GNSS
signal simulator that acts as spoofer, a real-time softwaredefined
GNSS receiver that plays the role of defender, and
post-processing versions of both the spoofer and defender. Two
recently-proposed authentication techniques are analytically and
experimentally evaluated: (1) a defense based on anomalous
received power in a GNSS band, and (2) a cryptographic
defense against estimation-and-replay-type spoofing attacks. The
evaluation reveals weaknesses in both techniques; nonetheless,
both significantly complicate a successful GNSS spoofing attackAerospace Engineering and Engineering Mechanic
A Graphical Approach to GPS Software-Defined Receiver Implementation
Global positioning system (GPS) software-defined
receivers (SDRs) offer many advantages over their hardwarebased
counterparts, such as flexibility, modularity, and upgradability.
A typical GPS receiver is readily expressible as a block
diagram, making a graphical approach a natural choice for
implementing GPS SDRs. This paper presents a real-time, graphical
implementation of a GPS SDR, consisting of two modes:
acquisition and tracking. The acquisition mode performs a twodimensional
fast Fourier transform (FFT)-based search over code
offsets and Doppler frequencies. The carrier-aided code tracking
mode consists of the following main building blocks: correlators,
code and carrier phase detectors, code and carrier phase filters,
a code generator, and a numerically-controlled oscillator. The
presented GPS SDR provides an abstraction level that enables
future research endeavors.Aerospace Engineering and Engineering Mechanic
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The Texas Spoofing Test Battery: Toward a Standard for Evaluating GPS Signal Authentication Techniques
A battery of recorded spoofing scenarios has been compiled
for evaluating civil Global Positioning System (GPS) signal
authentication techniques. The battery can be considered
the data component of an evolving standard meant to
define the notion of spoof resistance for commercial GPS
receivers. The setup used to record the scenarios is described.
A detailed description of each scenario reveals
readily detectable anomalies that spoofing detectors could target to improve GPS securityAerospace Engineering and Engineering Mechanic
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Tightly-Coupled Opportunistic Navigation for Deep Urban and Indoor Positioning
A strategy is presented for exploiting the frequency stability,
transmit location, and timing information of ambient radio-frequency “signals of opportunity” for the purpose of
navigating in deep urban and indoor environments. The
strategy, referred to as tightly-coupled opportunistic navigation
(TCON), involves a receiver continually searching
for signals from which to extract navigation and timing
information. The receiver begins by characterizing these
signals, whether downloading characterizations from a collaborative
online database or performing characterizations
on-the-fly. Signal observables are subsequently combined
within a central estimator to produce an optimal estimate
of position and time. A simple demonstration of the
TCON strategy focused on timing shows that a TCONenabled
receiver can characterize and use CDMA cellular
signals to correct its local clock variations, allowing it to
coherently integrate GNSS signals beyond 100 seconds.Aerospace Engineering and Engineering Mechanic
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CASES: A Smart, Compact GPS Software Receiver for Space Weather Monitoring
A real-time software-defined GPS receiver for the L1 C/A
and L2C codes has been developed as a low-cost space
weather instrument for monitoring ionospheric
scintillation and total electron content. The so-called
CASES receiver implements several novel processing
techniques not previously published that make it well
suited for space weather monitoring: (A) a differencing
technique for eliminating local clock effects, (B) an
advanced triggering mechanism for determining the onset of scintillation, (C) data buffering to permit observation
of the prelude to scintillation, and (D) data-bit prediction
and wipe-off for robust tracking. The receiver has been
tested in a variety of benign and adverse signal conditions
(e.g., severe ionospheric scintillation, both real and
simulated); the results are presented here. The custom
hardware platform on which the software runs is compact
while remaining flexible and extensible. The CASES
platform consists of a digital signal processor, an ARM
microcontroller, and a custom-built narrow-band dualfrequency
front end. Because the receiver is softwaredefined,
it can be remotely reprogrammed via the internet
or another communications link.Aerospace Engineering and Engineering Mechanic
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Indoor GPS: Tightly Coupled Opportunistic Navigation
Aerospace Engineering and Engineering Mechanic
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Receding Horizon Trajectory Optimization for Simultaneous Signal Landscape Mapping and Receiver Localization
A receiver with no a priori knowledge about its own states
is dropped in an unknown environment comprising multiple
signals of opportunity (SOPs) transmitters. Assuming
that the receiver could control its maneuvers in the
form of acceleration commands, two problems are considered.
First, the minimal conditions under which such
environment is completely observable are established. It is shown that receiver-controlled maneuvers reduce the
minimal required a priori information about the environment
for complete observability. Second, the trajectories
that the receiver should traverse in order to build a highfidelity
signal landscape map of the environment, while
simultaneously localizing itself within this map in space
and time with high accuracy are prescribed. To this end,
the one-step look-ahead (greedy) strategy is compared to
the multi-step look-ahead (receding horizon) strategy. The
limitations and achieved improvements in the map quality
and localization accuracy due to the receding horizon
strategy are quantified, and the associated computational
burden is discussed.Aerospace Engineering and Engineering Mechanic
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GPS Spoofing Detection System
A real-time method for detecting GPS spoofing in a
narrow-bandwidth civilian GPS receiver is being developed. It is
needed in order to detect malicious spoofed signals that seek to
deceive a C/A-code civilian GPS receiver regarding its position or
time. The ability to detect a spoofing attack is important to the
reliability of systems ranging from cell-phone towers, the power grid,
and commercial fishing monitors. The spoofing detector mixes and
accumulates base-band quadrature channel samples from two
receivers, one a secure reference receiver, and the other the
defended User Equipment (UE) receiver. The resulting statistic
detects the presence or absence of the encrypted P(Y) code that
should be present in both signals in the absence of spoofing.Aerospace Engineering and Engineering Mechanic
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Evaluation of Smart Grid and Civilian UAV Vulnerability to GPS Spoofing Attacks
Test results are presented from over-the-air civil GPS
spoofing tests from a non-negligible stand-off distance.
These tests were performed at White Sands
Missile Range (WSMR) against two systems dependent
on civil GPS, a civilian unmanned aerial vehicle
(UAV) and a GPS time-reference receiver used in
“smart grid” measurement devices. The tests against
the civil UAV demonstrated that the UAV could be
hijacked by a GPS spoofer by altering the UAV’s perceived
location. The tests against the time-reference
receiver demonstrated the spoofer’s capability of precisely
controlling timing from a distance, which means
a spoofer could manipulate measurements used for
smart grid control without requiring physical access
to the measurement devices. Implications of spoofing
attacks against each of these systems are also given.
Recommendations are presented for regulations regarding
GPS receivers used in critical infrastructure
applications. These recommendations include creating
a certification process by which receivers are declared
spoof-resistant if they are able to detect or mitigate
spoofing attacks in a set of canned scenarios.
The recommendations also call for a mandate that
only spoof-resistant receivers be used in applications
classified by the Department of Homeland Security
(DHS) as national critical infrastructure.Aerospace Engineering and Engineering Mechanic