摘要 :
Cross-correlations of unknown encrypted signals between two civilian GNSS receivers are used to detect spoofing of known open-source signals. This type of detection algorithm is the strongest known defense against sophisticated sp...
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Cross-correlations of unknown encrypted signals between two civilian GNSS receivers are used to detect spoofing of known open-source signals. This type of detection algorithm is the strongest known defense against sophisticated spoofing attacks if the defended receiver has only one antenna. The attack strategy of concern starts by overlaying false GNSS radio-navigation signals exactly on top of the true signals. The false signals increase in power, lift the receiver tracking loops off of the true signals, and then drag the tracking loops and the navigation solution to erroneous, but consistent results. This paper develops codeless and semi-codeless spoofing detection methods for use in inexpensive, narrow-band civilian GNSS receivers. Detailed algorithms and analyses are developed that use the encrypted military P(Y) code on the L1 GPS frequency in order to defend the open-source civilian C/A code. The new detection techniques are similar to methods used in civilian dualfrequency GPS receivers to track the P(Y) code on L2 by cross-correlating it with P(Y) on L1. Successful detection of actual spoofing attacks is demonstrated by off-line processing of digitally recorded RF data. The codeless technique can detect attacks using 1.2 sec of correlation, and the semi-codeless technique requires correlation intervals of 0.2 sec or less. This technique has been demonstrated in a narrow-band receiver with a 2.5 MHz bandwidth RF front-end that attenuates the P(Y) code by 5.5 dB.
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摘要 :
The Wide Area Augmentation System (WAAS), an
augmentation of the Global Positioning System (GPS),
measures ionospheric slant delays using dual frequency
receivers in a network of reference stations throughout
North America to comp...
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The Wide Area Augmentation System (WAAS), an
augmentation of the Global Positioning System (GPS),
measures ionospheric slant delays using dual frequency
receivers in a network of reference stations throughout
North America to compute and broadcast a vertical
Ionospheric Grid Delay (IGD) and its associated integrity
bound, the Grid Ionospheric Vertical Error (GIVE) at a
set of Ionospheric Grid Points (IGPs). The GIVEs are
used in receiver computations and significantly determine
the availability of navigation services. Since the initial
fielding beginning in July 2003, WAAS has computed the
vertical delay estimate and the integrity bound at each
IGP using a planar fit based on neighboring
measurements. In a recent WAAS release, fielded in
October 2011, the IGDs and the GIVEs at the IGPs are
computed using kriging estimation. Various tunable
ionospheric model parameters exist in the kriging
algorithm. These parameters were optimized over the full
range of possible ionospheric behaviors: from nominal to
storm ionosphere, and from solar maximum to solar
minimum. This paper documents the methodology,
implementation, and analysis for the trade study that
optimized kriging algorithm performance for the range of
ionospheric behaviors over the entire solar cycle. A set of
64 GIVE algorithm models with varying kriging and
irregularity detector parameters were traded over a 14 day
data set spanning the range of ionospheric behaviors. A
final set of GIVE algorithm kriging parameters was
determined through the trade study that dramatically
improved system availability performance and increased
system robustness against storm ionospheric activity
while maintaining nominal ionospheric performance. The
performance improvement is shown for a set of five
recent storm days.
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摘要 :
Future multi-constellation GNSS open the possibility to
fulfill stringent navigation integrity requirements
specified in safety-critical applications using receiver
autonomous integrity monitoring (RAIM). In this paper,
both the R...
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Future multi-constellation GNSS open the possibility to
fulfill stringent navigation integrity requirements
specified in safety-critical applications using receiver
autonomous integrity monitoring (RAIM). In this paper,
both the RAIM detector and its estimator are analyzed to
develop a new algorithm. In the first part of this paper,
the detector is selected by rigorously comparing two of
the most widely implemented methods. In particular, the
paper reveals fundamental differences between solution
separation (SS) and residual-based (RB) RAIM. SS
provides higher fault-detection performance than RB
RAIM because the SS test statistic is tailored to the fault
hypothesis, and to the state of interest. To prove these
results in presence of multi-measurement faults, which
occur in multi-constellation GNSS, analytical expressions
of the worst-case fault direction are derived for both SS
and RB RAIM. In the second part of the paper, a nonleast-
squares estimator is designed to reduce the integrity
risk at the cost of lower accuracy performance for
applications where integrity requirements are more
demanding than accuracy requirements. The new
estimator is numerically determined by solving an
integrity risk minimization problem that includes multiple
simultaneous fault hypotheses. Performance analyses
show a substantial drop in integrity risk using the new RAIM algorithm as compared to a SS method that uses a
least-squares estimator. In parallel, the decrease in
accuracy performance is quantified. Combined
availability of accuracy and integrity is evaluated at an
example location for a GPS/Galileo navigation system.
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摘要 :
The use of Global Navigation Satellite System (GNSS) technology in safety- and mission-critical services has raised the concern in recent times about possible GNSS Denial of Service (DoS) situations. In that sense, the GNSS vulner...
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The use of Global Navigation Satellite System (GNSS) technology in safety- and mission-critical services has raised the concern in recent times about possible GNSS Denial of Service (DoS) situations. In that sense, the GNSS vulnerability to interferences could become a real threat to the entire service integrity. In this work the interference mitigation capability of antenna arrays in GNSS signal acquisition process is addressed from a realistic implementation perspective. A two-fold objective is pursued: on one hand, we introduce the design and the implementation of a flexible FPGAbased GNSS antenna-array receiver platform, intended to be used as a reliable research tool tightly coupled with software defined GNSS receivers. In that sense, all the platform components, from the multichannel coherent front-end to the FPGA hardware accelerators and the embedded software defined processor, are presented in detail and validated using RF signal generators with realistic GPS-like waveforms. On the other hand, a novel array-based acquisition algorithm that uses the Generalized Likelihood Ratio Test is implemented using the platform. Statistics extraction modules such as the estimation of the array autocovariance matrix are included. Realistic interference scenarios were tested, including CW jamming and Long Term Evolution (LTE)-like in-band interferences. The results show that the algorithm offers excellent protection against uncorrelated directional interferences, even if the array is moderately uncalibrated.
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摘要 :
Advanced RAIM (ARAIM) for vertical guidance has
attracted considerable attention both from integrity
providers and receiver manufacturers, due to its potential
to achieve worldwide coverage of vertical guidance with a
reduced inve...
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Advanced RAIM (ARAIM) for vertical guidance has
attracted considerable attention both from integrity
providers and receiver manufacturers, due to its potential
to achieve worldwide coverage of vertical guidance with a
reduced investment on the ground segment compared to
Space-based Augmentation Systems. Several user
algorithms have been published, mostly variants of
solution separation and possible optimizations. These
descriptions have mostly focused on the definition of the
Vertical Protection Level (VPL), because that is what was
needed to simulate ARAIM availability as a function of
different parameters and constellation configurations.
However, an ARAIM user algorithm has many more
elements that need to be defined. The purpose of this
work is to describe an Advanced RAIM user algorithm
step-by-step including: the Integrity Support Message
(ISM) processing, the fault detection and exclusion, and
the Protection Level calculation – including the
Horizontal Protection Level. In this description, we
attempt to clarify areas that have remained undefined.
We give the contents of the ISM contents, and a
clarification of the interpretation of its parameters. These
parameters describe both the nominal error behavior and
the probability of fault of one or more satellites. The
nominal error is characterized by two sets of standard
deviation and maximum bias, the first one for integrity
purposes and the second one, less conservative, for
accuracy and continuity evaluation purposes. We show
how to compute the nominal error model as a function of
the ISM content, and how to determine which faults must
be monitored – including which subset solutions must be
computed and compared against the all-in-view solution.
In this paper, we make explicit under which conditions a
fault must be declared. In addition to the solution
separation statistics, we show why it is prudent to include
a chi-square test on the residuals as well. We also
describe the actions that follow the detection of a fault or
faults, and under which conditions fault exclusion can be
performed. Although this is not expected to be
fundamentally different than the current approaches in
horizontal RAIM, there are differences that arise.
As mentioned above, the Vertical Protection Level has
been defined in several publications (each with small
variations). In this paper we address the implementation
details in both the VPL and the HPL. First, in case a large
number of fault modes need to be monitored, a large
number of subset solutions must be computed. We show
how to efficiently compute the subset solutions. Second,
the PLs that provide good availability typically require an
iterative halving algorithm. We describe a method to
compute a tight upper bound with very few steps. In
addition, we provide the formulas for the Effective
Monitor Threshold, the fault free 10~(-7) error bound, and the
95% bound on the accuracy. A concrete numerical
example is given to facilitate the verification of the
provided formulas and algorithms.
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摘要 :
This paper presents the main results of the research
activities carried out by the Italian Air Force Flight Test
Centre in collaboration with the University of
Nottingham and Cranfield University in the area of
GNSS Avionics Based...
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This paper presents the main results of the research
activities carried out by the Italian Air Force Flight Test
Centre in collaboration with the University of
Nottingham and Cranfield University in the area of
GNSS Avionics Based Integrity Augmentation (ABIA).
This research included design, integration and
experimental flight test activities on the MB-339CD,
TORNADO and TYPHOON aircraft, as well as the
development of a novel approach to the problem of
GNSS ABIA for mission- and safety-critical air vehicle
applications and for multi-sensor avionics architectures
based on GNSS. As soon as the validity of the ABIA
concept was established, a prototype system was
developed for use in flight test applications. This system
is capable of alerting the pilot when the critical
conditions for GPS signal loss are likely to occur,
within a specified maximum time-to-alert. In this
ABIA prototype, the aircraft on-board sensors provide
information on the aircraft relevant flight parameters
(navigation data, engine settings, etc.) to an Integrity
Flag Generator (IFG), which is also connected to the
on-board GPS receiver. The IFG can be incorporated
into one of the existing airborne computers or can be a
dedicated processing unit. Using the available data on
GPS and the aircraft flight parameters, integrity signals
are generated which are displayed on one of the cockpit
displays and sent to an Aural Warning Generator
(AWG). At the same time, the deviation from the ideal
flight path is computed taking into account the
geometry and the tracking status of the available GPS
satellites, together with the flight test mission
requirements and the information provided by the onboard
avionic sensors. Current research is extending
the results obtained from flight tests to the design of a
more advanced ABIA system suitable for manned and
unmanned aircraft applications. Mathematical models
have been developed to describe the main causes of
GNSS signal outages and degradation in flight, namely:
antenna obscuration, multipath, fading due to adverse
geometry and Doppler shift. Adopting these models in
association with suitable integrity thresholds and
guidance algorithms, the ABIA system is able to
generate integrity cautions (predictive flags) and
warnings (reactive flags), as well as providing steering
information to the pilot and electronic commands to the
aircraft/UAV flight control system. These features
allow real-time avoidance of safety-critical flight
conditions and fast recovery of the required navigation
performance in case of GNSS data losses. In other
words, this novel ABIA system addresses all three
cornerstones of GNSS integrity augmentation in
mission- and safety-critical applications: prediction
(caution flags), reaction (warning flags) and correction
(alternate flight path computation).
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摘要 :
This paper presents methods for overbounding the
missed-detection probability for monitors with nominally
chi-square noise distributions. Such distributions occur
commonly for integrity monitors in safety-critical
applications, su...
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This paper presents methods for overbounding the
missed-detection probability for monitors with nominally
chi-square noise distributions. Such distributions occur
commonly for integrity monitors in safety-critical
applications, such as in Space-Based and Ground-Based
Augmentation Systems for GNSS. Robust bounding
methods are needed when noise parameters, such as the
measurement covariance matrix, are not precisely known.
Simple inflation of the covariance model for the input
data does not, typically, ensure conservative bounding for
all bias directions. To address this problem, conservative
models of monitor-statistic noise are introduced, which
ensure bounding for all covariance matrices in a specified
range, between lower and upper bounds.
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摘要 :
INS/GPS integrated system has been one of the most
popular methodologies in the research field of navigation
technology. This paper aims at developing a real time low
cost INS/GPS integrated navigator based on PC platform
and revi...
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INS/GPS integrated system has been one of the most
popular methodologies in the research field of navigation
technology. This paper aims at developing a real time low
cost INS/GPS integrated navigator based on PC platform
and reviewing many problems encountered in the
development of real time system. A MEMS IMU is
applied in the system in order to meet the requirements of
low cost and small size. The proposed system utilizes
Auto ZUPT/ZIHR as a accurate update source for
extended Kalman Filter (EKF) to improve the accuracy of
navigation solutions during GPS signal blockages. . In the
end, various field tests which include a pure INS aided by
ZUPT/ZIHR and NHC, a simple INS/GPS and an
INS/GPS aided by ZUPT/ZIHR and NHC are conducted
in one open field and roads in downtown. The
preliminary results presented in this study illustrates the
proposed system with frequent ZUPT/ZIHR provides a
stable performance which operates kinematically in free
inertial mode without the aiding of GPS by 15 minutes
with a low cost IMU and the INS/GPS integrated
solutions are improved by 50% when automatic
ZUPT/ZIHR and NHC are both applied in GNSS denied
environments.
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摘要 :
This paper investigates the simulation, Kalman Filter
tracking, and Kalman Filter geolocation of a chirp{
type civilian Global Positioning System (GPS) jam-
mer. The paper is divided into four parts. The rst
two parts present inf...
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This paper investigates the simulation, Kalman Filter
tracking, and Kalman Filter geolocation of a chirp{
type civilian Global Positioning System (GPS) jam-
mer. The paper is divided into four parts. The rst
two parts present information on the current genera-
tion of GPS jammers and propose a simple method
of jammer signal simulation. The third part outlines
a method by which a Kalman Filter can track the
state of the simulated signal at the output of a sim-
ulated radio frequency (RF) front end. The method
uses in-phase and quadrature accumulations, accu-
mulation models, and noise models. The paper also
considers the computational speed and numerical is-
sues of the proposed system. Results are presented
for the Kalman Filter signal tracker on data from
a truth-model simulation. The fourth part outlines
a particular implementation of a Time Di erence of
Arrival (TDOA) jammer geolocation system and its
associated state and measurements. A method of
Time{of{Arrival measurement formulation which re-
duces the required communication bandwidth between
di erent TDOA stations is also presented. A jammer
TDOA data collection campaign at White Sands Mis-
sile Range in June of 2012 is detailed. Results of the
proposed TDOA jammer geolocation system on two
sets of real data are compared to Inertial Navigation System (INS) position estimates.
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摘要 :
The resolution of GPS carrier-phase ambiguities when
carrying out Precise Point Positioning (PPP) has been a
major research challenge in recent years. There are two
main fixed-ambiguity PPP methods: Fractional Cycle
Bias (FCB) est...
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The resolution of GPS carrier-phase ambiguities when
carrying out Precise Point Positioning (PPP) has been a
major research challenge in recent years. There are two
main fixed-ambiguity PPP methods: Fractional Cycle
Bias (FCB) estimation and Integer-Recovery Clock
(IRC). Both methods should in theory provide a similar
level of performance.
The main challenge when using these existing fixedambiguity
PPP methods is the long time period (up to 60
minutes) required to obtain the ambiguity fixed PPP
solution. For a wide range of applications e.g. land
surveying, this long convergence period is not acceptable.
It has been shown that using GLONASS with GPS could
facilitate accurate float PPP solution with a lower
convergence time compared to GPS alone. Therefore, it is
interesting to explore if using GPS/GLONASS float
solution could improve fixed-ambiguity PPP. To date,
not much work has been done related to the effect of
using both GPS and GLONASS when performing fixedambiguity
PPP. In this paper, GPS ambiguities only are
attempted to be fixed to integers and GLONASS
ambiguities are kept as float values (i.e. the paper is
investigating the impact of using GLONASS float
solutions on ambiguity fixed GPS PPP).
Imperial College’s minimum constellation method which
is based on testing ambiguity fixing for all possible
combinations of satellites, is used in this paper when
attempting to fix ambiguities. The main aim is to reduce
the time required to the initial ambiguity resolution and
make ambiguity resolution possible in a larger number of
scenarios.
Ambiguity validation is carried out by using the ratio test
and variable threshold computation driven by the required
confidence level. Furthermore, integrity monitoring of the
solution is carried out using the state of the art Carrierphase
Autonomous Integrity Monitoring (CRAIM).
The results show that inclusion of GLONASS can reduce
the time required to obtain an initial GPS ambiguity
resolution, by at least, 10% compared to employing GPS
alone. In addition, using both GPS and GLONASS can
reduce the 3D and vertical position error at the initial
ambiguity resolution epoch.
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