39 research outputs found
GPS Carrier Tracking Loop Performance in the presence of Ionospheric Scintillations
The performance of several GPS carrier tracking loops
is evaluated using wideband GPS data recorded during
strong ionospheric scintillations. The aim of this study is
to determine the loop structures and parameters that enable
good phase tracking during the power fades and phase
dynamics induced by scintillations. Constant-bandwidth
and variable-bandwidth loops are studied using theoretical
models, simulation, and tests with actual GPS signals.
Constant-bandwidth loops with loop bandwidths near 15
Hz are shown to lose phase lock during scintillations. Use
of the decision-directed discriminator reduces the carrier
lock threshold by ∼1 dB relative to the arctangent and conventional Costas discriminators. A proposed variablebandwidth
loop based on a Kalman filter reduces the carrier
lock threshold by more than 7 dB compared to a 15-Hz
constant-bandwidth loop. The Kalman filter-based strategy
employs a soft-decision discriminator, explicitly models
the effects of receiver clock noise, and optimally adapts
the loop bandwidth to the carrier-to-noise ratio. In extensive
simulation and in tests using actual wideband GPS
data, the Kalman filter PLL demonstrates improved cycle
slip immunity relative to constant bandwidth PLLs.Aerospace Engineering and Engineering Mechanic
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Analysis of Ionospheric Scintillations using Wideband GPS L1 C/A Signal Data
A non-real-time GPS receiver has been developed and
tested for use in scintillation analysis. The receiver consists
of a digital storage receiver and non-real-time software
acquisition and tracking algorithms. The goal of
this work is to shed light on the behavior of strongly
scintillating signals: signals which cause conventional
GPS receivers to lose carrier lock.
The receiver collects wideband GPS L1 digital data sampled at 5.7 MHz using an RF front-end and stores it
on disk for post-processing. It processes the data off-line
to determine carrier signal amplitude and phase variations
during scintillations. The main processing algorithms
are traditional code delay and carrier frequency
acquisition algorithms and special signal processing algorithms
that effectively function as a delay-locked loop
and phase-locked loop. The tracking algorithms use
non-causal smoothing techniques in order to optimally
reconstruct the phase and amplitude variations of a
scintillating signal. These techniques are robust against
the deep power fades and strong phase fluctuations
characteristic of scintillating signals.
To test the receiver, scintillation data were collected
in Cauchoeira Paulista, Brazil, from December 4 to 6,
2003. The data set spans several hours and includes
times when one or more satellite signals are scintillating.
The smoothing algorithm has been used to determine
the carrier amplitude and phase time histories
of the scintillating signals along with the distortion of
the pseudorandom noise (PRN) code’s autocorrelation
function. These quantities provide a characterization
of scintillation that can be used to study the physics of
scintillations or to provide off-line test cases to evaluate
a tracking algorithm’s ability to maintain signal lock
during scintillations.Aerospace Engineering and Engineering Mechanic
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Assessing the Spoofing Threat: Development of a Portable GPS Civilian Spoofer
A portable civilian GPS spoofer is implemented on a digital
signal processor and used to characterize spoofing effects and develop defenses against civilian spoofing. This
work is intended to equip GNSS users and receiver manufacturers
with authentication methods that are effective
against unsophisticated spoofing attacks. The work also
serves to refine the civilian spoofing threat assessment
by demonstrating the challenges involved in mounting a
spoofing attack.Aerospace Engineering and Engineering Mechanic
Real-time software receiver
A real-time software receiver that executes on a general purpose processor. The software receiver includes data acquisition and correlator modules that perform, in place of hardware correlation, baseband mixing and PRN code correlation using bit-wise parallelism
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Evaluating GPS Receiver Robustness to Ionospheric Scintillation
A method for testing GPS receivers for ionospheric
scintillation robustness has been implemented using a
GPS signal simulator and a statistical model that captures
the characteristics of scintillation relevant to receiver
performance. This technique will help GNSS equipment
manufacturers and users prepare for the approaching solar
maximum by enabling repeatable receiver performance
tests under realistic scintillation conditions. Ionospheric
scintillation can impair the performance of phase tracking
loops in GNSS receivers by introducing deep amplitude
fades and abrupt phase changes in a signal. A statistical
model has been developed that accurately recreates these
effects by shaping the complex spectrum rather than
treating phase and amplitude individually. Generated
scintillation histories have been incorporated into the
output of a GPS signal simulator so that any compatible
receiver can be evaluated without modification. Such a
hardware-in-the-loop approach provides a controlled test
environment and the ability to characterize receiver
performance statistically by running many experiments.
It expands the range of possible test conditions beyond
those available during field testing. The method is simple
to implement, and its value has been demonstrated by a
variety of tests applied to four different receivers.Aerospace Engineering and Engineering Mechanic
ROMA (Rank-Ordered Multifractal Analysis) for intermittent fluctuations with global crossover behavior
Rank-Ordered Multifractal Analysis (ROMA), a recently developed technique
that combines the ideas of parametric rank ordering and one parameter scaling
of monofractals, has the capabilities of deciphering the multifractal
characteristics of intermittent fluctuations. The method allows one to
understand the multifractal properties through rank-ordered scaling or
non-scaling parametric variables. The idea of the ROMA technique is applied to
analyze the multifractal characteristics of the auroral zone electric field
fluctuations observed by SIERRA. The observed fluctuations span across
contiguous multiple regimes of scales with different multifractal
characteristics. We extend the ROMA technique such that it can take into
account the crossover behavior -- with the possibility of collapsing
probability distributions functions (PDFs) -- over these contiguous regimes.Comment: 24 pages, 18 figure
Modulation of enhancer looping and differential gene targeting by Epstein-Barr virus transcription factors directs cellular reprogramming
Epstein-Barr virus (EBV) epigenetically reprogrammes B-lymphocytes to drive immortalization and facilitate viral persistence. Host-cell transcription is perturbed principally through the actions of EBV EBNA 2, 3A, 3B and 3C, with cellular genes deregulated by specific combinations of these EBNAs through unknown mechanisms. Comparing human genome binding by these viral transcription factors, we discovered that 25% of binding sites were shared by EBNA 2 and the EBNA 3s and were located predominantly in enhancers. Moreover, 80% of potential EBNA 3A, 3B or 3C target genes were also targeted by EBNA 2, implicating extensive interplay between EBNA 2 and 3 proteins in cellular reprogramming. Investigating shared enhancer sites neighbouring two new targets (WEE1 and CTBP2) we discovered that EBNA 3 proteins repress transcription by modulating enhancer-promoter loop formation to establish repressive chromatin hubs or prevent assembly of active hubs. Re-ChIP analysis revealed that EBNA 2 and 3 proteins do not bind simultaneously at shared sites but compete for binding thereby modulating enhancer-promoter interactions. At an EBNA 3-only intergenic enhancer site between ADAM28 and ADAMDEC1 EBNA 3C was also able to independently direct epigenetic repression of both genes through enhancer-promoter looping. Significantly, studying shared or unique EBNA 3 binding sites at WEE1, CTBP2, ITGAL (LFA-1 alpha chain), BCL2L11 (Bim) and the ADAMs, we also discovered that different sets of EBNA 3 proteins bind regulatory elements in a gene and cell-type specific manner. Binding profiles correlated with the effects of individual EBNA 3 proteins on the expression of these genes, providing a molecular basis for the targeting of different sets of cellular genes by the EBNA 3s. Our results therefore highlight the influence of the genomic and cellular context in determining the specificity of gene deregulation by EBV and provide a paradigm for host-cell reprogramming through modulation of enhancer-promoter interactions by viral transcription factors
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Collaborative Research: Dynamics of Electrostatic Solitary Waves and their Effects on Current Layers
The contents of this final report require explanation, as the report cannot be written in a manner consistent with the usual guidelines for a final scientific technical report. The original PI on this grant was Professor Paul Kintner who passed away November 16, 2010. I, Charles E. Seyler, was asked by the Director of the School of Electrical and Computer Engineering to take over the grant last May and try to fulfill its obligations to the PIs at the lead institution (UNH). I have worked with Professor Kintner over the years and have published joint papers with him on the subject of this grant. Consequently, I was in the best position to carry out the remainder of the grant obligations at Cornell. When the grant was transferred to me, I immediately contacted the PI, Li-Jen Chen, and asked about the obligations of the Cornell collaboration and what plans Professor Kintner had made had done previously to meet them. I also offered my assistance in the way of contributing to the project in a way that my background would allow. I have considerable experience in interpretation of space-related data and I am somewhat familiar with LAPD. I have also performed plasma simulations related to electrostatic solitary waves, which is more directly related to my expertise. Dr. Chen's response was: 'Paul's role is to participate in the solitary wave experiments that we do at LAPD, and offer his experimentalist expertise during the experiments and related discussions. There is still the third experiment in a series of three to be carried out. The date is not set yet.' I later indicated that I could devote about two weeks of summer research to the project and asked the UNH group if there was anything that they would like me to do in the way of simulation or involvement in experiments or interpretation. I did not receive a response to this inquiry