52,043 research outputs found
Long-term monitoring of geodynamic surface deformation using SAR interferometry
Thesis (Ph.D.) University of Alaska Fairbanks, 2014Synthetic Aperture Radar Interferometry (InSAR) is a powerful tool to measure surface deformation and is well suited for surveying active volcanoes using historical and existing satellites. However, the value and applicability of InSAR for geodynamic monitoring problems is limited by the influence of temporal decorrelation and electromagnetic path delay variations in the atmosphere, both of which reduce the sensitivity and accuracy of the technique. The aim of this PhD thesis research is: how to optimize the quantity and quality of deformation signals extracted from InSAR stacks that contain only a low number of images in order to facilitate volcano monitoring and the study of their geophysical signatures. In particular, the focus is on methods of mitigating atmospheric artifacts in interferograms by combining time-series InSAR techniques and external atmospheric delay maps derived by Numerical Weather Prediction (NWP) models. In the first chapter of the thesis, the potential of the NWP Weather Research & Forecasting (WRF) model for InSAR data correction has been studied extensively. Forecasted atmospheric delays derived from operational High Resolution Rapid Refresh for the Alaska region (HRRRAK) products have been compared to radiosonding measurements in the first chapter. The result suggests that the HRRR-AK operational products are a good data source for correcting atmospheric delays in spaceborne geodetic radar observations, if the geophysical signal to be observed is larger than 20 mm. In the second chapter, an advanced method for integrating NWP products into the time series InSAR workflow is developed. The efficiency of the algorithm is tested via simulated data experiments, which demonstrate the method outperforms other more conventional methods. In Chapter 3, a geophysical case study is performed by applying the developed algorithm to the active volcanoes of Unimak Island Alaska (Westdahl, Fisher and Shishaldin) for long term volcano deformation monitoring. The volcano source location at Westdahl is determined to be approx. 7 km below sea level and approx. 3.5 km north of the Westdahl peak. This study demonstrates that Fisher caldera has had continuous subsidence over more than 10 years and there is no evident deformation signal around Shishaldin peak.Chapter 1. Performance of the High Resolution Atmospheric Model HRRR-AK for Correcting Geodetic Observations from Spaceborne Radars -- Chapter 2. Robust atmospheric filtering of InSAR data based on numerical weather prediction models -- Chapter 3. Subtle motion long term monitoring of Unimak Island from 2003 to 2010 by advanced time series SAR interferometry -- Chapter 4. Conclusion and future work
Circular polarization measurement in millimeter-wavelength spectral-line VLBI observations
This paper considers the problem of accurate measurement of circular
polarization in imaging spectral-line VLBI observations in the lambda=7 mm and
lambda=3 mm wavelength bands. This capability is especially valuable for the
full observational study of compact, polarized SiO maser components in the
near-circumstellar environment of late-type, evolved stars. Circular VLBI
polarimetry provides important constraints on SiO maser astrophysics, including
the theory of polarized maser emission transport, and on the strength and
distribution of the stellar magnetic field and its dynamical role in this
critical circumstellar region. We perform an analysis here of the data model
containing the instrumental factors that limit the accuracy of circular
polarization measurements in such observations, and present a corresponding
data reduction algorithm for their correction. The algorithm is an enhancement
of existing spectral line VLBI polarimetry methods using autocorrelation data
for calibration, but with innovations in bandpass determination,
autocorrelation polarization self-calibration, and general optimizations for
the case of low SNR, as applicable at these wavelengths. We present an example
data reduction at mm and derive an estimate of the predicted
accuracy of the method of m_c < 0.5% or better at lambda=7 mm and m_c < 0.5-1%
or better at lambda=3 mm. Both the strengths and weaknesses of the proposed
algorithm are discussed, along with suggestions for future work.Comment: 23 pages, 13 figure
Analyzing the effect of local rounding error propagation on the maximal attainable accuracy of the pipelined Conjugate Gradient method
Pipelined Krylov subspace methods typically offer improved strong scaling on
parallel HPC hardware compared to standard Krylov subspace methods for large
and sparse linear systems. In pipelined methods the traditional synchronization
bottleneck is mitigated by overlapping time-consuming global communications
with useful computations. However, to achieve this communication hiding
strategy, pipelined methods introduce additional recurrence relations for a
number of auxiliary variables that are required to update the approximate
solution. This paper aims at studying the influence of local rounding errors
that are introduced by the additional recurrences in the pipelined Conjugate
Gradient method. Specifically, we analyze the impact of local round-off effects
on the attainable accuracy of the pipelined CG algorithm and compare to the
traditional CG method. Furthermore, we estimate the gap between the true
residual and the recursively computed residual used in the algorithm. Based on
this estimate we suggest an automated residual replacement strategy to reduce
the loss of attainable accuracy on the final iterative solution. The resulting
pipelined CG method with residual replacement improves the maximal attainable
accuracy of pipelined CG, while maintaining the efficient parallel performance
of the pipelined method. This conclusion is substantiated by numerical results
for a variety of benchmark problems.Comment: 26 pages, 6 figures, 2 tables, 4 algorithm
MilliSonic: Pushing the Limits of Acoustic Motion Tracking
Recent years have seen interest in device tracking and localization using
acoustic signals. State-of-the-art acoustic motion tracking systems however do
not achieve millimeter accuracy and require large separation between
microphones and speakers, and as a result, do not meet the requirements for
many VR/AR applications. Further, tracking multiple concurrent acoustic
transmissions from VR devices today requires sacrificing accuracy or frame
rate. We present MilliSonic, a novel system that pushes the limits of acoustic
based motion tracking. Our core contribution is a novel localization algorithm
that can provably achieve sub-millimeter 1D tracking accuracy in the presence
of multipath, while using only a single beacon with a small 4-microphone
array.Further, MilliSonic enables concurrent tracking of up to four smartphones
without reducing frame rate or accuracy. Our evaluation shows that MilliSonic
achieves 0.7mm median 1D accuracy and a 2.6mm median 3D accuracy for
smartphones, which is 5x more accurate than state-of-the-art systems.
MilliSonic enables two previously infeasible interaction applications: a) 3D
tracking of VR headsets using the smartphone as a beacon and b) fine-grained 3D
tracking for the Google Cardboard VR system using a small microphone array
Efficient Downlink Channel Reconstruction for FDD Multi-Antenna Systems
In this paper, we propose an efficient downlink channel reconstruction scheme
for a frequency-division-duplex multi-antenna system by utilizing uplink
channel state information combined with limited feedback. Based on the spatial
reciprocity in a wireless channel, the downlink channel is reconstructed by
using frequency-independent parameters. We first estimate the gains, delays,
and angles during uplink sounding. The gains are then refined through downlink
training and sent back to the base station (BS). With limited overhead, the
refinement can substantially improve the accuracy of the downlink channel
reconstruction. The BS can then reconstruct the downlink channel with the
uplink-estimated delays and angles and the downlink-refined gains. We also
introduce and extend the Newtonized orthogonal matching pursuit (NOMP)
algorithm to detect the delays and gains in a multi-antenna multi-subcarrier
condition. The results of our analysis show that the extended NOMP algorithm
achieves high estimation accuracy. Simulations and over-the-air tests are
performed to assess the performance of the efficient downlink channel
reconstruction scheme. The results show that the reconstructed channel is close
to the practical channel and that the accuracy is enhanced when the number of
BS antennas increases, thereby highlighting that the promising application of
the proposed scheme in large-scale antenna array systems
The localization of single pulse in VLBI observation
In our previous work, we propose a cross spectrum based method to extract
single pulse signals from RFI contaminated data, which is originated from
geodetic VLBI postprocessing. This method fully utilizes fringe phase
information of the cross spectrum and hence maximizes signal power, however the
localization was not discussed in that work yet. As the continuation of that
work, in this paper, we further study how to localize single pulses using
astrometric solving method. Assuming that the burst is a point source, we
derive the burst position by solving a set of linear equations given the
relation between residual delay and offset to a priori position. We find that
the single pulse localization results given by both astrometric solving and
radio imaging are consistent within 3 sigma level. Therefore we claim that it
is possible to derive the position of a single pulse with reasonable precision
based on only 3 or even 2 baselines with 4 milliseconds integration. The
combination of cross spectrum based detection and the localization proposed in
this work then provide a thorough solution for searching single pulse in VLBI
observation. According to our calculation, our pipeline gives comparable
accuracy as radio imaging pipeline. Moreover, the computational cost of our
pipeline is much smaller, which makes it more practical for FRB search in
regular VLBI observation. The pipeline is now publicly available and we name it
as "VOLKS", which is the acronym of "VLBI Observation for frb Localization Keen
Searcher".Comment: 11 pages, 4 figures, 3 tables, accepted for publication in A
INTEGRAL timing and localization performance
In this letter we report on the accuracy of the attitude, misalignment, orbit
and time correlation which are used to perform scientific analyses of the
INTEGRAL data. The boresight attitude during science pointings has an accuracy
of 3 arcsec. At the center of the field, the misalignments have been calibrated
leading to a location accuracy of 4 to 40 arcsec for the different instruments.
The spacecraft position is known within 10 meters. The relative timing between
instruments could be reconstructed within 10 microsec and the absolute timing
within 40 microsec.Comment: 5 pages, 2 figures, accepted for publication in A+A letters, INTEGRAL
special issu
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