76,007 research outputs found
ISAR Autofocus Imaging Algorithm for Maneuvering Targets Based on Phase Retrieval and Gabor Wavelet Transform
The imaging issue of a rotating maneuvering target with a large angle and a high translational speed has been a challenging problem in the area of inverse synthetic aperture radar (ISAR) autofocus imaging, in particular when the target has both radial and angular accelerations. In this paper, on the basis of the phase retrieval algorithm and the Gabor wavelet transform (GWT), we propose a new method for phase error correction. The approach first performs the range compression on ISAR raw data to obtain range profiles, and then carries out the GWT transform as the time-frequency analysis tool for the rotational motion compensation (RMC) requirement. The time-varying terms, caused by rotational motion in the Doppler frequency shift, are able to be eliminated at the selected time frame. Furthermore, the processed backscattered signal is transformed to the one in the frequency domain while applying the phase retrieval to run the translational motion compensation (TMC). Phase retrieval plays an important role in range tracking, because the ISAR echo module is not affected by both radial velocity and the acceleration of the target. Finally, after the removal of both the rotational and translational motion errors, the time-invariant Doppler shift is generated, and radar returned signals from the same scatterer are always kept in the same range cell. Therefore, the unwanted motion effects can be removed by applying this approach to have an autofocused ISAR image of the maneuvering target. Furthermore, the method does not need to estimate any motion parameters of the maneuvering target, which has proven to be very effective for an ideal range–Doppler processing. Experimental and simulation results verify the feasibility of this approach
Millisecond and Binary Pulsars as Nature's Frequency Standards. II. Effects of Low-Frequency Timing Noise on Residuals and Measured Parameters
Pulsars are the most stable natural frequency standards. They can be applied
to a number of principal problems of modern astronomy and time-keeping
metrology. The full exploration of pulsar properties requires obtaining
unbiased estimates of the spin and orbital parameters. These estimates depend
essentially on the random noise component being revealed in the residuals of
time of arrivals (TOA). In the present paper, the influence of low-frequency
("red") timing noise with spectral indices from 1 to 6 on TOA residuals,
variances, and covariances of estimates of measured parameters of single and
binary pulsars are studied. In order to determine their functional dependence
on time, an analytic technique of processing of observational data in time
domain is developed which takes into account both stationary and non-stationary
components of noise. Our analysis includes a simplified timing model of a
binary pulsar in a circular orbit and procedure of estimation of pulsar
parameters and residuals under the influence of red noise. We reconfirm that
uncorrelated white noise of errors of measurements of TOA brings on gradually
decreasing residuals, variances and covariances of all parameters. On the other
hand, we show that any red noise causes the residuals, variances, and
covariances of certain parameters to increase with time. Hence, the low
frequency noise corrupts our observations and reduces experimental
possibilities for better tests of General Relativity Theory. We also treat in
detail the influence of a polynomial drift of noise on the residuals and
fitting parameters. Results of the analitic analysis are used for discussion of
a statistic describing stabilities of kinematic and dynamic pulsar time scales.Comment: 40 pages, 1 postscript figure, 1 picture, uses mn.sty, accepted to
Mon. Not. Roy. Astron. So
Active rotational and translational microrheology beyond the linear spring regime
Active particle tracking microrheometers have the potential to perform
accurate broad-band measurements of viscoelasticity within microscopic systems.
Generally, their largest possible precision is limited by Brownian motion and
low frequency changes to the system. The signal to noise ratio is usually
improved by increasing the size of the driven motion compared to the Brownian
as well as averaging over repeated measurements. New theory is presented here
which gives the complex shear modulus when the motion of a spherical particle
is driven by non-linear forces. In some scenarios error can be further reduced
by applying a variable transformation which linearises the equation of motion.
This allows normalisation which eliminates low frequency drift in the
particle's equilibrium position. Using this method will easily increase the
signal strength enough to significantly reduce the measurement time for the
same error. Thus the method is more conducive to measuring viscoelasticity in
slowly changing microscopic systems, such as a living cell.Comment: 9 pages, 2 figure
Linear Stability Analysis of a Levitated Nanomagnet in a Static Magnetic Field: Quantum Spin Stabilized Magnetic Levitation
We theoretically study the levitation of a single magnetic domain nanosphere
in an external static magnetic field. We show that apart from the stability
provided by the mechanical rotation of the nanomagnet (as in the classical
Levitron), the quantum spin origin of its magnetization provides two additional
mechanisms to stably levitate the system. Despite of the Earnshaw theorem, such
stable phases are present even in the absence of mechanical rotation. For large
magnetic fields, the Larmor precession of the quantum magnetic moment
stabilizes the system in full analogy with magnetic trapping of a neutral atom.
For low magnetic fields, the magnetic anisotropy stabilizes the system via the
Einstein-de Haas effect. These results are obtained with a linear stability
analysis of a single magnetic domain rigid nanosphere with uniaxial anisotropy
in a Ioffe-Pritchard magnetic field.Comment: Published version. 10 pages, 4 figures, 3 table
Assessing year to year variability of inertial oscillation in the Chukchi Sea using the wavelet transform
Master's Project (M.S.) University of Alaska Fairbanks, 2016Three years of ocean drifter data from the Chukchi Sea were examined using the wavelet transform to investigate inertial oscillation. There was an increasing trend in number, duration, and hence total proportion of time spent in inertial oscillation events. Additionally, the Chukchi Sea seems to facilitate inertial oscillation that is easier to discern using north-south velocity records rather than east-west velocity records. The data used in this analysis was transformed using wavelets, which are generally used as a qualitative statistical method. Because of this, in addition to measurement error and random ocean noise, there is an additional source of variability and correlation which makes concrete statistical results challenging to obtain. However, wavelets were an effective tool for isolating the specific period of inertial oscillation and examining how it changed over time
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