23,829 research outputs found
Wave Measurements Using GPS Velocity Signals
This study presents the idea of using GPS-output velocity signals to obtain wave measurement data. The application of the transformation from a velocity spectrum to a displacement spectrum in conjunction with the directional wave spectral theory are the core concepts in this study. Laboratory experiments were conducted to verify the accuracy of the inversed displacement of the surface of the sea. A GPS device was installed on a moored accelerometer buoy to verify the GPS-derived wave parameters. It was determined that loss or drifting of the GPS signal, as well as energy spikes occurring in the low frequency band led to erroneous measurements. Through the application of moving average skill and a process of frequency cut-off to the GPS output velocity, correlations between GPS-derived, and accelerometer buoy-measured significant wave heights and periods were both improved to 0.95. The GPS-derived one-dimensional and directional wave spectra were in agreement with the measurements. Despite the direction verification showing a 10° bias, this exercise still provided useful information with sufficient accuracy for a number of specific purposes. The results presented in this study indicate that using GPS output velocity is a reasonable alternative for the measurement of ocean waves
The use of GPS-arrays in detecting shock-acoustic waves generated during rocket launchings
This paper is concerned with the form and dynamics of shock-acoustic waves
(SAW) generated during rocket launchings. We have developed a method for
determining SAW parameters (including angular characteristics of the wave
vector, and the SAW phase velocity, as well as the direction towards the
source) using GPS-arrays whose elements can be chosen out of a large set of
GPS-stations of the global GPS network. The application of the method is
illustrated by a case study of ionospheric effects from launchings of launch
vehicles (LV) Proton and Space Shuttle from space-launch complexes Baikonur and
Kennedy Space Center (KSC) in 1998 and 1999 (a total of five launchings). The
study revealed that, in spite of a difference of LV characteristics, the
ionospheric response for all launchings had the character of an N - wave
corresponding to the form of a shock wave, regardless of the disturbance source
(rocket launchings, industrial explosions). The SAW period T is 270--360 s, and
the amplitude exceeds the standard deviation of TEC background fluctuations in
this range of periods under quiet and moderate geomagnetic conditions by
factors of 2 to 5 as a minimum. The angle of elevation of the SAW wave vector
varies from 30 degree to 60 degree, and the SAW phase velocity (900-1200 m/s)
approaches the sound velocity at heights of the ionospheric F-region maximum.Comment: EmTeX-386, 23 pages, 6 figure
The shock-acoustic waves generated by earthquakes
We investigate the form and dynamics of shock-acoustic waves generated by
earthquakes. We use the method for detecting and locating the sources of
ionospheric impulsive disturbances, based on using data from a global network
of receivers of the GPS navigation system and requiring no a priori information
about the place and time of associated effects. The practical implementation of
the method is illustrated by a case study of earthquake effects in Turkey
(August 17, and November 12, 1999), in Southern Sumatera (June 4, 2000), and
off the coast of Central America (January 13, 2001). It was found that in all
instances the time period of the ionospheric response is 180-390 s, and the
amplitude exceeds by a factor of two as a minimum the standard deviation of
background fluctuations in total electron content in this range of periods
under quiet and moderate geomagnetic conditions. The elevation of the wave
vector varies through a range of 20-44 degree, and the phase velocity
(1100-1300 m/s) approaches the sound velocity at the heights of the ionospheric
F-region maximum. The calculated (by neglecting refraction corrections)
location of the source roughly corresponds to the earthquake epicenter. Our
data are consistent with the present views that shock-acoustic waves are caused
by a piston-like movement of the Earth surface in the zone of an earthquake
epicenter.Comment: EmTeX-386, 30 pages, 4 figures, 3 tabl
Geomagnetic control of the spectrum of traveling ionospheric disturbances based on data from a global GPS network
In this paper an attempt is made to verify the hypothesis on the role of
geomagnetic disturbances as a factor determining the intensity of traveling
ionospheric disturbances (TIDs). To improve the statistical validity of the
data, we have used the based on the new GLOBDET technology method involving a
global spatial averaging of disturbance spectra of the total electron content
(TEC). To characterize the TID intensity quantitatively, we suggest that a new
global index of the degree of disturbance should be used, which is equal to the
mean value of the rms variations in TEC within the selected range of spectral
periods (of 20-60 min in the present case). It was found that power spectra of
daytime TEC variations in the range of 20-60 min periods under quiet conditions
have a power-law form, with the slope index k = -2.5. With an increase of the
level of magnetic disturbance, there is an increase in total intensity of TIDs,
with a concurrent kink of the spectrum caused by an increase in oscillation
intensity in the range of 20-60 min. It was found that an increase in the level
of geomagnetic activity is accompanied by an increase in total intensity of
TEC; however, it correlates not with the absolute level of Dst, but with the
value of the time derivative of Dst (a maximum correlation coefficient reaches
-0.94). The delay of the TID response of the order of 2 hours is consistent
with the view that TIDs are generated in auroral regions, and propagate
equatorward with the velocity of about 300-400 m/s.Comment: LaTeX2.09, 16 pages, 5 figures, 1 table, egs.cls, egs.bst (the style
files
Traveling wave packets of total electron content disturbances as deduced from global GPS network data
We identified a new class of mid-latitude medium-scale traveling ionospheric
disturbances (MS TIDs), viz. traveling wave packets (TWPs) of total electron
content (TEC) disturbances. For the first time, the morphology of TWPs is
presented for 105 days. Using the technique of GPS interferometry of TIDs we
carried out a detailed analysis of the spatial-temporal properties of TWPs by
considering an example of the most conspicuous manifestation of TWPs on October
18, 2001 over California, USA. The velocity and direction of TWPs correspond to
those of mid-latitude MS TIDs obtained previously from analyzing the phase
characteristics of HF radio signals as well as signals from geostationary
satellites and discrete cosmic radio sources.Comment: LaTeX2.09, 28 pages, 9 figure
Robust seismic velocity change estimation using ambient noise recordings
We consider the problem of seismic velocity change estimation using ambient
noise recordings. Motivated by [23] we study how the velocity change estimation
is affected by seasonal fluctuations in the noise sources. More precisely, we
consider a numerical model and introduce spatio-temporal seasonal fluctuations
in the noise sources. We show that indeed, as pointed out in [23], the
stretching method is affected by these fluctuations and produces misleading
apparent velocity variations which reduce dramatically the signal to noise
ratio of the method. We also show that these apparent velocity variations can
be eliminated by an adequate normalization of the cross-correlation functions.
Theoretically we expect our approach to work as long as the seasonal
fluctuations in the noise sources are uniform, an assumption which holds for
closely located seismic stations. We illustrate with numerical simulations and
real measurements that the proposed normalization significantly improves the
accuracy of the velocity change estimation
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