Terms Of Echoes Research Articles

Remote Sensing of the Polar Ice Zones with HF Radar

Radars operating in the HF band are widely used for over-the-horizon remote sensing of ocean surface conditions, ionospheric studies and the monitoring of ship and aircraft traffic. Several hundreds of such radars are in operation, yet only a handful of experiments have been conducted to assess the prospect of utilizing this technology for the remote sensing of sea ice. Even then, the measurements carried out have addressed only the most basic questions: is there ice present, and can we measure its drift? Recently the theory that describes HF scattering from the dynamic sea surface was extended to handle situations where an ice cover is present. With this new tool, it becomes feasible to interpret the corresponding radar echoes in terms of the structural, mechanical, and electrical properties of the ice field. In this paper we look briefly at ice sensing from space-borne sensors before showing how the persistent and synoptic wide area surveillance capabilities of HF radar offer an alternative. The dispersion relations of different forms of sea ice are examined and used in a modified implementation of the electromagnetic scattering theory employed in HF radar oceanography to compute the corresponding radar signatures. Previous and present-day HF radar deployments at high latitudes are reviewed, noting the physical and technical challenges that confront the implementation of an operational HF radar in its ice monitoring capability.

A morphology-independent data analysis method for detecting and characterizing gravitational wave echoes

The ability to directly detect gravitational waves has enabled us to empirically probe the nature of ultra-compact relativistic objects. Several alternatives to the black holes of classical general relativity have been proposed which do not have a horizon, in which case a newly formed object (e.g. as a result of binary merger) may emit echoes: bursts of gravitational radiation with varying amplitude and duration, but arriving at regular time intervals. Unlike in previous template-based approaches, we present a morphology-independent search method to find echoes in the data from gravitational wave detectors, based on a decomposition of the signal in terms of generalized wavelets consisting of multiple sine-Gaussians. The ability of the method to discriminate between echoes and instrumental noise is assessed by inserting into the noise two different signals: a train of sine-Gaussians, and an echoing signal from an extreme mass-ratio inspiral of a particle into a Schwarzschild vacuum spacetime, with reflective boundary conditions close to the horizon. We find that both types of signals are detectable for plausible signal-to-noise ratios in existing detectors and their near-future upgrades. Finally, we show how the algorithm can provide a characterization of the echoes in terms of the time between successive bursts, and damping and widening from one echo to the next.

Ultrasonic measurement of lubricant film thickness in sliding Bearings with overlapped echoes

The lubricant film thickness in sliding bearings with a thin liner have previously been immeasurable due to the overlapping of ultrasonic signals making the use of the ultrasonic spring model impossible. In this paper, we firstly modeled the overlapped echoes in terms of superimposed Gaussian echoes corrupted by noise and used the EM algorithms to extract the interested echo. Then the extracted echo was used to obtain the lubricant film thickness by an ultrasonic spring model. A high precision calibrated rig which could form a series of known film thicknesses was set up and the performance of the method was shown to work well.

Day-time F region echoes observed by the São Luís radar

This brief report presents unique day-time F region coherent scatter echoes observed by the 30MHz radar over São Luís in eastern Brazil. The radar observation was part of the July 2011 radar campaign designed to detect echoes from day-time F region irregularities. These rare day-time F region radar echoes, scattered from short wavelength plasma irregularities, have quasi patchy features. They are characterized by large upward Doppler velocities (∼100–200m/s). They are very weak (by more than 10dB) when compared to radar echoes observed over São Luís scattered from the night-time F plasma irregularities known as equatorial spread F. Simultaneous GPS vertical total electron content (TEC), and F2 peak frequency (f○F2) and F2 peak height (hmF2) estimated from ionosonde measurements over São Luís have provided additional evidence that the received day-time F region radar echoes were from genuine ionospheric scatterers. This report suggests physical explanations for the day-time radar echoes in terms of the generalized Rayleigh–Taylor instability mechanism operating on large magnetic declination (∼20° westward) of the geomagnetic field geometry in São Luís, conductivity distribution in the winter southern hemisphere and summer northern hemisphere, and the resulting magnetic flux tube electric field configurations.

Neural coding of echo-envelope disparities in echolocating bats

The effective use of echolocation requires not only measuring the delay between the emitted call and returning echo to estimate the distance of an ensonified object. To locate an object in azimuth and elevation, the bat's auditory system must analyze the returning echoes in terms of their binaural properties, i.e., the echoes' interaural intensity and time differences (IIDs and ITDs). The effectiveness of IIDs for echolocation is undisputed, but when bats ensonify complex objects, the temporal structure of echoes may facilitate the analysis of the echo envelope in terms of envelope ITDs. Using extracellular recordings from the auditory midbrain of the bat, Phyllostomus discolor, we found a population of neurons that are sensitive to envelope ITDs of echoes of their sonar calls. Moreover, the envelope-ITD sensitivity improved with increasing temporal fluctuations in the echo envelopes, a sonar parameter related to the spatial statistics of complex natural reflectors like vegetation. The data show that in bats envelope ITDs may be used not only to locate external, prey-generated rustling sounds but also in the context of echolocation. Specifically, the temporal fluctuations in the echo envelope, which are created when the sonar emission is reflected from a complex natural target, support ITD-mediated echolocation.

Decoherence and Loschmidt echoes: quantum against classical

Some recent theoretical results on the stability of quantum dynamics against perturbations of the Hamiltonian - the so-called Loschmidt echoes and their relation to various decoherence measures are reviewed. We show that the representation of Loschmidt echoes in terms of the Wigner function can explain some seemingly paradoxical behavior of the quantum-classical correspondence.

Model-based estimation of ultrasonic echoes. Part I: analysis and algorithms.

The patterns of ultrasonic backscattered echoes represent valuable information pertaining to the geometric shape, size, and orientation of the reflectors as well as the microstructure of the propagation path. Accurate estimation of the ultrasonic echo pattern is essential in determining the object/propagation path properties. In this study, we model ultrasonic backscattered echoes in terms of superimposed Gaussian echoes corrupted by noise. Each Gaussian echo in the model is a nonlinear function of a set of parameters: echo bandwidth, arrival time, center frequency, amplitude, and phase. These parameters are sensitive to the echo shape and can be linked to the physical properties of reflectors and frequency characteristics of the propagation path. We address the estimation of these parameters using the maximum likelihood estimation (MLE) principle, assuming that all of the parameters describing the shape of the echo are unknown but deterministic. In cases for which noise is characterized as white Gaussian, the MLE problem simplifies to a least squares (LS) estimation problem. The iterative LS optimization algorithms when applied to superimposed echoes suffer from the problem of convergence and exponential growth in computation as the number of echoes increases. In this investigation, we have developed expectation maximization (EM)-based algorithms to estimate ultrasonic signals in terms of Gaussian echoes. The EM algorithms translate the complicated superimposed echoes estimation into isolated echo estimations, providing computational versatility. The algorithm outperforms the LS methods in terms of independence to the initial guess and convergence to the optimal solution, and it resolves closely spaced overlapping echoes.

First observations of summer polar mesospheric backscatter with a 224 MHz radar

Exceptionally strong echoes from the mesopause region have been observed with the EISCAT 224 MHz radar during the summer of 1987. The echoes are so strong that they cannot be explained by incoherent scatter theory but may instead be associated with neutral air turbulence. We discuss the variability of these echoes in terms of time and height distribution, and outline some characteristics of vertical velocity oscillations observed at the echo height.

A Frequency-Domain Interpretation of Echoes and their Effect on Teletext Data Reception

The interpretation of echoes in terms of ripples in the amplitude and group delay of a system is analyzed and discussed. Simple relationships are given, valid for most practical purposes for echo levels up to at least 10-20 percent. Scre general applications are considered, including determination of SAW filter triple transit frcm group delay ripple rate and amplitude. For this paper the relationships were used to represent echoes due to multipath. A computer program for television system analysis used these echoes as an input and eye diagrams were plotted for different echo levels and tines, for both ideal and real systems. Computer simulation of a data decoder, which extends the analytical approach to include direct assessrent of errors as a function of decoding threshold, is described but not used herein. Ihe analytical approach appears to be promising and will be applied to a greater number of echo and real system combinations as time permits.

Radar auroral echo characteristics as seen by a 398-MHz phased array radar operated at Homer, Alaska

In this paper we present the first results of the mapping of the spatial distribution of auroral echoes by a 398-MHz phased array radar recently installed at Homer, Alaska. The results are shown in the form of two-dimensional spatial maps of auroral echoes. These maps are classified into types that are extensions of the ‘diffuse’ and ‘discrete’ echo types introduced by Leadabrand et al. (1959) and Presnell et al. (1959) for narrow beam radars. The auroral echo signatures typically seen during the afternoon and evening sectors are then described and discussed in relation to radar auroral echo characteristics found by previous investigations and other auroral processes. We show that interpretation of the occurrence of auroral echoes in terms of control by an electric field and by electron density in the radar scattering volume produces a consistent morphology of the radar aurora in relation to the other observed auroral processes. Finally, we include a dynamic auroral echo signature that may be related to the growth phase of an auroral substorm.

Analysis of 174 ventricular premature beats, 172 retrograde P waves and 36 ventricular echoes with reference to atrial echoes through the S-A node.

Electrocardiograms were taken from a 36-year-old female with vegetative neurosis to obtain 174 ventricular premature beats, 172 retrograde P waves, 36 ventricular echoes and 8 atrial echoes through the S-A node or by way of the S-A junctional region. The atrial echo through the S-A node were differentiated only in esoplrageal leads which showed 4 types of P waves; a sinus P wave, a retrograde (nodal) P wave, a retrograde P wave followed by a ventricular echo and an antegrade P wave of an atrial echo. 1. An R-R' interval (an interval between the sinus-conducted beat and the ventricular premature beat) of 0.40 to 0.48 second was observed in 100 per cent of ventricular premature beats with ventricular echoes and in 81 per cent of those without them. 2. An R'-P' interval (an interval between the ventricular premature beat and the following retrograde P wave) of 0.24 to 0.31 second was observed in 78 per cent of ventricular premature beats with ventricular echoes. While, an R'-P' interval of 0.16 to 0.23 second was observed in 79 per cent of ventricular premature beats with-out ventricular echoes. 3. The R-R' interval was inversely proportional to the R'-P' interval. The ventricular echo occur-red when the R-R' interval was less than 0.48 second and the R'-P' interval greater than 0.22 second. 4. When the R-R'-R' interval (an interval between the sinus-conducted beat preceding the ventricular premature beat and the ventricular echo response) was the same, a shorter R-R' interval was associated with a longer R'-P' interval and a shorter P'-R' interval (an interval between the retrograde P wave and the ventricular echo response). While, a longer R-R' interval was associated with a shorter R'-P' interval and a longer P'-R' interval. 5. When the R-R' interval was the same, a longer R'-P' interval was ordinarily preceded by a longer R'p-R interval (an interval between the preceding ventricular premature beat and the following sinus-conducted beat) and followed by a ventricular echo. When the R-R' interval was preceded by a shorter Rp-R interval (an interval between the preceding sinus-conducted beat and the sinus-conducted beat followed by R'), it was followed by a shorter R'-P' interval and not followed by a ventricular echo. 6. An effort was made to explain ventricular echoes in terms of re-entry by retrograde stimulation. The ectopic focus in the A-V junctional region may be discharged by a retrograde impulse and made fire again antegrade in the same way as in the premature beats. 7. In esophageal leads, when the ventricular premature beat was neither interpolated nor followed by a fully compensatory pause, the following P wave showed aberrant intra-atrial conduction. The temporal relation between the regular sinus impulse and this aberrant P wave seemed to indicate that the preceding sinus impulse was blocked by retrograde propagation of the ventricular premature impulse, that the succeeding sinus impulse was abolished by pre-mature firing of the S-A node by re-entry of this retrograde impulse and that a slower conduction in the S-A node afforded incomplete recovery to the atria and enabled aberrant re-excitation of the atria.