Atmospheric Propagation Effects Research Articles

Effects of atmospheric scattering and refraction on the performance of acoustic direction‐finding arrays.

Refraction, turbulent scattering, and other atmospheric propagation effects complicate the performance of outdoor microphone arrays used to infer source bearings. In principle, with good knowledge of the atmospheric profiles, one can compensate for the refraction effects. However, the random angle‐of‐arrival variations induced by atmospheric turbulence limit array performance even when the signal‐to‐noise ratio is high. The turbulent scattering effect can be viewed as a coherence loss between elements of the beamforming array. Most research to date on acoustic signal coherence in the atmosphere has dealt with line‐of‐sight (LOS) propagation paths. A number of complications characteristic of real turbulence have been introduced; these include statistical inhomogeneity, anisotropy, and intermittency of the turbulent eddies. The LOS theory has also been extended to include sensor displacements longitudinal to the propagating wavefronts as well as transverse displacements. Formulations for the Cramer–Rao lower bound (CRLB) on the angle‐of‐arrival accuracy have been developed from the LOS theory. Based on limited experimental data and modeling, actual performance appears to be substantially worse than the CRLB. Recently, numerical methods have been introduced for solving second‐moment parabolic equations, which offer the possibility of incorporating non‐LOS effects into coherence calculations.

Assessment of Atmospheric Propagation Effects in SAR Images

TerraSAR-X, the first civil German synthetic aperture radar (SAR) satellite, was successfully launched on June 15, 2007. After 4.5 days, the first processed image was obtained. The overall quality of the image was outstanding; however, suspicious features could be identified which showed precipitation-related signatures. These rain-cell signatures are thoroughly investigated, and the physical background of the related propagation effects is provided. In addition, rain-cell signatures from former missions like SIR-C/X and the Shuttle Radar Topography Mission are provided for comparison. During the commissioning phase of TerraSAR-X, a total of 12 000 scenes were investigated for potential propagation effects, and about 100 scenes revealed atmospheric effects to a visible extent. Some of the particularly interesting events were selected and are discussed in greater detail. An interesting case of data acquisition over New York will be presented, which shows typical rain-cell signatures, and the SAR image will be compared with weather-radar data acquired nearly simultaneously (within the same minute). By comparing the images, it can be clearly seen that reflectivities in the weather-radar image of 50 dBZ may cause visible artifacts in SAR images. Furthermore, in this paper, we discuss the influence of the atmosphere (troposphere) on the external calibration of TerraSAR-X. By acquiring simultaneous weather-radar data over the test site and the SAR acquisition, it was possible to flag affected SAR images and to exclude them from the procedure to derive the absolute calibration constant. Thus, it was possible to decrease the 1 sigma uncertainty of the absolute calibration factor by 0.15 dB.

The source of infrasound associated with long‐period events at Mount St. Helens

During the early stages of the 2004–2008 Mount St. Helens eruption, the source process that produced a sustained sequence of repetitive long‐period (LP) seismic events also produced impulsive broadband infrasonic signals in the atmosphere. To assess whether the signals could be generated simply by seismic‐acoustic coupling from the shallow LP events, we perform finite difference simulation of the seismo‐acoustic wavefield using a single numerical scheme for the elastic ground and atmosphere. The effects of topography, velocity structure, wind, and source configuration are considered. The simulations show that a shallow source buried in a homogeneous elastic solid produces a complex wave train in the atmosphere consisting of P/SV and Rayleigh wave energy converted locally along the propagation path, and acoustic energy originating from the source epicenter. Although the horizontal acoustic velocity of the latter is consistent with our data, the modeled amplitude ratios of pressure to vertical seismic velocity are too low in comparison with observations, and the characteristic differences in seismic and acoustic waveforms and spectra cannot be reproduced from a common point source. The observations therefore require a more complex source process in which the infrasonic signals are a record of only the broadband pressure excitation mechanism of the seismic LP events. The observations and numerical results can be explained by a model involving the repeated rapid pressure loss from a hydrothermal crack by venting into a shallow layer of loosely consolidated, highly permeable material. Heating by magmatic activity causes pressure to rise, periodically reaching the pressure threshold for rupture of the “valve” sealing the crack. Sudden opening of the valve generates the broadband infrasonic signal and simultaneously triggers the collapse of the crack, initiating resonance of the remaining fluid. Subtle waveform and amplitude variability of the infrasonic signals as recorded at an array 13.4 km to the NW of the volcano are attributed primarily to atmospheric boundary layer propagation effects, superimposed upon amplitude changes at the source.

Atmospheric Radiowave Propagation Effects on Spatial Resolution of Spaceborne Synthetic Aperture Radar

摘要: 利用合成孔径原理和大气折射理论推导了大气层给SAR方位向分辨率带来的误差公式,针对不同时间、地点、高低年的电离层电子浓度和低层大气折射率剖面,对不同频段进行了计算,并对计算结果进行了分析讨论.得出了大气层中星载SAR的方位向分辨率误差为毫米量级,其绝对值随频率增高而减小的结论. 关键词: SAR / 方位向分辨率 / 误差

Improved detection of atmospheric turbulence with SLODAR

We discuss several improvements in the detection of atmospheric turbulence using SLOpe Detection And Ranging (SLODAR). Frequently, SLODAR observations have shown strong ground-layer turbulence, which is beneficial to adaptive optics. We show that current methods which neglect atmospheric propagation effects can underestimate the strength of high altitude turbulence by up to ~ 30%. We show that mirror and dome seeing turbulence can be a significant fraction of measured ground-layer turbulence, some cases up to ~ 50%. We also demonstrate a novel technique to improve the nominal height resolution, by a factor of 3, called Generalized SLODAR. This can be applied when sampling high-altitude turbulence, where the nominal height resolution is the poorest, or for resolving details in the important ground-layer.

Atmospheric electromagnetic pulse propagation effects from thick targets in a terawatt laser target chamber

Generation and effects of atmospherically propagated electromagnetic pulses (EMPs) initiated by photoelectrons ejected by the high density and temperature target surface plasmas from multiterawatt laser pulses are analyzed. These laser radiation pulse interactions can significantly increase noise levels, thereby obscuring data (sometimes totally) and may even damage sensitive probe and detection instrumentation. Noise effects from high energy density (approximately multiterawatt) laser pulses (approximately 300-400 ps pulse widths) interacting with thick approximately 1 mm) metallic and dielectric solid targets and dielectric-metallic powder mixtures are interpreted as transient resonance radiation associated with surface charge fluctuations on the target chamber that functions as a radiating antenna. Effective solutions that minimize atmospheric EMP effects on internal and proximate electronic and electro-optical equipment external to the system based on systematic measurements using Moebius loop antennas, interpretations of signal periodicities, and dissipation indicators determining transient noise origin characteristics from target emissions are described. Analytic models for the effect of target chamber resonances and associated noise current and temperature in a probe diode laser are described.

Reducing noise from an oil refinery catalytic distillation column

This paper concerns the reduction of the exhaust noise from a Catalytic Distillation Column, commonly called a Cat Cracker. A Cat Cracker is used to convert heavy oil into gasoline products. Following the upgrading of the Cat Cracker, there were persistent community complaints of an irregularly varying noise that sounded like an overflying jet aircraft. The paper describes a detailed study of the Cat Cracker noise involving: field tests on-plant and in the community; scale model tests in the laboratory; theoretical predictions of the in-stack sound power level; and a study of atmospheric propagation effects using the Parabolic Equation method. The objectives of the study were to i) identify methods of reducing the noise levels, and ii) establish the cause of the irregularity of the noise levels in the community. The laboratory tests used a 1 3 scale model to explore qualitatively the nature of any potential interaction between the two principal elements in the stack. The study concluded that the cause of the noise was an interaction between the turbulent flow from a valve and a nearby Multi-Holed Orifice (MHO) plate downstream of the valve. The irregular variations in the noise were predicted to be atmospheric effects. Following the investigation the valve and the MHO were subsequently replaced by three MHO's in series which gave a reduction in noise levels at the stack tip of up to 14 dB. Noise measurements in the community demonstrated a similar level of noise reduction. In some weather conditions the Cat Cracker noise can still be heard, albeit at a much reduced level.

Numerical analysis of the scaling laws about focused beam spreading induced by the atmosphere

Using typical sets of numerical parameters under characteristic laser beam propagation conditions, focused beam spreading induced by atmospheric turbulence and thermal blooming is analyzed by numerical simulation. In the analysis of the combined effect, we assume that the thermal blooming effect arises in consequence of the increase in beam radius induced by atmospheric turbulence and beam jitter. The scaling relations between the laser propagation effects and characteristic parameters describing the propagation in atmosphere are obtained. Based upon these relations, we can efficiently estimate atmospheric propagation effects and learn the performance of a particular system design under various environmental conditions, as well as conduct laser system design optimization.

L-band radiometers measuring salinity from space: atmospheric propagation effects

Microwave radiometers can measure sea surface salinity from space using L-band frequencies around 1.4 GHz. However, requirements to the accuracy of the measurements, in order to be satisfactory for the user, are so stringent that the influence of the intervening atmosphere cannot be neglected. The present paper will describe and quantify the effect of losses in the atmosphere caused by oxygen, water vapor, clouds, and rain, and indicate possible correction actions to be taken.

Azimuth Resolution of Spaceborne P,VHF-Band SAR

A degradation of spatial resolution performance of spaceborne synthetic aperture radar (SAR) images, which is due to the atmospheric radio waves propagation effects, is considered in this letter. A special calculation procedure of spatial resolution performances, in particular of the SAR azimuth resolution, is offered. Herein, the analysis of a potential spatial resolution, resulting from the influence of the atmosphere, on SAR images is presented.

Cell-Site Diversity Performance of LMDS Systems Operating in Heavy Rain Climatic Regions

In this paper, the potential performance advantages of ce ll-site diversity in millimeter-wave fixed cellular systems located in heavy rain climatic regions are examined. Cell-site diversity is considered an efficient technique to mitigate the aggravating effects of atmospheric propagation of radiowaves at millimeter frequency bands and, particularly, to combat rain attenuation. The paper is focused on the analytical estimation of the performance improvement of LMDS systems due to diversity reception. A physical prediction model for rain attenuation, properly modified to take into account the behaviour of the rainfall medium in tropical and subtropical areas, is the basis of the analysis. Implementations of the proposed model are presented along with an investigation of its various aspects. This work was mainly stimulated by the recent strong interest to provide technically and economically feasible solutions for broadband wireless access to the developing countries.

Simulation of the performance of a Ka‐band VSAT videoconferencing system with uplink power control and data rate reduction to mitigate atmospheric propagation effects

AbstractThe purpose of this paper is to evaluate the impact of propagation effects at 20/30 GHz on the performances of the radio interface of a VSAT videoconferencing system and to demonstrate the interest to implement Fade Mitigation Techniques in order to improve the link availability. Specifications and system requirements of the VSAT network are briefly described. System performances of the physical layer are simulated using time‐series data measured with the ESA's OLYMPUS satellite. The simulations enable to implement and optimize Up‐Link Power Control and Data Rate Reduction, and to assess the related link performance improvement. Moreover, the introduction of propagation data sets allows to have a better idea of some system parameters which directly impact on the quality of service such as the switching rate or the link availability that such a system should have when deployed in operational conditions. Copyright © 2002 John Wiley & Sons, Ltd.

Detection algorithms for hyperspectral imaging applications

We introduce key concepts and issues including the effects of atmospheric propagation upon the data, spectral variability, mixed pixels, and the distinction between classification and detection algorithms. Detection algorithms for full pixel targets are developed using the likelihood ratio approach. Subpixel target detection, which is more challenging due to background interference, is pursued using both statistical and subspace models for the description of spectral variability. Finally, we provide some results which illustrate the performance of some detection algorithms using real hyperspectral imaging (HSI) data. Furthermore, we illustrate the potential deviation of HSI data from normality and point to some distributions that may serve in the development of algorithms with better or more robust performance. We therefore focus on detection algorithms that assume multivariate normal distribution models for HSI data.

Four years of experimental results from the New Mexico ACTS propagation terminal at 20.185 and 27.505 GHz

The Advanced Communications Technology Satellite (ACTS) propagation experiment has collected four years of propagation data at 20.185 and 27.505 GHz. The objective of the experiment is to develop long-term statistics and modeling techniques for predicting atmospheric propagation effects in the Ka band. The experiment includes seven identical earth stations at different locations in North America. Each location is meant to characterize a unique rain region. This paper presents the data collected in White Sands, NM. The data from this site provide an excellent resource for validating rain attenuation models due to its unique arid climate with occasional high rain-rate storms. The seasonal and cumulative four-year attenuation statistics for the 20.2 and 27.5 GHz beacons are presented. The attenuation with respect to clear air (ACA) is compared to five different rain attenuation models and seven different frequency scaling models. The results illustrate how well each model predicts rain attenuation in a desert climate region.

Coherent laser radar at 2 mu m using solid-state lasers

The development of a coherent laser radar system using 2- mu m Tm- and Tm, Ho-doped solid-state lasers, which is useful for the remote range-resolved measurement of atmospheric winds, aerosol backscatter, and differential absorption lidar (DIAL) measurements of atmospheric water vapor and CO/sub 2/ concentrations, is described. Measurements made with the 2- mu m coherent laser radar system, advances in the laser technology, and atmospheric propagation effects on 2- mu m coherent lidar performance are discussed. Results include horizontal atmospheric wind measurements to >20 km. vertical wind measurements to >5 km, near-horizontal cloud returns to 100 km, and hard target (mountainside) returns from 145 km.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The effect of atmospheric sound absorption on signal bandwidth and energy and some consequences for bat echolocation

The degradation of broadband pulses due to atmospheric absorption was examined as a potential explanation for flexible echolocation behavior. It was found that absorption can explain flexible echolocation behavior in FM/CF bats but not in CF/FM bats. The effects of absorption on target detection and the potential importance to bats of other atmospheric propagation effects are examined and discussed.

Ionospheric effects of atmospheric gravity waves: Doppler shift of radio signals

Spectra of Doppler shifts of ionospheric reflected radio signals have been obtained from the computer processing of observations made on an experimental ionospheric propagation path between England and Greece. The analysis of these Doppler shifts showed that systematic variations exist in the ionosphere and that these variations or «oscillations» of ionospheric electron density consist mainly of two approximately sinusoidal components with a mean period of 180 and 70 minutes, respectively. The analysis also showed that these oscillations have a seasonal variation. These results are compared to similar results obtained from a time and space analysis of ionospheric data from different European stations. It is concluded that these systematic ionospheric density oscillations could be considered as local effects of atmospheric gravity waves propagation at ionospheric heights.

Laser altimetry measurements from aircraft and spacecraft

The techniques involved in the design and application of laser altimeter instruments are reviewed, including a description of the instrument subsystems required for the range and waveform measurements. Laser pulse transmitters based on the relatively novel technology of diode-pumped solid-state lasers are considered. Various factors affecting laser altimeter instrument performance are discussed. These include the receiver signal-to-noise ratio, atmospheric propagation, and altimeter platform effects. Some examples of laser altimeter data are presented to illustrate the variety of possible instrument applications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Atmospheric turbulence-induced signal fades on optical heterodyne communication links

The three basic atmospheric propagation effects, absorption, scattering, and turbulence, are reviewed. A simulation approach is then developed to determine signal fade probability distributions on heterodyne-detected satellite links which operate through naturally occurring atmospheric turbulence. The calculations are performed on both angle-tracked and nonangle-tracked downlinks, and on uplinks, with and without adaptive optics. Turbulence-induced degradations in communication performance are determined using signal fade probability distributions, and it is shown that the average signal fade can be a poor measure of the performance degradation.

Uniqueness of statistics derived from moments of irradiance fluctuations in atmospheric optical propagation

This paper describes a new and very simple technique to examine the uniqueness criteria of some probability density functions (PDF) of irradiance fluctuations used to model the atmospheric optical propagation effects. By plotting the normalized function ( m −1 (2k) k m −1 2 1 ) derived from the kth moment m k , the rate of growth of m k is studied. Examples to assess the applicability and utility of the technique include scintillations due to speckles and turbulence as well as experimental results for laboratory-simulated and outdoor atmospheric turbulence.