Atmospheric Propagation Effects Research Articles

Examining Infrasound Propagation at High Spatial Resolution Using a Nodal Seismic Array

AbstractInfrasound—acoustic waves in the atmosphere below 20 Hz—is a useful monitoring tool. Topography and atmospheric structure strongly control infrasound propagation, and at common source–receiver distances neither of these effects can be ignored when quantitative source constraints are sought. Detailed spatial measurements of the infrasound wavefield would inform propagation models and improve source estimates. However, the “large‐N” deployment strategy now well‐known in seismology has not yet been realized for infrasound studies. Here, we use the 900‐node seismic array from the 2014 Imaging Magma Under St. Helens (iMUSH) experiment as a proxy for a large‐N infrasound network, by leveraging acoustic–seismic coupled arrivals. The active‐source component of iMUSH consisted of 23 shallowly buried explosions around Mount Saint Helens volcano; these explosions produced epicentral infrasound recorded on the nodes. We find that the bulk presence of ground‐coupled infrasound on the nodes is controlled by wind noise and source–receiver distance, with observed arrivals for eight explosions. Explosions with the most extensive coupling produce complex spatial waveform patterns across the array. These patterns are related to both topographic and atmospheric propagation effects, as well as spatially variable site (coupling) effects. We compare our observations to simple topographic diffraction and high‐resolution wind advection models, and full‐wave numerical simulations. We find strong spatial correlations between (a) coupled arrival strength and modeled topographic obstruction and (b) coupled arrival time and along‐path winds. Our seismoacoustic analyses and results are applicable to other existing and future nodal seismic data sets and can expand the utility of such deployments.

A Study of Tropospheric and Ionospheric Propagation Conditions during Differential Interferometric SAR Measurements Applied on Zagreb 22 March 2020 Earthquake

The differential interferometric synthetic aperture radar (DInSAR) method is based on phase variation between the complex value of pixels of timely separated scenes in interferometric SAR pairs. This phase variation has five components: surface topography, curvature of planet’s surface, terrain displacement, volume scatterers, and atmospheric propagation effects. The terrain displacement is the main product of the DInSAR method, while the last two effects are unpredictable and bring inaccuracy into the terrain displacement measurements. In this work, the propagation conditions in the troposphere and ionosphere were studied during two DInSAR measurements examining the Zagreb 22 March 2020 earthquake, with terrain raising of up to +3 cm at the epicenter. For the troposphere, the vertical profile of the modified refraction index, which incorporates local curvature change with height, was reconstructed using aerological balloon probing data. Ionospheric conditions were determined based on total electron content (TEC) calculated from the Croatian positioning system (CROPOS) and global navigation satellite system (GNSS) reference stations’ measurements. One of the DInSAR measurements was conducted in unfavorable tropospheric refractive conditions, which resulted in an overall bias of −2 cm. The variability of propagation conditions indicates the need for examining the atmospheric propagation effects when calculating terrain displacements using the DInSAR method. The results of DInSAR indicate slight displacements, comparable with the amplitude of atmospheric variations, and should therefore be approached with caution.

Squint Spotlight SAR Imaging by Two-Step Scaling Transform-Based Extended PFA and 2-D Autofocus

In this article, a novel imaging algorithm by combining two-step scaling transform (TSST) with structure-aided 2-D autofocus is proposed for the squint spotlight synthetic aperture radar (SAR). First, on the basis of planar wavefront assumption, a modified range-frequency linear scaling transform (MRFLST) and an azimuth-time nonlinear scaling transform (ATNST) are proposed to eliminate the coupling between range-frequency and azimuth-time of the received echo. Furthermore, to improve the efficiency, the MRFLST is implemented by using the principle of chirp scaling (PCS), which involves only complex multiplications and fast Fourier transforms (FFTs) without any interpolation, meanwhile, a constant scaling factor (CSF) selecting criteria is defined to avoid range spectrum aliasing. Then, to correct the phase error caused by the range measurement error and atmospheric propagation effects, the prior 2-D phase error structure implied in the TSST is analyzed. Finally, by integrating the derived 2-D phase error structure and range frequency fragmentation technique, a new 2-D autofocus algorithm is presented to improve the image quality. Simulated and real data experiments are carried out to verify the proposed algorithm.

Identifying structured light modes in a desert environment using machine learning algorithms.

The unique orthogonal shapes of structured light beams have attracted researchers to use as information carriers. Structured light-based free space optical communication is subject to atmospheric propagation effects such as rain, fog, and rain, which complicate the mode demultiplexing process using conventional technology. In this context, we experimentally investigate the detection of Laguerre Gaussian and Hermite Gaussian beams under dust storm conditions using machine learning algorithms. Different algorithms are employed to detect various structured light encoding schemes including the use of a convolutional neural network (CNN), support vector machine, and k-nearest neighbor. We report an identification accuracy of 99% under a visibility level of 9 m. The CNN approach is further used to estimate the visibility range of a dusty communication channel.

Improved Parametric Models for Explosion Pressure Signals Derived From Large Datasets

Abstract Accurate recording and characterization of explosion-induced pressure signals are key components of the forensic analysis of explosion events in the atmosphere. Parametric overpressure models based on several key waveform features (peak overpressure, positive pulse duration, and impulse) are widely used to estimate explosion energy in terms of trinitrotoluene equivalent yield. However, those models are often developed by a limited dataset, including only a few events or recordings at relatively short propagation distances. Here, we develop empirical waveform-parameter models based on a regression analysis of a large set of data curated from four chemical explosion experiments including 16 detonations. We measured peak overpressure and impulse for positive and negative phases from ∼1000 pressure signals recorded at local ranges (<20 km) with scaled distance up to 8000 m/kg1/3. The measured waveform parameters showed large variation with respect to observing distances indicating the effects of atmospheric propagation. In this study, a second-order polynomial model was used in a least-squares regression to account for those propagation effects and to improve data fitting. In addition to model parameters for waveform features, we also determined range-dependent model uncertainties based on data variance. The model uncertainty represents the prediction error of our models and can be critical to evaluating the uncertainty of yield estimate.

The Alphasat Aldo Paraboni scientific and communication experiments at Ka and Q/V bands in Austria

SummaryThe use of higher frequencies for satellite multimedia communication systems calls for research of the atmospheric propagation effects at these bands (rain, cloud and gaseous attenuation, scintillation, and depolarization). Alphasat was successfully launched on 25 July 2013. This largest and most powerful European telecommunication satellite carries, besides a commercial payload belonging to the mobile satellite communication provider Inmarsat, several Technology Demonstration Payloads (TDPs) from ESA. One of them is the Aldo Paraboni payload (TDP5) for Q/V‐band communication and Ka/Q‐band propagation experiments. These experiments explore future applications in satellite communication and measure how the Earth's atmosphere affects the propagation of electromagnetic waves. Under ESA contract, JOANNEUM RESEARCH designed, developed, and operates a Q/V‐band communication ground station and a Ka/Q‐band propagation terminal. The experimental site is equipped with ancillary equipment including a multifrequency radiometer profiler, a 2D video disdrometer (2DVD), and meteorological stations. This paper reports on the experimental setup, data processing, and obtained results.

高能激光大气传输性能评估技术

The actual effect of laser propagation in the atmosphere is not only related to its own propagation mechanism, but also closely related to atmospheric factors. Therefore, to accurately evaluate the effect of laser atmospheric propagation, it is not only necessary to carry out the mechanism research and establish the mathematical model of laser atmosoheric propagation, but also mecessary to carry out measurement, analysis and prediction of the optical characteristics of the atmosphere in the propagating process. Considering the complexity of the real atmospheric environment and the interaction of many factors, most of the researches adopt the physical experiment or the numerical simulation with controlled parameters to quantitatively reveal the influence of various atmospheric factors on laser propagation. Based on a large number of field measurements and the physical correlation analysis, the research of atmospheric optical characteristics focuses on the parametric modeling of optical turbulence. The development of the performance evaluation technology for the high energy laser was briefly introduced when propagating in the atmosphere at home and aboard, and its development trend for the practical application of high energy laser system in the future was pointed out.

Gamma Ray Showers Observed at Ground Level in Coincidence With Downward Lightning Leaders

AbstractBursts of gamma ray showers have been observed in coincidence with downward propagating negative leaders in lightning flashes by the Telescope Array Surface Detector (TASD). The TASD is a 700‐km2 cosmic ray observatory located in southwestern Utah, USA. In data collected between 2014 and 2016, correlated observations showing the structure and temporal development of three shower‐producing flashes were obtained with a 3‐D lightning mapping array, and electric field change measurements were obtained for an additional seven flashes, in both cases colocated with the TASD. National Lightning Detection Network information was also used throughout. The showers arrived in a sequence of 2–5 short‐duration (≤10 μs) bursts over time intervals of several hundred microseconds and originated at an altitude of ≃3–5 km above ground level during the first 1–2 ms of downward negative leader breakdown at the beginning of cloud‐to‐ground lightning flashes. The shower footprints, associated waveforms and the effect of atmospheric propagation indicate that the showers consist primarily of downward‐beamed gamma radiation. This has been supported by GEANT simulation studies, which indicate primary source fluxes of ≃1012–1014 photons for 16° half‐angle beams. We conclude that the showers are terrestrial gamma ray flashes, similar to those observed by satellites, but that the ground‐based observations are more representative of the temporal source activity and are also more sensitive than satellite observations, which detect only the most powerful terrestrial gamma ray flashes.

Radio Wave Satellite Propagation in Ka/Q Band

In 2013 the European Space Agency, in cooperation with Inmarsat, launched the Alphasat communication satellite hosting four Technology Demonstration Payloads (TDPs). One of them is the Aldo Paraboni payload, supported by the Italian Space Agency (ASI) and executed by ESA in the framework of the Advanced Research in Telecommunications Systems (ARTES) 8 Telecom program. In addition to the Communication experiment, it includes the Alphasat Scientific Experiment transmitting coherent beacon signals at Ka-band (19.701 GHz) and Q-band (39.402 GHz). The satellite supports a Europe-wide experiment to investigate the atmospheric propagation effects occurring in Ka and Q bands. The demand for increasing bandwidth in the satellite radio communication domain is moving the communication channels to the higher frequency bands. Hence for both research and commercial purposes is especially important to effectively explore the Q band that is affected by attenuation, depolarization and scintillation due to different atmospheric effects. In 2014 the Department of Broadband Infocommunications and Electromagnetic Theory at Budapest University of Technology and Economics joined the ASAPE (AlphaSat Aldo Paraboni Experimenters) group and developed a ground station to be installed in Budapest. This work was supported by the European Space Agency under its Plan for European Cooperating States program. Our paper gives the background of the Alphasat Scientific Experiment and overviews the design phases of the receiver station in Budapest. We present also the processing and validation of data recorded so far and our future experimenting plans.

The Power of Simultaneous Multi-frequency Observations for mm-VLBI: Beyond Frequency Phase Transfer

Abstract Atmospheric propagation effects at millimeter wavelengths can significantly alter the phases of radio signals and reduce the coherence time, putting tight constraints on high-frequency Very Long Baseline Interferometry (VLBI) observations. In previous works, it has been shown that non-dispersive (e.g., tropospheric) effects can be calibrated with the frequency phase transfer (FPT) technique. The coherence time can thus be significantly extended. Ionospheric effects, which can still be significant, remain however uncalibrated after FPT as well as the instrumental effects. In this work, we implement a further phase transfer between two FPT residuals (i.e., so-called FPT-square) to calibrate the ionospheric effects based on their frequency dependence. We show that after FPT-square, the coherence time at 3 mm can be further extended beyond 8 hr and the residual phase errors can be sufficiently canceled by applying the calibration of another source, which can have a large angular separation from the target ( ) and significant temporal gaps. Calibrations for all-sky distributed sources with a few calibrators are also possible after FPT-square. One of the strengths and uniqueness of this calibration strategy is the suitability for high-frequency all-sky survey observations including very weak sources. We discuss the introduction of a pulse calibration system in the future to calibrate the remaining instrumental effects, allowing the possibility of imaging the source structure at high frequencies with FPT-square, where all phases are fully calibrated without involving any additional sources.

Coastal Sea-Level Measurements Based on GNSS-R Phase Altimetry: A Case Study at the Onsala Space Observatory, Sweden

The characterization of global mean sea level is important to predict floods and to quantify water resources for human use and irrigation, especially in coastal regions. Recently, the application of global navigation satellite system reflectometry (GNSS-R) for water level monitoring has been successfully demonstrated. This paper focuses on the retrieval of sea surface height within a field experiment that was conducted at the Onsala Space Observatory (OSO) using the phase-based altimetry method. A continuous phase tracking algorithm, which relies on the GNSS amplitude and phase observations is proposed and works even under rough sea conditions at OSO’s coast. Factors impacting the phase-based altimetry model, i.e., atmospheric propagation effects of the GNSS signals and influence of the GNSS-R observation instrument, are discussed. The relationship between the yield of coherent GNSS-R compared to the overall recorded events and the wind speed is investigated in detail. Ground-based sea-level measurements from June 10 to July 3, 2015 demonstrate that altimetric information about the reflecting water surface can be obtained with a root mean square error of 4.37 cm with respect to a reference tide gauge (TG) data set. The sea surface changes, derived from our field experiment and the reference TG, are highly correlated with a correlation coefficient of 0.93. The altimetric information can be retrieved even when the sea surface is very rough, corresponding to wind speeds up to 13 m/s. Moreover, the use of inexpensive conventional GNSS antennas shows that the system is useful for future large-scale sea level monitoring applications including numerous low-cost coastal ground stations.

What colour is penguin guano?

AbstractThe identification and quantification of Antarctic Pygoscelis penguin colonies depends increasingly on recognition of the characteristic optical properties of guano deposits, but almost all knowledge of these properties until now has been compromised by resolution and atmospheric propagation effects. Here we present hyperspectral reflectance data in the range 350–2500 nm, collected in situ from fresh guano deposits in Pygoscelis penguin colonies on Signy Island, South Orkney Islands. The period of data collection included the transition from predominantly white guano to the pink coloration characteristic of a krill-rich diet. The main identifiable features in the spectra are a broad absorption feature centred around 550 nm, responsible for the pink coloration and identified with the pigment astaxanthin, as well as several water absorption features. Variations in these features are responsible for differentiation between spectra. From these results we propose two spectral indices suitable for use with satellite data, one of which responds to the presence of astaxanthin in the guano and the other to water. Our results do not allow us to differentiate between penguin species from their guano, but do suggest that the breeding phenology of Pygoscelis penguins could be determined from a time series of multispectral imagery.

Linearization theory of small scale thermal blooming effect in non-Kolmogorov turbulent atmosphere

High energy laser beams propagating in the atmosphere are subjected to a variety of effects, such as the absorption and scattering of molecule and aerosol, atmospheric turbulence effects, thermal blooming effects, and the interaction between turbulence and thermal blooming. In general, these atmospheric propagation effects degrade laser beam quality and reduce the beam power concentration at the target. With adaptive optics compensation, the beam quality can be modified. But small-scale perturbation has developed and the phase compensation becomes unstable in some conditions. The performance of adaptive-optics system is degraded, which effects can be well explained by small-scale linear theory of thermal blooming. However previous theoretical studies of small-scale thermal blooming focused on the Kolmogorov turbulence. In the past decade, experimental evidence has shown significant deviations from Kolmogorov model in certain portions of the atmosphere. An generalized power-law of non-Kolmogorov turbulence model has been introduced, which becomes quite popular in the optical propagation community. Numerous theoretical and developmental efforts have been made based on non-Kolmogorov turbulence model in recent years. Thus it is very meaningful and imperative to explore the theoretical mechanism of high energy laser phase compensation with non-Kolmogorov turbulence.In this study, the Strehl ratio of the thermal blooming phase compensation is generalized with the non-Kolmogorov turbulence spectrum, and the analytical expression is obtained based on the linear theory of small-scale thermal blooming. The influence of the turbulence spectrum on the phase compensation of the high energy laser is analyzed. The results show that the turbulence spectrum has an important influence on the phase compensation of turbulent thermal blooming effect. Under the same turbulence Fresnel number condition, the compensation effect is worse when the spectral index is closer to 3 and the compensation effect is better when the spectral index is close to 4. Under the same atmospheric coherence length condition or under the same turbulence refractive index constant condition, the Strehl ratio decreases with the increase of the thermal blooming effect when the spectral index is close to 3 and the decline rate of the Strehl ratio is slower when the turbulence spectrum index is close to 4. This is because as the turbulence spectrum exponent increases, the logarithmic amplitude fluctuation slows down due to the interaction between turbulence and thermal blooming. These theoretical results can provide some scientific bases and theoretical guidance for the practical applications of high energy laser transmission.

Investigation of Resonator BWO-Based Spectroscopy of the Atmosphere in the THz Region

This paper intends to give a code about atmospheric propagation effects affecting terahertz (THz) communication system. The main focus is on attenuation caused by atmospheric gases with the radiation transmission theory and the empirical continuum absorption based on the HITRAN database. Theoretical aspects about them are presented, emphasizing on those which deserve special attention as frequency increases. Laboratory measurements of the absorption spectra of laboratory air and major atmospheric gases mixed with water vapor in the 250 - 350 GHz frequency range at atmospheric pressure and room temperature on a basis of backward wave oscillators (BWOs) are obtained. The results of experiments are compared with the calculations. It is found that the water vapor transmittance is greater than the calculation. Data of these measurements agree with the results of analysis of atmospheric spectra with in statistical accuracy of experiments. Accurate measurements are also needed for further studies of the physics of the molecules and their interactions. The investigation makes it significant for enhancing accuracy of models of radiation propagation in the atmosphere.

Atmospheric Attenuation in Wireless Communication Systems at Millimeter and THz Frequencies [Wireless Corner

This paper intends to give an overview about atmospheric propagation effects affecting millimeter and terahertz (THz) communication systems. The main focus is on attenuation caused by atmospheric gases and liquid water droplets, either in the form of suspended particles into clouds or rain falling hydrometeors. Theoretical aspects about each of them are presented, emphasizing on those that deserve special attention as frequency increases. Statistics of attenuation estimated from meteorological data and some experimental results, as in the case of rain attenuation, obtained in Madrid, Spain, are presented throughout the paper, thus providing further insights about the phenomena discussed.

Influence of atmospheric structure and topography on infrasonic wave propagation

The effects of topography and atmospheric structures on infrasonic wave propagation from a volcanic source were investigated using observations and numerical modeling. This paper presents the first long‐term observational data set showing spatiotemporal variations in patterns of infrasound propagation at distances of up to 60 km from a persistently active infrasound source (Sakurajima Volcano, Japan). The data show that the amplitudes of infrasonic waves received at distant stations relative to those received at a reference station close to the source can vary up to an order of magnitude over short time intervals and short distances and that they do not follow the theoretical geometric decay expected for homogeneous media. Moreover, waveforms also change significantly in both time and space. Numerical simulations were performed using a two‐dimensional finite difference time domain (2‐D FDTD) method. Effects of atmospheric structure and topography are included in a vertical section parallel to the wave propagation direction. The simulation successfully reproduced the variations of amplitudes and waveforms. Results are interpreted in terms of wave refraction due to sound and wind speed gradients and wave diffraction at topographic barriers. Our numerical results indicate that both atmospheric and topographic propagation effects are nonnegligible. To evaluate the propagation effects and determine source processes in spatially and temporally varying infrasound data, atmospheric data with a time resolution higher than is currently available are required. If the data are available, the present results suggest that the propagation effects could be evaluated using 2‐D FDTD modeling at realistic calculation times.

Validation of atmospheric propagation models in littoral waters

Various atmospheric propagation effects are limiting the long-range performance of electro-optical imaging systems. These effects include absorption and scattering by molecules and aerosols, refraction due to vertical temperature gradients and scintillation and blurring due to turbulence. In maritime and coastal areas, ranges up to 25 km are relevant for detection and classification tasks on small targets (missiles, pirates). From November 2009 to October 2010 a measurement campaign was set-up over a range of more than 15 km in the False Bay in South Africa, where all of the propagation effects could be investigated quantitatively. The results have been used to provide statistical information on basic parameters as visibility, air-sea temperature difference, absolute humidity and wind speed. In addition various propagation models on aerosol particle size distribution, temperature profile, blur and scintillation under strong turbulence conditions could be validated. Examples of collected data and associated results are presented in this paper.

Comparison of hourly variations of radio refractivity for quiet and disturbed days during dry and rainy seasons at Minna

In principle, the EM wave propagation characteristics during quiet or disturbed air conditions are straightforwardly determined by the state of atmospheric refractivity. Nevertheless, atmospheric refractivity varies in time and space more or less randomly. Therefore the statistics of atmospheric refractivity and related propagation effects are of main interest. This work clearly investigates the major difference between radio refractivity changes for quiet and disturbed days during dry and rainy season for Minna. Radio refractivity values were calculated from measured meteorological parameters (relative humidity, temperature and pressure) at Minna, in 2008. The result showed that, there is a sharp decrease in radio refractivity at the early hour of the day (6:00hr) during dry season, while during rainy season, the sharp decrease occurred at the noon time (13:00hr). The result equally showed that radio refractivity has higher values ~388N-units during rainy season than during dry season with the highest value of ~318N-units. This is due to decrease in relative humidity during dry season than during rainy season.

Three-Dimensional Positioning of Point Scatterers Based on Radargrammetry

This paper presents a new technique for the retrieval of 3-D point scatterer (PS) location and resolution cell configuration exploiting only the intensity of synthetic aperture radar (SAR) images. The implemented method is based on the principle of radargrammetry and makes use of Bayesian inference, wherein, directed graphs are utilized to represent dependencies of random variables and probability density functions are modeled by particle filter representations. Compared to published radargrammetric work, the newly developed algorithm optimally exploits stacks of acquisitions taken with at least three different incidence angles. Due to the large angular diversity, the method is insensitive to atmospheric propagation effects and motion of the scatterers in contrast to the coherent phase-based techniques such as persistent scatterer interferometry and SAR Tomography. Additionally, the method estimates absolute height, which is a big advantage compared to the relative estimates provided by the phase-based techniques, which moreover encounter phase unwrapping errors and temporal decorrelation. High-resolution spotlight TerraSAR-X data of Berlin central station is used as a processing example for this technique. The test case demonstrates the unambiguous absolute height estimation of PSs and resolving of complicated scattering situations (e.g., layover) in urban areas.

Atmospheric range correction for two-frequency SLR measurements

It has been widely known that the use of two-frequency Satellite Laser Ranging (SLR) system is limited by stringent precision requirements of the range measurements and the proper atmospheric model. Owing to the stringent requirements, this SLR system is impractical for the current requirement of SLR measurements within the framework of global geodetic observing system (GGOS). If in the future this stringent requirement could be met, this SLR system would be an attractive tool to reduce atmospheric propagation effects of SLR and would be of great benefit for the next generation of GGOS design. To anticipate possible future developments of the two-frequency SLR systems, we have developed a new atmospheric correction formula for the two-frequency SLR measurements. The new formula eliminates the total atmospheric density effect including its gradient and provides two terms to calculate the curvature effect and the water vapor distribution effect. While the curvature effect can be calculated by an accurate model, the required information about the water vapor distribution along the propagation path can be calculated using previous developments of optical delay modeling or alternatively using results from microwave measurements. Theoretical simulations using the two-frequency systems of the Graz and TIGO-Concepcion stations shows that the new formula completely reduces all propagation effects at any elevation angle above 3° with an accuracy better than 1 mm. However, the required precision for the difference of the two-frequency SLR measurements, i.e. better than 45 μm for a single epoch, exceeds the capability of the current state of the art SLR systems.