Vertical Arrival Angle Research Articles

Multi‐Frequency SuperDARN Interferometer Calibration

AbstractThe ground‐based, high‐frequency radars of the Super Dual Auroral Radar Network (SuperDARN) observe backscatter from ionospheric field‐aligned plasma irregularities and features on the Earth's surface out to ranges of several thousand kilometers via over‐the‐horizon propagation of transmitted radio waves. Interferometric techniques can be applied to the received signals at the primary and secondary antenna arrays to measure the vertical angle of arrival, or elevation angle, for more accurate geolocation of SuperDARN observations. However, the calibration of SuperDARN interferometer measurements remains challenging for several reasons, including a 2π phase ambiguity when solving for the time delay correction factor needed to account for differences in the electrical path lengths between signals received at the two antenna arrays. We present a new technique using multi‐frequency ionospheric and ground backscatter observations for the calibration of SuperDARN interferometer data, and demonstrate its application to both historical and recent data.

Climatology of HF Propagation Characteristics at Very High Latitudes From SuperDARN Observations

AbstractConventional forecasting of high‐frequency (HF, 3–30 MHz) radio wave propagation is based on a combination of ionospheric and propagation models. However, at very high latitudes this approach is seriously undermined by the intrinsically dynamic ionospheric conditions regularly perturbed by energetic particle precipitations and strong electric fields. From this perspective, the multi‐year observations of HF propagation characteristics by Super Dual Auroral Radar Network (SuperDARN) radars across auroral and polar cap regions represent a unique opportunity for systematic validation of the conventional approach, as well as for creating an empirical propagation model directly from the radar observations. Qualitative identification and quantitative characterization of the propagation modes requires an accurate knowledge of the vertical angle of arrival (elevation angle) across the high‐latitude part of the radar network. This information has become available only in recent years, facilitated by the development of reliable data‐based calibration techniques for SuperDARN interferometry. We present the solar‐cycle/seasonal/diurnal climatology of HF propagation characteristics at very high latitudes derived from two‐frequency observations by the Rankin Inlet SuperDARN radar.

Depth estimation as a sequential process following array invariant range estimation

Range estimation based on the array invariant concept exploits the inherent properties of shallow-water waveguide propagation. Multipath arrivals observed on a vertical line array, indexed on vertical arrival angle and time difference of arrival, form what is known as a beam migration pattern in the shape of an ellipse that is a function of source range and the acoustic invariant beta. Fitting the beam migration pattern to an ellipse yields the source range. A property of this beam migration pattern is that it is independent of source depth. Source depth determines how the multipath arrivals are distributed along that ellipse. We will extend previous work on estimating range and present how to estimate depth. This decouples the two estimates and reduces a 2D search process to two 1D processes. We will explore the CRLB structure of the range and depth estimates and demonstrate the results of processing experiment data from the RADAR 2007 experiment in shallow water off the coast of Portugal.

Passive broadband source depth estimation in the deep ocean using a single vector sensor.

For an acoustic receiver deployed at the bottom of the direct arrival zone of a submerged source at short horizontal ranges in deep ocean, the interference pattern of the direct and surface-reflected acoustic arrivals shows periodic modulation, which is directly related to the source depth, source frequency, and vertical arrival angle. In this work, the interference cycle presented in the frequency domain is used to extract the broadband source depth, with the vertical arrival angle obtained from the ratio of vertical acoustic intensity and horizontal acoustic intensity from the signal recorded by a single vector sensor. Experimental results demonstrate the source depth estimation without requiring knowledge of the ocean environment.

Observation of out-of-plane ambient noise on two vector sensor moorings in Lake Travis.

Two Autonomous Underwater Multi-Dimensional Acoustic Recorders (AUMDAR) were deployed in the southeastern part of Lake Travis during the summer of 2018. Each AUMDAR system possessed a three-axis acoustic vector sensor capable of estimating the azimuthal and vertical arrival angles from discrete sound sources. A unique and complicated propagation environment existed during the experiment due to the mooring locations and the range-dependent lake bathymetry. The AUMDAR systems were almost entirely shielded from sound emanating from surface vessels to the south and southeast of the deployment location, while a larger, yet limited, direct acoustical field of view was realized to the north and northeast. During the evening hours, the low-frequency received level increased without a corresponding increase in the number of detected discrete surface vessels. During the same time, the predominant direction of the received sound pointed toward the bend in the river channel. A three-dimensional ray model was employed to assess the various arrival angles from a grid of source positions located throughout the lake. The model results are consistent with the observations and suggest that the ambient noise field originated from vessels physically located to the northwest of the sensors, but arriving at angles consistent with out-of-plane sound propagation.

Ion drift meter aboard China seismo-electromagnetic satellite

A lot of electromagnetic anomalies observed by satellites before earthquakes indicate that there is interrelation between earthquake and ionosphere.China seismo-electromagnetic satellite (CSES) is the first Chinese space-based platform of three-dimensional earthquake monitoring system.The scientific payload of plasma analyzing package (PAP) aboard CSES is designed to study the possible influence of the seismic activity on the ionospheric plasma and thereby to monitor the earthquakes from space.The PAP is made up of three sensors,retarding potential analyzer (RPA),ion drift meter (IDM),and ion capture meter (ICM).The main objective of IDM is to detect the ion bulk velocity from-3 to 3 km/s with a precision better than 20 m/s,perpendicular to the sensor-look direction. The IDM sensor consists of six-layer grids and a collector.The grid is made of beryllium copper,plated with gold.Polyimide is used to achieve electrical insulation between grids.The grid transmission rate of signal layer is designed to be 82.64%,and total transmission rate of six layers is 31.85%.To ensure the performance of IDM,the side length of the square aperture and the depth of the sensor are designed to be 40 and 20 mm,respectively.The radius of segmented planar collector is 50 mm.The arrival angle of the ions is determined by measuring the ratio between the currents from the different electrically isolated collector segments.Accordingly,velocity perpendicular to the sensor-look direction is calculated,based on arrival angle and ion velocity parallel to the sensor-look direction which is measured by the RPA. In addition,a wide-range and high-precision current measurement circuit is designed to measure the current of IDM.The preamplifier circuit has three measurement ranges,providing different amplification factors.The right measurement range is chosen automatically by the field programmable gate array (FPGA).The test results show that the circuit provides a total measuring dynamic range from 20 pA to 6A with an accuracy better than 0.4%. Finally,the method of testing in the plasma environment and the measurement results are discussed.The plasma environment test of the IDM flight model is carried out in the Institute for Space Astrophysics and Planetology,National Institute of Astrophysics (INAF-IAPS).Since the plasma source is fixed to a large volume vacuum tank,the arrival angle of the plasma with respect to the sensor-look direction is changed by horizontally or vertically mounted IDM on the rotating platform in the vacuum tank.As the platform rotates,the performance of IDM is proved by testing different ion arrival angles in vacuum tank.The ion velocities along the Y and Z axes of the spacecraft are validated by testing the horizontal arrival angle and the vertical arrival angle respectively.The IDM test data are consistent with those obtained under the setting angle of the rotating platform.The experimental results show that the detector has good performance and will fulfill the mission goal of monitoring the bulk velocity of ion,perpendicular to the sensor-look direction.

Joint distribution of power and angle of arrival for three‐dimensional channel model

With the development of multi-antenna systems, the channel mode is crucial for estimating the potential of multiple-antenna, wireless communication systems. Various channel models have been proposed. However, most of them assume that multipath signals arrive with the same power. Therefore, they can only be used to investigate a very small number of all of the received signals. In this study, the joint distribution of power of arrival, horizontal angle of arrival (AOA θ ) and vertical angle of arrival (AOA φ ) is derived. Micro-cell and macro-cell environments are investigated, respectively. The results indicate that the received signals with different power/path loss have different angle distributions. Simulations are also performed to verify the derived results. Moreover, the influence on the channel model caused by antenna height and surrounding objectives is also discussed in this study.

Method of determining the vertical angle of arrival of a signal to a towed array

The paper demonstrates the possibility of determining the vertical angle of arrival of a signal to a towed horizontal array. The estimate accuracy is determined by the size of the change in the horizontal angle of direction to the signal source during aperture synthesis. The method is checked by the results of a field experiment. Under conditions of finding the receiver array in a shadow zone with respect to the emitter and scatterer, we reveal the fact of formation of a uniray signal in the waveguide with the angle of reflection of the signal from the bottom at an angle on the order to 50° toward the horizontal.

Application of ground scatter returns for calibration of HF interferometry data

Information on the vertical angle of arrival (elevation) is crucial in determining propagation modes of high-frequency (HF, 3–30 MHz) radio waves travelling through the ionosphere. The most advanced network of ionospheric HF radars, SuperDARN (Super Dual Auroral Radar Network), relies on interferometry to measure elevation, but this information is rarely used due to intrinsic difficulties with phase calibration as well as with the physical interpretation of the measured elevation patterns. In this work, we propose an empirical method of calibration for SuperDARN interferometry. The method utilises a well-defined dependence of elevation on range of ground scatter returns. “Fine tuning” of the phase is achieved based on a detailed analysis of phase fluctuation effects at very low elevation angles. The proposed technique has been successfully applied to data from the mid-latitude Hokkaido East SuperDARN radar. It can also be used at any other installation that utilises HF interferometry.

Measurements of three-dimensional acoustic propagation in a scale-model canyon

Scale-model acoustic propagation experiments were conducted in a laboratory tank to investigate three-dimensional (3D) propagation effects induced by range-dependent bathymetry. The model bathymetry, patterned after measured bathymetric data, represents a portion of the Hudson Canyon at 1:7500 scale and was fabricated from closed-cell polyurethane foam using a computer-numerically controlled (CNC) milling machine. In the measurement apparatus, a computer-controlled positioning system locates the receiving hydrophone at user-defined points in 3D space while the stationary source hydrophone emits broadband pulsed waveforms. Precise control of the receiving hydrophone permits the creation of synthetic arrays from which horizontal and vertical beamforming is performed. Results are shown for propagation paths along and across the axis of the canyon where the received time series are post-processed to estimate travel times, transmission loss, and horizontal and vertical arrival angles. [Work supported by ONR.]

Estimation of aircraft angular coordinates using a directional-microphone array--An experimental study.

Ground reflections cause problems when estimating the direction of arrival of aircraft noise. In traditional methods, based on the time differences between the microphones of a compact array, they may cause a significant loss of accuracy in the vertical direction. This study evaluates the use of first-order directional microphones, instead of omnidirectional, with the aim of reducing the amplitude of the reflected sound. Such a modification allows the problem to be treated as in free field conditions. Although further tests are needed for a complete evaluation of the method, the experimental results presented here show that under the particular conditions tested the vertical angle error is reduced ∼10° for both jet and propeller aircraft by selecting an appropriate directivity pattern. It is also shown that the final level of error depends on the vertical angle of arrival of the sound, and that the estimates of the horizontal angle of arrival are not influenced by the directivity pattern of the microphones nor by the reflective properties of the ground.

Beam forming of the underwater sound field from impact pile driving

Observations of underwater noise from impact pile driving were made with a vertical line array. Previous studies [Reinhall and Dahl, J. Acoust. Soc. Am. 130, 1209-1216 (2011)] show that the dominant underwater noise from impact driving is from the Mach wave associated with the radial expansion of the pile that propagates down the pile at supersonic speed after impact. Here precise estimates of the vertical arrival angles associated with the down- and up-going Mach wave are made via beam forming, and the energy budget of the arrival structure is quantified.

On the Mach wave effect in impact pile driving, its observation, and its influence on transmission loss

Pile driving in water produces extremely high sound pressure levels in the surrounding underwater environment of order 10 kPa at ranges of order 10 m from the pile that can result in deleterious effects on both fish and marine mammals. In Reinhall and Dahl [J. Acoust. Soc. Am. 130, 1209-1216, Sep. 2011] it is shown that the dominant underwater noise from impact driving is from the Mach wave associated with the radial expansion of the pile that propagates down the pile at speeds in excess of Mach 3 with respect to the underwater sound speed. In this talk we focus on observations of the Mach wave effect made with a 5.6 m-length vertical line array, at ranges 8-15 m in waters of depth ~12.5 m. The key observation is the dominant vertical arrival angle associated with the Mach wave, ~17 deg., but other observations include: its frequency dependence, the ratio of purely waterborne energy compared with that which emerges from the sediment, and results of a mode filtering operation which also points to the same dominant angle. Finally, these observations suggest a model for transmission loss which will also be discussed. [Research supported by the Washington State Department of Transportation.]

Evidence of Doppler-shifted Bragg scattering in the vertical plane by ocean surface waves

A set of narrowband tones (280, 370, 535, and 695 Hz) were transmitted by an acoustic source mounted on the ocean floor in 10 m deep water and received by a 64-element hydrophone line array lying on the ocean bottom 1.25 km away. Beamformer output in the vertical plane for the received acoustic tones shows evidence of Doppler-shifted Bragg scattering of the transmitted acoustic signals by the ocean surface waves. The received, scattered signals show dependence on the ocean surface wave frequencies and wavenumber vectors, as well as on acoustic frequencies and acoustic mode wavenumbers. Sidebands in the beamformer output are offset in frequency by amounts corresponding to ocean surface wave frequencies. Deviations in vertical arrival angle from specular reflection agree with those predicted by the Bragg condition through first-order perturbation theory using measured directional surface wave spectra and acoustic modes measured by the horizontal hydrophone array.

Depth and range tracking of sperm whales in the Gulf of Alaska using a two-element vertical array, satellite and bioacoustic tags

A two-element vertical array was deployed between August, 15 and 17 2010, on the continental slope off Southeast Alaska, in 1200 m water depth. The instruments were attached to a longline fishing anchorline, deployed at 300 m depth, close to the sound-speed minimum of the deep water profile. The anchorline also served as a decoy, attracting seven depredating sperm whales to the area. Three animals were tagged with a satellite tag and one of them was tagged with both a satellite and bioacoustic “BProbe” tag. Both tags recorded dive depth information. Relative arrival times of surface- and bottom-reflected paths are used to estimate animal range and depth on a single hydrophone, and compared with tagging results. The two-element array is then used to estimate vertical arrival angles of the direct and surface-reflected paths to determine whether range and depth localization can occur without the use of bottom multipath. This data will be useful in determining whether long-range tracking of sperm whales is possible using a single compact instrument deployment. Potential applications include observing what ranges whales are willing to travel to depredate. [Work conducted under the SEASWAP program, supported by the National Oceanic and Atmospheric Administration and the North Pacific Research Board.]

Underwater Mach wave radiation from impact pile driving: Theory and observation

The underwater noise from impact pile driving is studied using a finite element model for the sound generation and parabolic equation model for propagation. Results are compared with measurements using a vertical line array deployed at a marine construction site in Puget Sound. It is shown that the dominant underwater noise from impact driving is from the Mach wave associated with the radial expansion of the pile that propagates down the pile after impact at supersonic speed. The predictions of vertical arrival angle associated with the Mach cone, peak pressure level as function of depth, and dominant features of the pressure time series compare well with corresponding field observations.

Acoustic response of rigid pipe wind-noise reducers for infrasound measurement.

A network of rigid pipes with multiple inlets is one commonly employed strategy to filter out pressure fluctuations generated by wind when making infrasound measurements. This talk presents a model of the transfer function of such a rosette filter using a computer model (DeltaEC) that includes thermo-viscous effects at the walls of the pipes. Measurements of this transfer function have been made utilizing the coherence between a calibrated reference microphone and a calibrated microphone shielded by the wind-reduction rosette and these data will be compared to the model. Rosettes with pipes of varied lengths were tested and modeled. The response of the rosette as a function of the vertical arrival angle of the incident plane wave will also be presented. [Work supported by US Army Space and Missile Defense Command.]

Utilization of the pressure gradient along a sparse vertical line array to determine vertical arrival angle from partial reflections from a layered seabed.

Pressure signals measured by two four-element vertical line arrays, centered at depths 25 and 50 m, during an experiment off the New Jersey coast in Summer 2006 (SW06, depth 80 m) are combined to determine the pressure gradient of the received signal. The pressure gradient is converted to an estimate of vertical particle velocity, which combined with an estimate of the mean-squared pressure, and the plane wave impedance establishes a non-dimensional quantity. This quantity is inverted to determine arrival angle estimates from separate multipath arrivals of the transmitted signals (1 kHz). A geoacoustic model for the seabed [Choi et al., J. Acoust. Soc. Am. 124 EL128–EL134 (2008)] includes a prominent sub-bottom reflector at depth 20 m below water sediment interface, which produces an energetic arrival time separated from the bottom arrival. The arrival angle estimates are examined as the source moves out in range from 50 to 300 m, and results are compared to arrival angle estimates derived from PE simulations. The performance of this gradient-based estimator is evaluated for various types of noise sources and levels, utilizing both real and PE-simulated time series. [Work supported by ONR.]

Underwater acoustic measurements of Doppler-shifted Bragg scattering in the vertical plane from interaction with the ocean surface.

High resolution beamformer output in the vertical plane for received fields from narrowband acoustic tones transmitted at 280, 370, 535, and 695 Hz shows evidence of Doppler-shifted Bragg scattering by the ocean surface waves. The sidebands in the beamformer output are offset in frequency by amounts corresponding to ocean surface wave frequencies, while deviations in vertical arrival angle agree with those predicted by the Bragg condition through first-order perturbation theory using measured directional surface wave spectra and measured acoustic mode wave numbers. The acoustic measurements were made by a horizontal hydrophone array on the ocean bottom in a shallow water waveguide under a variety of ocean surface conditions. [Work supported by ONR, Code 321(US).]

The evolution of vertical spatial coherence with range from source.

Vertical spatial coherence for shallow water propagation at frequencies 1–10 kHz is studied as function of range (50 to 5000 m), as part of the Shallow‐Water 2006 program that took place off the coast of New Jersey in August 2006 in waters 80 m deep. An acoustic source was deployed from the R/V Knorr at depths 30 and 40 m and signals were recorded on a moored receiving system consisting of two 1.4 m long vertical line arrays (VLA) centered at depths 25 and 50 m. At all ranges, spatial coherence, Γ (normalized spatial correlation), is locally stationary and depends on element vertical separation d up to the maximum kd (59) afforded by the VLA, where k is acoustic wave number. For range normalized by depth, r*, less than about 10, Γ is oscillatory, with non‐zero imaginary part, reflecting the inclusion of multipaths for which no single path dominates. For r* greater than 10, Γ tends to exhibit a monotonic decay with kd and the imaginary part vanishes reflecting symmetry about 0 deg vertical arrival angle. T...