Receiver Positions Research Articles

Study on the Near-Field Holography Measurements of Slant-Axis Terahertz Antennas under Cassegrain Mode

The surface accuracy of the reflector antenna directly affects the antenna efficiency, and is one of the key performance indicators of the telescopes. Near-field radio holography, which features the advantages of high measurement accuracy and convenience, is one of the most commonly used surface alignment methods for millimeter and submillimeter wave radio telescopes. Near-field radio holography under Cassegrain mode can measure the overall deformation error in the optical path of the telescope. The slant-axis mount is more adaptable to the extreme site environment of terahertz telescopes, and can provide better thermal protection and control for the overall telescope. However, the unique rotation characteristics of the slant-axis antennas will introduce additional error factors during the holography measurements, and devoted study on the near-field holography of slant-axis terahertz under Cassegrain mode has not been reported. In this paper, the influence of the reference receiver position and diffraction of the secondary mirror has been analyzed. The near-field holography measurements have been conducted on a 1.2 m slant-axis terahertz antenna. The repeatability of the experimental system has been evaluated to be better than 2.0 μm RMS (Root Mean Square), and the comparison between near-field holography and photography has been performed to verify the correctness of the error analyses and corrections.

Investigation of sonic boom waveforms and metrics at elevated receiver locations

Microphones used for aircraft noise measurements are sometimes positioned above local ground level, for example to accommodate environmental protection or to comply with certification regulations. Acoustic waves reflected from the ground can interfere with waves propagating directly to elevated receivers. A modeling study was conducted of sonic boom waveforms received at ground level versus at elevated positions, using source characteristics for an early X-59 low-boom aircraft configuration (C609) and a variety of atmospheric conditions. Ground impedance effects from different ground cover types were also considered. Among several acoustic metrics, it was found that perceived level of loudness (PL) and A-weighted sound exposure level were in general the most sensitive to differences due to receiver position. Location within the boom carpet also affected receiver levels based on the angle between the incoming ray and the ground: the difference between metrics at ground and elevated receivers was generally higher at undertrack locations where that angle was largest. Depending on atmospheric conditions and receiver location within the carpet, predicted PL at an elevated receiver was lower than the level at the ground by up to 5.0 dB for a receiver height of 4 ft and 2.2 dB lower for a 1.6-ft receiver.

How much COVID-19 face protections influence speech intelligibility in classrooms?

The ongoing pandemic caused by the COVID-19 virus is challenging many aspects of daily life. Several personal protective devices have become essential in our lives. Face protections are mostly used in order to stop the air aerosol coming out of our mouths. Nevertheless, this fact may also have a negative effect on speech transmission both in outdoor and indoor spaces. After a severe lockdown, classes have now started again. The adoption of face protection by teachers is either recommended or mandatory even though this is affecting speech intelligibility and thus students’ comprehension. This study aims to understand how protections may affect the speech transmission in classrooms and how this could be influenced by the several typologies of face protections. An experimental campaign was conducted in a classroom in two different reverberant conditions, measuring and comparing the variation in speech transmission and sound pressure level at different receiver positions. Furthermore, a microphone array was used to investigate the distribution of the indoor sound field, depending on the sound source. Results clearly show how different types of personal protection equipment do affect speech transmission and sound pressure level especially at mid-high frequency and that the source emission lobes vary when wearing certain types of personal devices.

Seismic detection and characterization of a man-made karst analog — A feasibility study

At the site of a water drainage shaft on the campus of Qatar University that serves as a man-made karst analog, two seismic imaging techniques were adapted to use resonant scattered waves recorded during active-source seismic surveys and during passive ambient-noise surveys. Data acquisition included two seismic transmission surveys that encompassed the shaft and a passive ambient-noise survey that extended across the top of the shaft. Seismic imaging of band-pass-filtered resonance waves correctly estimated the location and dimension of the shaft. Furthermore, the method detected the presence and the location of a horizontal drainage pipe and gravel bed connecting neighboring water shafts. Ambient-noise data were analyzed by computing amplitude values of the seismic records in spectral passbands. The results indicated an amplification of seismic amplitudes above the shaft for low-frequency passbands and a sharp decrease in amplitude values for high-frequency passbands. The high- and low-amplitude values displayed as a function of the receiver position allowed for accurate detection and location of the shaft in space. Ground truthing of the imaging results confirmed the accuracy of the seismic techniques, whereas numerical modeling supported the interpretation of the ambient-noise data. The techniques used do not require knowledge of the seismic velocities in the subsurface, but they depend on a priori information about the approximate location of the target.

Exploring Signals on L5/E5a/B2a for Dual-Frequency GNSS Precise Point Positioning.

Due to its nature, Precise Point Positioning (PPP) depends on the GNSS measurements and quality of satellite correction products used to relatively quickly provide precise and accurate positions. With the rapid evolution of Global Navigation Satellite Systems (GNSSs), new frequencies and signals are being broadcast, which have a positive impact on PPP performance. This paper presents, for the first time, a comprehensive analysis of PPP performance from these new GPS, Galileo and BeiDou-2/3 signals, which are not yet commonly used for PPP, with correct mitigation of errors such as the estimation of GPS Block-IIF L5 variations. Satellite orbits and clocks, as well as GPS Block-IIF L5 corrections, are estimated in real-time using DLR’s RETICLE engine, while the user processing is performed with York University’s PPP engine. First, as a reference, PPP performance is assessed on widely used signals: GPS L1/L2, Galileo E1/E5a, and BeiDou-2/3 B1-2/B3. Horizontal and vertical rms of 2.3 and 2.6 cm, respectively, are achieved in static processing and 5.4 and 7.5 cm in kinematic processing after 1 h of processing using real-time satellite correction products. The compatibility of BeiDou-2 and BeiDou-3 on the shared B1-2/B3 frequencies is analyzed and discrepancies in the receiver clock are found. Next, since all three constellations share two common frequencies, the paper focuses on analyzing PPP performance of GPS, Galileo and BeiDou-3 on [L1, E1, B1] at 1575.42 MHz and [L5, E5a, B2a] at 1176.45 MHz. Horizontal and vertical rms of 6.9 and 7.1 cm are achieved in kinematic processing. The effect of the known GPS Block-IIF L5 biases is studied as well, as it is shown to affect the receiver position and clock, as well as the ionospheric estimates and ambiguities. Average improvements of 15% and 20% in the horizontal and vertical rms, respectively, are observed when these biases are mitigated.

Locating Image Sources from Multiple Spatial Room Impulse Responses

Measured spatial room impulse responses have been used to compare acoustic spaces. One way to analyze and render such responses is to apply parametric methods, yet those methods have been bound to single measurement locations. This paper introduces a method that locates image sources from spatial room impulse responses measured at multiple source and receiver positions. The method aligns the measurements to a common coordinate frame and groups stable direction-of-arrival estimates to find image source positions. The performance of the method is validated with three case studies—one small room and two concert halls. The studies show that the method is able to locate the most prominent image sources even in complex spaces, providing new insights into available Spatial Room Impulse Response (SRIR) data and a starting point for six degrees of freedom (6DoF) acoustic rendering.

QZSS SLAS의 국내 가용성 검토

In this paper, the availability of SLAS information broadcast by Japanese QZS in South Korea was reviewed. First, SLAS signal characteristics, structure and target performance are described to use augmentation information. SLAS signal is constructed similarly with GPS L1 signal so that SLAS information can be used at GPS receiver. SLAS have 9 types of messages which consist of 8-bit preamble, 6-bit message type indicator, 212-bit data filed, and 24-bit CRC. The augmentation information consist of satellite PRNs, IODs, and DGPS correction data is transmitted through the messages. Then, the algorithm for calculating receiver position are described in detail. Finally the availability of SLAS signal at South Korea is verified by comparing the GPS L1-only, SLAS-augmented, and MSAS-augmented navigation performances of at Japan and South Korea through post processing.

Design and simulation of bionic compound eye with electrowetting liquid lens

To solve the problem that the bionic compound eye system can't zoom adaptively, a zoomable bionic compound eye system based on electrowetting-on-dielectric liquid lens cambered array is proposed. The influence of the system structure on the imaging performance is analyzed, and the adaptive zoom capability of the system and the moving range of the corresponding image plane are calculated. The results show that the field of view angle increases with the increase of the curvature of the base. Compared with the non-uniform lens array, the uniform lens array can significantly reduce the defocus aberration of the system. Reducing the size of the lens unit can also decrease the defocus aberration of the edge lens. When the object distance or receiver position is changed, the defocus aberration of the system will be reduced by adjusting the focal length of the lens unit. The movable range of the system receiver is 1.9 mm~15 mm.

A New UTD Based Time-Domain Solution for UWB Diffraction in 3-D Environments

In this paper, a new time-domain (TD) solution is proposed for analyzing the diffraction of ultra wideband (UWB) signals in three dimensional (3-D) scenarios. In this sense, the solution is based on uniform theory of diffraction (UTD) where the diffraction has been considered by obstacles like 3-D wedge and building. The TD solution is derived with the help of frequency-domain (FD) diffraction coefficient existing in the literature that can be used for arbitrary positions of transmitter (Tx) and receiver (Rx). Considering soft and hard polarizations, the TD expressions are obtained for both amplitude diffraction (i.e., single diffraction) and slope diffraction (i.e., double diffraction). The accuracy of the proposed solution is confirmed by comparing the TD results with the rigorous Maliuzhinets solution and also with the numerical inverse fast Fourier transform (IFFT) of the corresponding FD results. The results are shown for diffraction by objects made up of perfectly electrical conducing (PEC) and dielectric materials. It is obserevd that the TD solution exhibits symetery and reciprocity property and thus can predict the diffracted field for any arbitrary position of Tx and Rx. The generalized behavior of the TD solution is confirmed by employing different kinds of excitation pulses at the Tx. Channel impulse response (CIR) is also studied to analyze the pulse shape distortion. Finally it is shown that the proposed TD solution is computationally more efficient than the FD solution.

Single-Channel Multiple-Receiver Sound Source Localization System with Homomorphic Deconvolution and Linear Regression.

The conventional sound source localization systems require the significant complexity because of multiple synchronized analog-to-digital conversion channels as well as the scalable algorithms. This paper proposes a single-channel sound localization system for transport with multiple receivers. The individual receivers are connected by the single analog microphone network which provides the superimposed signal over simple connectivity based on asynchronized analog circuit. The proposed system consists of two computational stages as homomorphic deconvolution and machine learning stage. A previous study has verified the performance of time-of-flight estimation by utilizing the non-parametric and parametric homomorphic deconvolution algorithms. This paper employs the linear regression with supervised learning for angle-of-arrival prediction. Among the circular configurations of receiver positions, the optimal location is selected for three-receiver structure based on the extensive simulations. The non-parametric method presents the consistent performance and Yule–Walker parametric algorithm indicates the least accuracy. The Steiglitz–McBride parametric algorithm delivers the best predictions with reduced model order as well as other parameter values. The experiments in the anechoic chamber demonstrate the accurate predictions in proper ensemble length and model order.

Improving the Performance of Time-Relative GNSS Precise Positioning in Remote Areas.

Global navigation satellite systems (GNSS) can attain centimeter level positioning accuracy, which is conventionally provided by real-time precise point positioning (PPP) and real-time kinematic (RTK) techniques. Corrections from the data center or the reference stations are required in these techniques to reduce various GNSS errors. The time-relative positioning approach differs from the traditional PPP and RTK in the sense that it does not require external real-time corrections. It computes the differences in positions of a single receiver at different epochs using phase observations. As the code observations are not used in this approach, its performance is not affected by the noise and multipath of code observations. High reliability is another advantage of time-relative precise positioning because the ambiguity resolution is not needed in this approach. Since the data link is not required in the method, this approach has been widely used in remote areas where wireless data link is not available. The main limitation of time-relative positioning is that its accuracy degrades over time between epochs because of the temporal variation of various errors. The application of the approach is usually limited to be within a time interval of less than 20 min. The purpose of this study was to increase the time interval of time-relative positioning and to extend the use of this method to applications with a longer time requirement, especially in remote areas without wireless communication. In this paper, the main error sources of the time-relative method are first analyzed in detail, and then the approach to improve the accumulated time relative positioning method is proposed. The performance of the proposed method is assessed using both static and dynamic observations with a duration as long as several hours. The experiments presented in this paper show that, among the four scenarios tested (i.e., GPS, GPS/Galileo, GPS/Galileo/BeiDou, and GPS/Galileo/BeiDou/GLONASS), GPS/Galileo/BeiDou performed best and GPS/Galileo/BeiDou/GLONASS performed worst. The maximum positioning errors were mostly within 0.5 m in the horizontal direction, even after three hours with GPS/Galileo/BeiDou. It is expected that the method could be used for positioning and navigation for as long as several hours with decimeter level horizontal accuracy in remote areas without wireless communication.

Extended versus point light source: where does the difference in the illuminance exist?

In this paper we derive and analyse the expressions to find the illuminance from luminous ball, disc and line in the case of general position of the light receiver. We show that one can always replace a luminous ball with a point light source located at its center and having the appropriate luminous intensity. Any luminous disc or line can be considered, with reasonable accuracy (the relative error in the determination of the illuminance is less than $5 \%$), as the point light source with anisotropic (cosine) luminous intensity and placed at their center, if the distance to the observation point is approximately four times larger than their characteristic sizes. The issues outlined in this article will be useful for undergraduate students, who study the basics of photometry.

Source-domain full-waveform inversions

Conventional full-waveform inversion (FWI) updates a velocity model by minimizing the data residuals between the predicted and observed data at the receiver positions. We have developed a new FWI to update the velocity model by minimizing virtual source artifacts at the receiver positions in the source-domain FWI (SFWI). Virtual source artifacts are created by replacing the propagating source wavefield by the forward-time observed data at the receiver positions as a data-residual constraint. Therefore, no matter whether the velocity model is correct or not, the data residuals at the receiver positions always are forced to be zero. If the velocity model is correct, this data-residual constraint has no effect on the wavefield because the predicted data are the same as the observed data. However, if the estimated velocity model is incorrect, the mismatch between the replaced forward-time observed data and the incorrect predicted upgoing waves (e.g., the reflected waves) at the receiver positions will produce downgoing artifact waves. Thus, the data-residual constraint behaves as a virtual source to create artifact wavefields. By minimizing the virtual source artifacts (equivalent to producing the artifact wavefield), the velocity model can be iteratively updated toward the true velocity model. Similar to conventional FWI, SFWI can be implemented in either the frequency or time domain, which is unlike previous source-domain solutions which have to be implemented only in the frequency domain to solve the normal equations. SFWI does more overfitting of noisy observed data than conventional FWI does because noise is amplified by the differential operators when calculating the virtual source artifacts. Tests on synthetic data indicate that SFWI inverts for the velocity model more accurately than conventional FWI for noise-free or low-noise data.

Measurement and Prediction of Short-Range Path Loss between 27 and 40 GHz in University Campus Scenarios

In this paper, we present the results of short-range path loss measurement in the microwave and millimetre wave bands, at frequencies between 27 and 40 GHz, obtained in a campaign inside a university campus in Rio de Janeiro, Brazil. Existing empirical path loss prediction models, including the alpha-beta-gamma (ABG) model and the close-in free space reference distance with frequency-dependent path loss exponent (CIF) model, are tested against the measured data, and an improved prediction method that includes the path loss dependence on the height difference between transmitter and receiver is proposed. The main contribution of this paper is the use of the Fuzzy technique to perform path loss predictions for short links in the millimetre wave range, from 27 to 40 GHz, providing lower errors when compared to the traditional ABG and CIF models. However, it should be noted that the Fuzzy technique uses a set of equations to perform the prediction and the attenuation coefficient is not explicit as in the classical models. Also, a non-negligible correlation between the difference in height between transmitter and receiver positions and the path loss in such short links (i.e., the path inclination) has been observed and requires further investigation. If confirmed, it could provide an additional parameter to improve the accuracy of the traditional ABG model.

High-Impedance Wireless Power Transfer Transmitter Coils for Freely Positioning Receivers

In this letter, a wireless power transfer (WPT) system based on two high-impedance coil (HIC) - cable loop antennas with a modified shield - as transmitters (Tx)s and a spiral coil as a receiver (Rx) is proposed and discussed. Utilizing features of HIC at its parallel-circuit resonance frequency, we design the Tx in a way that in the absence of the Rx, the input impedance of the Tx is very high compared to the case when the Rx is near the Tx. This feature offers a possibility for free positioning of the Rx over an array of Txs and auto self-activation and de-activation of the Txs, leading to highly efficient performance. To verify the proposed solution, we have designed, fabricated, and experimentally tested a WPT system based on two HICs as Txs and one Rx. The proposed system operates in a high-frequency range (around 280 MHz) in the near-field coupling regime. The measured averaged efficiency of the prototype is higher than 93%. The proposed system is simple, cheap, and does not require any control circuit system for tuning the system when the receiver position changes.

Room Acoustical Parameter Estimation From Room Impulse Responses Using Deep Neural Networks

We describe a new method to estimate the geometry of a room and reflection coefficients given room impulse responses. The method utilizes convolutional neural networks to estimate the room geometry and multilayer perceptrons to estimate the reflection coefficients. The mean square error is used as the loss function. In contrast to existing methods, we do not require the knowledge of the relative positions of sources and receivers in the room. The method can be used with only a single RIR between one source and one receiver. For simulated environments, the proposed estimation method can achieve an average of 0.04 m accuracy for each dimension in room geometry estimation and 0.09 accuracy in reflection coefficients. For real-world environments, the room geometry estimation method achieves an accuracy of an average of 0.065 m for each dimension.

Direction-of-Arrival Estimation With Virtual Antenna Array: Observability Analysis, Local Oscillator Frequency Offset Compensation, and Experimental Results

Estimating the direction-of-arrival (DoA) of radio frequency sources is essential in many wireless applications. Traditional DoA estimation based on bearing measurements requires multiantenna arrays, which is not suitable for portable electronics for its large form factor. In this article, we improved a novelty single-antenna-based DoA estimation technique, called virtual multiantenna arrays. While the device is moving and continuously receiving signals, the DoA can be estimated by measuring the intercepted signals at several positions along the receiver's trajectory. Previous virtual array technology left two unsolved problems. Some receiver movements are not able to estimate the DoA, without a theoretical basis to screen feasible trajectories; the virtual array requires precise, relative coordinates of the receiver, which is challenging in reality. This article investigates the feasibility of the virtual array and improves its robustness by addressing these two unsolved problems. A theoretical foundation for feasible receiver trajectory determination is provided; the receiver has to move with accelerations to make the DoA observable. Also, we prove the nature that the DoA could be estimated by measuring the accelerations only, where precise receiver position is not mandatory. These results indicate that the virtual array is feasible with simple receiver movements and low-cost commercial equipment, thus significantly reducing system complexity and cost. Simulations are conducted to validate our theoretical predictions. The proof of concept implemented on a software-defined radio testbed also proves the validity and suitability of the improved DoA estimation technology in applications with form-factor constraints.

Navigation in GPS Spoofed Environment Using M-Best Positioning Algorithm and Data Association

Intentionally misguiding a global positioning system (GPS) receiver has become a potential threat to almost all civilian GPS receivers in recent years. GPS spoofing is among the types of intentional interference, in which a spoofing device transmits spoofed signals towards the GPS receiver to alter the GPS positioning information. This paper presents a robust positioning algorithm, followed by a track filter, to mitigate the effects of spoofing. It is proposed to accept the authentic GPS signals and spoofed GPS signals into the positioning algorithm and perform the robust positioning with all possible combinations of authentic and spoofed pseudorange measurements. The pseudorange positioning algorithm is accomplished using an iterative least squares (ILS). Further, to efficiently represent the robust algorithm, the M-best position algorithm is proposed, in which a likelihood-based cost function optimizes the positions and only provides M-best positions at a given epoch. However, during robust positioning, the positions evolved due to spoofed pseudorange measurements are removed to overcome GPS spoofing. In order to remove the fake positions being evolved owing to wrong measurement associations in the ILS, a gating technique is applied within the Kalman filter (KF) framework. The navigation filter is a three-dimensional KF with a constant velocity (CV) model, all the position estimates evolved at a specific epoch are observations. Besides, to enhance this technique’s performance, the track to position association is performed by using two data association algorithms: nearest neighbor (NN) and probabilistic data association (PDA). Simulations are carried out for GPS receiver positioning by injecting different combinations of spoofed signals into the receiver. The proposed algorithm’s efficiency is given by a success rate metric (defined as the navigation track to follow the true trajectory rather than spoofing trajectory) and position root mean square error (PRMSE).

Beamforming Using Uniform Spherical Arrays: Array Construction, Beam Characteristics, and Multi-Rank Transmission

5G new radio (NR) provides features such as enhanced mobile broadband (eMBB) services and massive machine-type communications (mMTC). Multiple-input multiple-output (MIMO) transmission is an essential technique to enable these features, using antenna arrays with a sufficient number of elements to generate accurate and high-gain beams to desired user locations. In newer services, various forms of mobile devices are expected to appear in diverse locations, and beamforming need to accommodate such directional diversification. However, utilization of existing array structures has a limitation in transmitting signals to arbitrary receiver positions in the three-dimensional space. Therefore, a new type of array capable of forming accurate beams in isotropic fashion is desired, and the corresponding transmission strategies need to be identified. In this paper, the construction and utilization of the three-dimensional uniform spherical array (USA) are investigated. Beamforming vectors and the basis codebook for such arrays are defined, and characteristics of the generated beams are analyzed via comparison to the beams produced by conventional arrays. Array parameters to minimize the inter-beam interference are determined, and multiuser transmission performance is evaluated using the proper configuration of the array. We also apply the radiation pattern to the antenna elements to present the sum-rate performance in practical settings.

Exploration of room acoustics coupling in Hagia Sophia of İstanbul for its different states.

İstanbul's Hagia Sophia is a monumental structure with multiple sub-spaces coupled to one another through arches. Its architectural elements have undergone alterations as its function has changed from that of a church to a mosque, a mosque to a museum, and back to a mosque. This study makes use of Hagia Sophia's rich formal and material characteristics to conduct a comprehensive investigation of room acoustics coupling. The methodology involves the application of the diffusion equation model (DEM) for sound energy flow analysis. Energy flow decays and energy flow dips are examined for almost 1000 receiver positions distributed throughout the various sub-spaces of the building. Ray-tracing (Ray-t) simulations are used to support the energy flow decay analysis conducted using DEM. The Ray-t data are subjected to Bayesian analysis to identify the decay parameters and the degree of acoustical coupling. Among the many variables, the source-receiver distance and positioning within different sub-spaces appear to be the underlying determinant of multi-slope sound decay pattern. On the other hand, the cases of multi-slope decays identified within the structure tend to weaken and single-slope cases increase when the overall absorption area increases in the mosque state due to the carpeted floor.