Reflector Height Research Articles

Raytracing atmospheric delays in ground-based GNSS reflectometry

Several studies have recognized that Global Navigation Satellite System Reflectometry (GNSS-R) is subject to atmospheric propagation delays. Unfortunately, there is little information in the peer-reviewed literature about the methods and algorithms involved in correcting for this effect. We have developed an atmospheric ray-tracing procedure to solve rigorously the three-point boundary value problem of ground-based GNSS-R observations. We defined the reflection-minus-direct or interferometric delay in terms of vacuum distance and radio length. We clarified the roles of linear and angular refraction in splitting the total delay in two components, along-path and geometric. We have introduced for the first time two subcomponents of the atmospheric geometric delay, the geometry shift and the geometric excess. We have defined corresponding atmospheric altimetry corrections necessary for unbiased altimetry retrievals. Using simulations, we examined the interferometric atmospheric delay for a range of typical scenarios, where it attained centimeter-level values at low satellite elevation angles ~ 5° for a 10-m high station. We found a linear and exponential dependence on reflector height and satellite elevation angle, respectively. A similar trend was found for the atmospheric altimetry correction, albeit with an amplified meter-level magnitude. The two delay components were similar near the horizon while the angular one vanished at zenith. For the altimetry correction components, both remained non-zero at zenith. We thus quantified the atmospheric bias in GNSS-R sea level retrievals.

Effects of Parameter Selections on Soil Moisture Retrieval Using GNSS-IR

Soil moisture is an important geophysical quantity for evaluating soil conditions. Traditional measurement methods such as remote sensing and field measurements cannot meet the requirements with high spatial and temporal resolution. Soil moisture retrieval based on Global Navigation Satellite System– Interferometry and Reflectometry (GNSS-IR) can be applied on the existing GNSS networks to solve these problems and has the great potential to complement existing soil moisture monitoring networks. Parameters (satellite elevation ranges, priori reflector height, and signal frequency) play a very important role in soil moisture retrieval using GNSS-IR and different selections could have great influence on the results. Previous research often relied on experience in the parameter selections, which was not conducive to the application and promotion of the technology. This study aims to investigate the effects of soil moisture retrieval using GNSS-IR based on different parameter selections. After describing the basic theory of soil moisture retrieval, the influence of the satellite elevation ranges, the priori reflector height, and the chosen signal frequency are investigated with National Science Foundation’s (NSF) Plate Boundary Observatory (PBO) data and experimental data obtained in Nanjing, China. The results show that using data within an elevation range of [5°, 30°] and arc lengths greater than 20° can reduce the mean RMSE and MAE by 1%-31.5% and 2%-31%. The selection of the priori reflector height does not have a significant influence. L2 signals (compared to L1 signals) can improve the accuracy of soil moisture retrieval and can reduce the mean RMSE and MAE by 20.9% and 20.4%.

Real-time sea-level monitoring using Kalman filtering of GNSS-R data

Current GNSS-R (GNSS reflectometry) techniques for sea surface measurements require data collection over longer periods, limiting their usability for real-time applications. In this work, we present a new, alternative GNSS-R approach based on the unscented Kalman filter and the so-called inverse modeling approach. The new method makes use of a mathematical description that relates SNR (signal-to-noise ratio) variations to multipath effects and uses a B-spline formalism to obtain time series of reflector height. The presented algorithm can provide results in real time with a precision that is significantly better than spectral inversion methods and almost comparable to results from inverse modeling in post-processing mode. To verify the performance, the method has been tested at station GTGU at the Onsala Space Observatory, Sweden, and at the station SPBY in Spring Bay, Australia. The RMS (root mean square) error with respect to nearby tide gauge data was found to be 2.0 cm at GTGU and 4.8 cm at SPBY when evaluating the output corresponding to real-time analysis. The method can also be applied in post-processing, resulting in RMS errors of 1.5 cm and 3.3 cm for GTGU and SPBY, respectively. Finally, based on SNR data from GTGU, it is also shown that the Kalman filter approach is able to detect the presence of sea ice with a higher temporal resolution than the previous methods and traditional remote sensing techniques which monitor ice in coastal regions.

Land surface characterization using BeiDou signal-to-noise ratio observations

China's BeiDou Navigation Satellite System (BDS) is providing new opportunities for GNSS reflectometry-related applications. We give the first and comprehensive description of the feasibility and potential of using BDS signal-to-noise ratio (SNR) data to characterize land surface in terms of the volumetric soil moisture (VSM), vegetation water content (VWC) and snow depth. BDS SNR-derived interferogram metrics (phase ?, amplitude A, and effective reflector height h) are investigated, and their correlations to the corresponding land surface parameters are established. Data collected from a geodetic-quality BDS/GPS compatible receiver for approximately 300-day period were used to validate the VSM retrieval. Results show that both BDS B1 and B2 frequencies can perform well to reflect the fluctuations of the VSM. Specifically, the B2-derived phase ? exhibits a slightly higher correlation with in situ VSM than that of B1 (R?=?0.83 vs. R?=?0.80), and the B2-derived amplitude A also exhibits a higher correlation with MODIS NDVI than that of B1 (R?=?0.49 vs. R?=?0.53); whilst for snow, the B1 and B2 results indicate qualitative agreement with concurrent in situ snow depth measurements. Furthermore, similar estimation performance can be obtained by comparing the results of BDS B1 and B2 against GPS L2C and L5. Therefore, BDS could be a new and powerful data source with comparable potential as GPS for effectively characterizing high-temporal resolution land surface.

Sea Level Estimation Based on GNSS Dual-Frequency Carrier Phase Linear Combinations and SNR

Ground-based GNSS-R (global navigation satellite system reflectometry) can provide the absolute vertical distance from a GNSS antenna to the reflective surface of the ocean in a common height reference frame, given that vertical crustal motion at a GNSS station can be determined using direct GNSS signals. This technique offers the advantage of enabling ground-based sea level measurements to be more accurately determined compared with traditional tide gauges. Sea level changes can be retrieved from multipath effects on GNSS, which is caused by interference of the GNSS L-band microwave signals (directly from satellites) with reflections from the environment that occur before reaching the antenna. Most of the GNSS observation types, such as pseudo-range, carrier-phase and signal-to-noise ratio (SNR), suffer from this multipath effect. In this paper, sea level altimetry determinations are presented for the first time based on geometry-free linear combinations of the carrier phase at low elevation angles from a fixed global positioning system (GPS) station. The precision of the altimetry solutions are similar to those derived from GNSS SNR data. There are different types of observation and reflector height retrieval methods used in the data processing, and to analyze the performance of the different methods, five sea level determination strategies are adopted. The solutions from the five strategies are compared with tide gauge measurements near the GPS station, and the results show that sea level changes determined from GPS SNR and carrier phase combinations for the five strategies show good agreement (correlation coefficient of 0.97–0.98 and root-mean-square error values of <0.2 m).

Tropospheric delays in ground‐based GNSS multipath reflectometry—Experimental evidence from coastal sites

AbstractRecent studies have demonstrated the utility of ground‐based Global Navigation Satellite Systems‐Multipath Reflectometry (GNSS‐MR) for sea level studies. Typical root‐mean‐square (RMS) differences of GNSS‐MR‐derived sea level time series with respect to nearby tide gauges are on the order of 6–40 cm, sufficiently accurate to estimate tidal and secular sea level variations but are possibly biased due to delay of the signal through the troposphere. In this study we investigate the tropospheric effect from more than 20 GNSS coastal sites located from several meters up to 280 m above sea level. We find a bias in the estimated heights that is elevation and height dependent and can reach orders of 1 m for a 90 m site. Without correcting for tropospheric delay we find that GNSS‐MR‐estimated tidal coefficients will be smaller than their true amplitudes by around 2% while phases seem unaffected. Correcting for the tropospheric delay also improves leveling results as a function of reflector height. Correcting for the tropospheric delay in GNSS‐MR for sea level studies is therefore highly recommended for all sites no matter the height of the antenna above the sea surface as it manifests as a scale error.

Avoiding errors attributable to topography in GPS-IR snow depth retrievals

Global Positioning System (GPS) interferometric reflectometry represents a potential source of new snow data for climate scientists and water managers with spatial and temporal sensitivity. Generally, the snow layer fluctuation is considered to be correlated with the ground surface fluctuation. The reflector heights in terms of the signal-to-noise ratio (SNR) are quite close to the vertical distance between the antenna and the ground or snow level at the corresponding Fresnel zone. The reflector heights at different zones were represented by a grid model in this work, which can reflect and overcome some of the problems caused by the topography. The proposed method for snow depth retrievals looks for good quality reflector height values of the horizontal reflecting zone in the grid model, and with this method improvements in snow depth retrieval accuracy were achieved (RMSE: 7.40cm, Corr.: 0.99) compared to the PBO H2O group calculation results (RMSE: 16.58cm, Corr.: 0.99).

Vegetation Sensing Using GPS-Interferometric Reflectometry: Theoretical Effects of Canopy Parameters on Signal-to-Noise Ratio Data

The potential to use GPS signal-to-noise ratio (SNR) data to estimate changes in vegetation surrounding a ground-based antenna is evaluated. A 1-D plane-stratified model that simulates the response of GPS SNR data to changes in both soil moisture and vegetation is presented. The model is validated against observations of SNR data from four field sites with varying vegetation cover. Validation shows that the average correlation between modeled and observed SNR data is higher than the average correlation between concurrent SNR observations from different satellite tracks at a site. The model also reproduces variations in the SNR metrics amplitude, phase, and effective reflector height over a range of vegetation wet weights from 0 to 4 $\hbox{kg}\cdot\hbox{m}^{-2}$ , with $r^{2}$ values of 0.79, 0.84, and 0.62, respectively. Model simulations indicate that the amplitude of SNR oscillations may be used to estimate vegetation amount when vegetation wet weight is below 1.5 $\hbox{kg}\cdot\hbox{m}^{-2}$ . When vegetation wet weight exceeds 1.5 $\hbox{kg}\cdot\hbox{m}^{-2}$ , the sensitivity of amplitude to changes in vegetation amount decreases. Phase of SNR oscillations also varies consistently with vegetation up to 1.5 $\hbox{kg}\cdot\hbox{m}^{-2}$ . However, phase is also very sensitive to soil moisture variations, thus limiting its utility for estimating vegetation. Effective reflector height is not a consistent indicator of vegetation change. Beyond 1.5 $\hbox{kg}\cdot\hbox{m}^{-2}$ , the constant frequency assumption used to characterize SNR fluctuations does not adequately describe observed data. A more complex approach than the standard SNR metrics used here is required to extend GPS-Interferometric Reflectometry sensing to thicker canopies.

Assessment of modernized GPS L5 SNR for ground-based multipath reflectometry applications

Global Positioning System (GPS) signal-to-noise ratio (SNR) measurements can be employed to retrieve environmental variables in multipath reception conditions, whereby direct or line-of-sight transmission is received simultaneously with coherent reflections thereof. Previous GPS SNR multipath studies of soil moisture and snow depth have focused on the legacy GPS L1 and L2 bands. In the present paper, short-delay, near-grazing incidence multipath from the L5-band GPS SNR is assessed for its value in detecting soil moisture and snow depth. The L5 signal will become more important in the future because of compatibility and interoperability among the different global satellite navigation systems. The L5 results are compared with L2C estimates to determine whether the L2C–L5 differences (given their differing power budgets and their modulation properties) are significant. To address these questions, measurements and simulations were employed. A physically-based multipath simulator was enhanced to investigate the differences between parameter retrievals for the L2C and the L5 GPS signals. Parameter retrievals from synthetic observations for different scenarios were compared. Comparisons included varying reflector height, surface material, and surface roughness. Measurements from two GPS stations in Colorado, USA, were used to retrieve soil moisture and snow depth conditions. Over a 153-day period that encompassed the catastrophic 2013 Colorado flooding event, L2-derived volumetric soil moisture had an RMS difference of 0.042cm3/cm3 while the L5 RMS difference was 0.034cm3/cm3 with respect to in-situ data (values of volumetric soil moisture range between 0.04 and 0.34cm3/cm3). In a separate 483-day campaign, L5-derived snow depth estimates were compared to L2C-derived values and found strongly correlated, deviating from a one-to-one relationship by only 0.00001±0.0064cm/cm. These results indicate the absence of any detectable biases in L5 as compared to L2C for retrieving soil moisture and snow depth from GPS SNR multipath observations.

Easy installation CPW-fed technique for fan-beam array antenna using grounded reflector for wireless applications

A fan-beam linear array antenna employing a non-parasitic grounded reflector, a new Coplanar Waveguide (CPW)-fed network, and Grounded Coplanar Waveguide (GCPW) radiation elements is presented. This fan-beam antenna is achieved using a four-element conventional non-planar monopole integrated with CPW line, which is perpendicularly connected to CPW feed network. In addition, the grounded reflector is vertically connected to the ground part of CPW feed network. These three main parts together produce a fan-beam antenna that can satisfy beneficial input impedance, radiation patterns, and gain bandwidth requirements in the frequency range 3.2–3.66 GHz. This array covers applications of 3.3 and 3.5 GHz WiMAX (Worldwide Interoperability for Microwave Access) operating bands. Also, combining a non-planar GCPW monopole antenna as an array with a non-parasitic grounded reflector and CPW feed network provides designers with an advantageous fan-beam pattern to be generated. In addition, the reflector height is reduced to half by the concept of image theory. As a consequence, the spatial dimension of the antenna will be reduced. Therefore, this combination results in a cheap, lightweight, and easy installing non-planar printed array antenna at high frequency bands. Comparisons are made between results obtained with and without a reflector array antenna.

Effects of Near-Surface Soil Moisture on GPS SNR Data: Development of a Retrieval Algorithm for Soil Moisture

Global Positioning System (GPS) multipath signals can be used to infer volumetric soil moisture around a GPS antenna. While most GPS users concentrate on the signal that travels directly from the satellite to the antenna, the signal that is reflected by nearby surfaces contains information about the environment surrounding the antenna. The interference between the direct and reflected signals produces a modulation that can be observed in temporal variations of the signal-to-noise ratio (SNR) data recorded by the GPS receiver. Changes in the dielectric constant of the soil, which are associated with fluctuations in soil moisture, affect the effective reflector height, amplitude, and phase of the multipath modulation. Empirical studies have shown that these changes in SNR data are correlated with near-surface volumetric soil moisture. This study uses an electrodynamic single-scattering forward model to test the empirical relationships observed in field data. All three GPS interferogram metrics (effective reflector height, phase, and amplitude) are affected by soil moisture in the top 5 cm of the soil; surface soil moisture ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$&lt; 1\hbox{-}\hbox{cm}$</tex></formula> depth) exerts the strongest control. Soil type exerts a negligible impact on the relationships between GPS interferogram metrics and soil moisture. Phase is linearly correlated with surface soil moisture. The slope of the relationship is similar to that observed in field data. Amplitude and effective reflector height are also affected by soil moisture, although the relationship is nonlinear. Phase is the best metric derived from GPS data to use as a proxy for soil moisture variations.

Analytical model for solar irradiance near a planar vertical diffuse reflector – Formulation, validation, and simulations

An analytical model is formulated for the irradiance on a surface (collector) with a rear (opposite the sun) planar vertical diffuse reflector, as is common for a lower roof on a multi-story building. The vast majority of research on solar reflectors has been for specular, or mirror, reflectors, with any diffuse reflections modeled about the specular reflection angle. This model is capable of calculating incident and reflected direct, diffuse, and ground-reflected radiation using components borrowed from the Hay, Davies, Klucher, Reindl (HDKR) irradiance model, and is easily implemented in any computation programming software capable of numeric integration. The model accounts for reflector edge effects and shading of diffuse and ground-reflected radiation by the reflector, but it does not account for shading of beam radiation by the reflector.The model shows good overall agreement with experimental tests, and is three percentage points more accurate than a standard radiation model for tilted surfaces. The model indicates that a planar vertical diffuse reflector increases the irradiance at high clearness indices and low reflector incidence angles, and decreases the irradiance otherwise. Increasing the reflector height and decreasing the collector pitch and distance between the collector and reflector increases the irradiance during clear periods, but decreases the irradiance, to a lesser absolute extent, during cloudy periods. Annual simulations show a gain in winter insolation and a loss in summer insolation for an average collector/reflector, with an increase in annual insolation for collectors near high albedo reflectors.

Wideband Fan-Beam Low-Sidelobe Array Antenna Using Grounded Reflector for DECT, 3G, and Ultra-Wideband Wireless Applications

A low-sidelobe fan-beam wideband linear array antenna employing a nonparasitic grounded reflector is presented. This fan-beam antenna is achieved using a six-element conventional planar monopole array antenna and feed network, which together produce sidelobes at - 26 dB. Dolph-Tschebyscheff distribution is employed, and a broadband array feed network is designed to satisfy beneficial input impedance bandwidth requirements in the frequency range 1.7-2.2 GHz. Therefore, this array covers applications of GSM-DCS2, DECT, and 3G. Combining planar monopole antenna as an array with nonparasitic grounded reflector allows an advantageous fan beam pattern to be generated. H-plane 3-dB beamwidths are controllable using grounded reflector height. In addition, the results are compared with and without reflector array antenna.

조명용 LED 반사컵의 설계변수 의존도 분석 및 기능성 설계의 단순화 기법

The design methods of the reflecting cup employed in an LED lamp are presented. The beam patterns of an LED lamp can be adjusted by functional segmentation of the reflector height. Thus, the reflecting angle of slope was designed according to the height. The analysis of the designed reflecting cup shows that precise adjustable beam patterns can be achieved.

Acoustic levitator for contactless motion and merging of large droplets in air

Large droplet transport in a line-focussed acoustic manipulator in terms of maximum droplet size is achieved by employing a driving voltage control mechanism. The maximum volume of the transported droplets in the order of few microliters is thereby increased by three orders of magnitude compared to the constant voltage case, widening the application field of this method significantly. A drop-on-demand droplet generator is used to supply the liquid droplets into the system. The ejected sequence of picoliter-size droplets is guided along trajectories by the acoustic field and accumulates at the selected pressure node, merging into a single large droplet. Droplet movement is achieved by varying the reflector height. This also changes the intensity of the radiation pressure during droplet movement, which in turn could atomise the droplet. The acoustic force is adjusted by regulating the driving voltage of the actuator to keep the liquid droplet suspended in air and to prevent atomisation. In the herein presented levitation concept, liquids with a wide range of surface tension (water and tetradecane were tested) can be transported over distances of several mm. The aspect ratio of the droplet in the acoustic field is shown to be a good indicator for radiation pressure intensity and is kept between 1.1 and 1.4 during droplet transport. Despite certain limitations with volatile liquids, the presented acoustic levitator concept has the potential to expand the range of analytical characterisation and manipulation methods in applications ranging from chemistry and biology.

Modeling and Fabrication of CMOS Surface Acoustic Wave Resonators

A fully integrated two-port surface acoustic wave (SAW) resonator, fabricated using a standard 0.6-mum complementary metal-oxide semiconductor (CMOS) process is described in this paper. Only three micromachining processes, namely, reactive ion etching, zinc-oxide deposition, and wet etching, implemented subsequent to the standard process, are required to realize these resonators. Three design examples of these resonators are given to demonstrate the characteristics of these resonators at different operating frequencies. Experimental measurements of the S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> transmission characteristics were conducted on the fabricated resonators and they were found to have parallel resonant frequencies of 1.02 GHz, 941 MHz, and 605 MHz and quality (Q) factors of 44, 86, and 285, respectively. Based on these measurements and the fabrication layers of the device, an equivalent-circuit model tailored specifically for standard CMOS two-port resonators was developed. Finite-element modeling of the SAW resonators was performed to verify the measured series resonant frequency. Comparison between the developed model and measurement characteristics was also presented. Improvement in Q factor was observed when reflector height was increased

Design and analysis of small multisector antenna for wireless LANs based on monopole Yagi‐Uda elements

This paper proposes a new small multisector antenna called MS-MPYA (multisector monopole Yagi-Uda array) for wireless LANs. MS-MPYA consists of a multisector monopole Yagi-Uda array on a circular finite ground plane. The horizontal radiation pattern of the MS-MPYA is largely determined by the multisector array setting effect. A numerical analysis shows that setting metallic fins between arrays is effective in eliminating the strong reflection that would otherwise cause pattern instability; when the metallic fins are set, the beamwidth is reduced in the horizontal plane due to mutual sector interaction. The MS-MPYA design method considers the variables of array length, ground plane length, reflector height, and fin length. We clarify that the MS-MPYA offers a much lower profile than corner reflector or horn antennas. © 1998 Scripta Technica, Electron Comm Jpn Pt 1, 81(12): 80–90, 1998

Modern acoustic measurements on three types of concert platforms

Recent studies, particularly those by Gade and Naylor, have begun to quantify the acoustical requirements of performers on stage as expressed in measurable acoustic parameters. These parameters, originally based on laboratory findings, are now being measured in real concert halls. The purpose of this investigation is to add to this database by measuring the conditions on three different types of concert platforms: A proscenium theatre stage, a multipurpose room with a reflector above the stage, and finally, a traditional concert hall stage. Measurements in the multipurpose room will also quantify the effect of reflector height above the stage in terms of support (ST100) and modulation transfer functions. Measurements in the proscenium theatre will quantify conditions on stage, in the orchestra pit, and between the stage and the pit.

A Multipath RF Propagation Model for Computer Simulation of Complex Impulse Responses of Broadcast Teletext Channels

To accurately predict the performance of broadcast teletext systems, one must know the characteristics of typical over-the-air broadcast television channels. Consequently, ensembles of typical complex impulse responses of broadcast teletext channels in the VHF and UHF frequency ranges are required for computer performance simulations. To circumvent the lack of comprehensive measured channel data, a theoretical multipath propagation model for suburban and urban environments was developed to gain insight into the nature of typical television multipath channels. In this model, both the direct and scatter paths consist of a line-of-sight and ground-reflected component. The basis of the propagation model is an expression for the ratio of the scattered path to direct path field strength caused by vertically oriented dielectric slabs of specified thickness, height, and width. The reflector location, orientation, width, height, and thickness are randomized for each Monte Carlo simulation cycle. A scatter table of relative amplitude versus multipath delay is maintained for subsequent analysis and formation of typical multipath channel impulse responses. A perceived desired-to-undesired ratio (PDUR), which is essentially a signal-to-clutter ratio, is computed to eliminate channels of unacceptable video quality, as these would be of no interest to the teletext application. A modification to account for near-field scattering is described. Finally, mobile channel applications are discussed.