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  • 2015-2019  (449)
  • 2010-2014  (490)
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  • 21
    Publication Date: 2018-01-28
    Description: This paper presents a design methodology of multisine signals, so that they can be used as phase references or standards convenient for modulated measurements. To achieve an extendable measurement bandwidth, the spectrum of each multisine is designed with three successive segments, where the left (lower-frequency) and right (higher-frequency) ending ones have the same phase spectra. In this way several multisines, one by one overlapped at the ending segments, can jointly serve as an integrated phase reference/standard signal, which in theory has an arbitrarily extendable bandwidth. By the example of “Schroeder” phase relationship, the proposed method is experimentally verified for wideband modulated measurements, where the designed multisines are used as the phase references and standards, respectively, of a nonlinear vector network analyzer test bed. In this work, a phase measurement error less than ±1 deg. is experimentally confirmed for 1601-tone measurements with 160 MHz modulation bandwidth around 2 GHz.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
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  • 22
    Publication Date: 2018-01-28
    Description: A broadband multilayer beam steering antenna array with a modified mushroom-like radiating element is introduced. The two-layer aperture-coupled array consists of four equal sets of mushroom-like elements, each of which is made of 16 unit cells in a 4×4 configuration. The feeding network is a modified quasi-Butler matrix which benefits from improved double-box branch line couplers. The network has an isolation bandwidth between 4 and 7.9 GHz (65%) and a transmission bandwidth from 4.8 to 7.1 GHz (39%) which operates acceptably in most of the C-band, especially at WLAN and ITS. A broadband Butler matrix uses broadband 3-dB couplers and a middle network. The geometry of the element and feeding network is utilized in array form to enhance the impedance bandwidth and radiation efficiency of the antenna. The results show that the antenna impedance bandwidth covers 3.82 to 8.2 GHz (port 1) and 3.8 to 8.28 GHz (port 2) for S nn ≤ -10 dB, which covers the C-band fully (4-8 GHz). Moreover, it has a 3-dB gain-bandwidth of 2.25 GHz that extends from 4.65 to 6.9 GHz with a peak value of 13.65 dBi. The antenna array has a 3-dB axial-ratio bandwidth of 1.65GHz that extends from 4.9 to 6.55 GHz and involves WLAN (5.15-5.825 GHz) and ITS (5.795-6.400 GHz). The radiation efficiency of the array for both port excitations is in excess of 75% when circular polarization is achieved. Two firing beams at elevation angles of 20 o and -35 o are obtained for ports 1 and 2 upon excitation, respectively.
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  • 23
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    Wiley-Blackwell
    Publication Date: 2018-01-19
    Description: No abstract is available for this article.
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  • 24
    Publication Date: 2018-01-16
    Description: In this paper, a frequency selective surface with tunable bandstop performance by integrating graphene is discussed. The graphene implanted in the circular frequency selective surface unit makes the transmission and reflection bands of the proposed FSS tunable owing to its alterable complex conductivity controlled by the chemical potential. The transmission zero of the proposed FSS performing as a reflection plane happens at the frequency from 1.65 THz to 2.19 THz when the chemical potentials of graphene change from 0 eV to 1 eV. Moreover, reconfigurable radiation pattern of a traditional dipole antenna is accomplished loaded with the proposed FSS by altering the complex conductivity of graphene. The dipole has a directional radiation pattern with enhanced directivity at 2.1 THz by adding the proposed FSS with μ c = 0.8 eV. However, the omnidirectional radiation pattern is maintained by changing the value of chemical potential to 0 eV while the FSS is nearly transparent. Therefore, a radiation diversity property could be achieved by controlling the chemical potential of graphene loaded in FSS.
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  • 25
    Publication Date: 2018-01-16
    Description: This paper presents a simple technique to extend the aperture of planar composite right/left-handed (CRLH) metamaterial antennas with minimal impact on the antenna's dimensions. Unlike most CRLH antenna structures the proposed antenna is via-free. The proposed antenna is shown to operate over a wideband from UHF to C-band with good radiation characteristics. The antenna configuration consists of a vertically stacked CRLH unit-cells comprising of a patch and meandered lines, where the patch is engraved with an S-shaped slot. The design uses minimal ground plane area. The meander line inductance is grounded using CPW ground which eliminates conventional CRLH TL metallic via into ground plane. The antenna is feed through a coplanar waveguide (CPW) match stub that is electromagnetically coupled to the unit cells. Antenna dimensions are 17.5×32.15×1.6 mm 3 , which corresponds to 0.204 λ 0 ×0.375 λ 0 ×0.018 λ 0  where free-space wavelength ( λ 0 ) is 3.5GHz. Parametric study enabled the optimization of the antenna performance in terms of impedance bandwidth, radiation gain and radiation efficiency. Measured results confirm the antenna can operate from 850 MHz to 7.90 GHz, which is equivalent to a fractional bandwidth of 161.14%. The antenna has a maximum gain and radiation efficiency of 5.12 dBi and ~80%, respectively, at 3.5GHz.
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  • 26
    Publication Date: 2018-01-09
    Description: This paper presents validation of ionospheric Global Positioning System (GPS) radio occultation (RO) measurements of the GPS Attitude, Positioning, and Profiling Experiment occultation receiver (GAP-O). GAP is one of eight instruments comprising the Enhanced Polar Outflow Probe (e-POP) instrument suite onboard the Cascade Smallsat and Ionospheric Polar Explorer (CASSIOPE) satellite. One of the main error sources for certain GAP-O data products is the receiver differential code bias (rDCB). A minimization of standard deviations (MSD) technique has shown the most promise for rDCB estimation, with estimates ranging primarily from -40 to -28 total electron content units (TECU; 21.6 to 15.1 ns), including a long-term decrease in rDCB magnitude and variability over the first three years of instrument operation. In application of the MSD method, the sensitivity of bias estimates to ionospheric shell height are as large as 4.5 TECU per 100 km. MSD calculations also agree well with the “assumption of zero topside TEC” method for rDCB estimate at satellite apogee. Bias-corrected topside TEC of GAP-O was validated by statistical comparison with topside TEC obtained from ground-based GPS TEC and ionosonde measurements. Although GAP-O and ground-based topside TEC had similar variability, GAP-O consistently underestimated the ground-derived topside TEC by up to 7 TECU. Ionospheric electron density profiles obtained from Abel inversion of GAP-O occultation TEC showed good agreement with F-region densities of ground-based incoherent scatter radar (ISR) measurements. Comparison of GAP-O and ionosonde measurements revealed correlation coefficients of 0.78 and 0.79, for peak F-region density and altitude, respectively.
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  • 27
    Publication Date: 2018-01-07
    Description: The Super Dual Auroral Radar Network (SuperDARN) fitted data products (e.g. spectral width and velocity) are produced using weighted least-squares fitting. We present a new First-Principles Fitting Methodology (FPFM) that utilizes the first-principles approach of Reimer et al. [2016] to estimate the variance of the real and imaginary components of the mean autocorrelation functions (ACF) lags. SuperDARN ACFs fitted by the FPFM do not use ad hoc or empirical criteria. Currently, the weighting used to fit the ACF lags are derived from ad hoc estimates of the ACF lag variance. Additionally, an over-cautious lag filtering criterion is used that sometimes discards data that contains useful information. In low signal-to-noise (SNR) and/or low signal-to-clutter regimes the ad hoc variance and empirical criterion lead to underestimated errors for the fitted parameter because the relative contributions of signal, noise, and clutter to the ACF variance is not taken into consideration. The FPFM variance expressions include contributions of signal, noise, and clutter. The clutter is estimated using the maximal power-based self-clutter estimator derived by Reimer and Hussey [2015]. The FPFM was successfully implemented and tested using synthetic ACFs generated with the radar data simulator of Ribeiro et al. [2013a]. The fitted parameters and the fitted-parameter errors produced by the FPFM are compared with the current SuperDARN fitting software, FITACF. Using self-consistent statistical analysis, the FPFM produces reliable or trustworthy quantitative measures of the errors of the fitted parameters. For an SNR in excess of 3 dB and velocity error below 100 m/s, the FPFM produces 52% more data points than FITACF.
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  • 28
    Publication Date: 2018-01-06
    Description: Atmospheric radars have the ability to continuously observe the height profiles of wind velocity vectors, across a wide height range, with high temporal resolution. They have been used for research, such as atmospheric gravity waves and meteorological phenomena, as well as for weather forecasting. For atmospheric radars, strong clutter echoes from surrounding hard targets, such as mountains or buildings, are the major source of interference. As they utilize very weak scattering from radio refractive index fluctuations produced by atmospheric turbulence, even weak echoes from low-level sidelobes become a serious problem. In order to reject or suppress clutter echoes, it is effective to use the norm constrained-directionally constrained minimization of power (NC-DCMP) method, which creates a null toward the direction of the clutter, while maintaining the main lobe shape. We successfully implemented the NC-DCMP adaptive processing in the real-time processing system of the MU (Middle and Upper atmosphere) radar. This study demonstrates the effectiveness of real-time processing of the clutter rejection. The primary advantage is enabling the estimation of wind velocity profiles in real-time and the reduction of the amount of recorded data. The improvement of the wind estimation accuracy, especially for vertical flow, was confirmed from observation data recorded over a long period.
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  • 29
    Publication Date: 2018-01-06
    Description: We investigate the potentials of open plasmonic resonators (meta-atoms) with different shapes in the context of light trapping. Consistent with the theory of [M. G. Silveirinha, Phys. Rev. A , 89, 023813, 2014] it is found that in some conditions complex-shaped dielectric cavities may support discrete light states screened by volume plasmons that in the limit of a vanishing material loss have an infinite lifetime. The embedded eigenstates can be efficiently pumped with a plane wave excitation when the meta-atom core has a nonlinear response, such that the trapped light energy is precisely quantized.
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  • 30
    Publication Date: 2018-01-04
    Description: In double-satellite interference localization system, the position of the interference source is computed by estimating the time difference of arrival (TDOA) and the frequency difference of arrival (FDOA) between the signals retransmitted by two satellites. The traditional cross ambiguity function (CAF) based TDOA/FDOA estimation method requires the constant TDOA/FDOA between the two received signals, which can be satisfied only when the signal length is short. Therefore, the CAF estimation method cannot be applied in long time accumulation (LTA). To solve this problem, a novel LTA method is proposed based on signal segmentation. In the proposed method, the two received signals are first divided into several short signal segments. Subsequently, the TDOA, the initial FDOA, and the FDOA change rate of each signal segment are estimated by a modified CAF (MCAF) method. After compensating the FDOA difference between the signal segments, the results are spliced again to form a complete signal without FDOA variation. Consequently, the signal energy can be concentrated and the estimation accuracy of FDOA can be improved. Experimental results of real data demonstrate that greater signal-to-noise ratio (SNR) gain is achieved by the proposed method, compared to the traditional CAF based method.
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