Microwave radiometric technique to retrieve vapor, liquid and ice.I. Development of a neural network-based inversion method

With the advent of the microwave radiometer, passive remote sensing of clouds and precipitation has become an indispensable tool in a variety of meteorological and oceanographical applications. There is wide interest in the quantitative retrieval of water vapor, cloud liquid, and ice using brightness temperature observations in scientific studies such as Earth's radiation budget and microphysical processes of winter and summer clouds. Emission and scattering characteristics of hydrometeors depend on the frequency of observation. Thus, a multifrequency radiometer has the capability of profiling cloud microphysics. Sensitivities of vapor, liquid, and ice with respect to 20.6, 31.65 and 90 GHz brightness temperatures are studied. For the model studies, the atmosphere is characterized by vapor density and temperature profiles and layers of liquid and ice components. A parameterized radiative transfer model is used to quantify radiation emanating from the atmosphere. It is shown that downwelling scattering of radiation by an ice layer results in enhancement at 90 GHz brightness temperature. Once absorptive components such as vapor and liquid are estimated accurately, then it is shown that the ice water path can be retrieved using ground-based three-channel radiometer observations. In this paper the authors developed two- and three-channel neural network-based inversion models. Success of a neural network-based approach is demonstrated using a simulated time series of vapor, liquid, and ice. Performance of the standard explicit inversion model is compared with an iterative inversion model. In part II of this paper, actual radiometer, and radar field measurements are utilized to show practical applicability of the inverse models     

The equivalence of the electric and magnetic surface current approaches in microstrip antenna studies

It is shown that the fields produced by the electric surface currents on the conducting patch of a microstrip antenna are equivalent to those produced by a magnetic surface current sheet bounding the patch in the substrate only when dyadic Green's functions for the two-layer stratified medium are used.     

Gain and carrier lifetime measurements in AlGaAs single quantum well lasers

We have measured the optical gain and carrier lifetime (τ) at threshold in AlGaAs single quantum well lasers in the temperature range10-70degC. The small shift of the spectral position of the gain peak with increasing injection is evidence for the two-dimensional-like density of states in a quantum well. The net gainGis found to vary linearly with the currentI. The observed slow decrease ofdG/dIand τ with increasing temperature suggests the absence of a large temperature-dependent nonradiative carrier loss or optical absorbtion loss in these lasers. This is also supported by the observed low temperature dependence of the external differential quantum efficiency.     

Characterization of mass-transported p-substrate GaInAsP/InPburied-heterostructure lasers with analytical solutions for electricaland thermal resistances

Studies have been carried out to evaluate mass-transported p-substrate GaInAsP/InP buried-heterostructure lasers, which have a number of potential advantages over the more conventional n-substrate lasers. Devices have been fabricated with series resistances as low as 3 Ω, in good agreement with the p-substrate spreading resistance calculated using conformal mapping. A further development of this theory yields simple formulas of thermal resistances of heat generated both in the active region and in the p-InP. The presently fabricated p-substrate lasers also showed CW threshold currents as low as 4.5 mA, differential quantum efficiencies as high as 34% per facet, output powers as high as 33 mW per facet, and a maximum total electrical-to-optical power conversion efficiency of 36%     

Nanoporous polymeric transmission gratings for high-speed humidity sensing

Nanoporous polymeric transmission gratings are demonstrated to be an excellent platform for high-speed optical humidity sensing. The grating structures were fabricated with a modified holographic, polymer-dispersed liquid crystal (H-PDLC) system. The sensing mechanism was based on changes in the relative transmission associated with the adsorption and desorption of water vapour by nanopores. The spectral changes due to varying humidity levels were measured by a spectrometer and compared with the calculated results based on the coupled wave theory. When the relative humidity (RH) changed from 40% to 95%, the relative transmission at 475?nm increased from 6.3% to 46.6% and that at 702?nm increased from 4%?to 64%; these results indicate the sensor's high sensitivity. In addition, the sensor demonstrated excellent reversibility and reproducibility over a large RH range (from 20% to 100% RH). Moreover, the response time of the sensor was measured to be less than 350?ms, making it suitable for many high-speed humidity-sensing applications.     

FA‐Assistant Iodide Coordination in Organic–Inorganic Wide‐Bandgap Perovskite with Mixed Halides

Mixed‐halide wide‐bandgap perovskites are key components for the development of high‐efficiency tandem structured devices. However, mixed‐halide perovskites usually suffer from phase‐impurity and high defect density issues, where the causes are still unclear. By using in situ photoluminescence (PL) spectroscopy, it is found that in methylammonium (MA⁺)‐based mixed‐halide perovskites, MAPb(I_0.6Br_0.4)₃, the halide composition of the spin‐coated perovskite films is preferentially dominated by the bromide ions (Br^?). Additional thermal energy is required to initiate the insertion of iodide ions (I^?) to achieve the stoichiometric balance. Notably, by incorporating a small amount of formamidinium ions (FA⁺) in the precursor solution, it can effectively facilitate the I^? coordination in the perovskite framework during the spin‐coating and improve the composition homogeneity of the initial small particles. The aggregation of these homogenous small particles is found to be essential to achieve uniform and high‐crystallinity perovskite film with high Br^? content. As a result, high‐quality MA_0.9FA_0.1Pb(I_0.6Br_0.4)₃ perovskite film with a bandgap (E_g) of 1.81 eV is achieved, along with an encouraging power‐conversion‐efficiency of 17.1% and open‐circuit voltage (V_oc) of 1.21 V. This work also demonstrates the in situ PL can provide a direct observation of the dynamic of ion coordination during the perovskite crystallization.     

Generalized Least Squares With Ignored Errors in Variables

We present data, both real and simulated, that show generalized least squares (GLS) estimation, intended to account for correlated response error structure, can produce gross biasing in regression parameter estimates under misspecified models with ignored errors in explanatory-variable measurements. The bias, and its subsequent effect on mean squared error (MSE), can be much more severe than the apparently less appropriate ordinary least squares (OLS) estimator. This article provides a theoretical basis for these effects by deriving expressions for the bias and MSE for the general GLS estimator through Taylor-series expansions. The results are compared with simulations for two specific weight matrices and applied to a dataset relating atmospheric pollutant levels in Los Angeles with average recorded wind speed. We show that the bias (with subsequent implications for the MSE) is always worse for the exponential correlation model with equally spaced explanatory-variable observations and present a simple test to decide a preference for OLS or GLS in practice.     

Stability analysis of modulated tool path turning

A new periodic sampling-based method for identifying the stability of modulated tool path turning (MTP) is presented. A metric is defined that provides a numerical value to indicate stability; it is nominally zero for forced vibration and large for self-excited vibration. Tests were performed using ASM 6061-T6 aluminum tubes with varying wall thicknesses to control stability, where MTP was applied to create discrete chips by superimposing sinusoidal oscillation in the feed direction. Results are compared for the new periodic sampling metric and the traditional frequency-domain approach, where the frequency spectrum is analyzed to identify the chatter frequency (should it exist).     

Temporal Scale Selection in Time-Causal Scale Space

When designing and developing scale selection mechanisms for generating hypotheses about characteristic scales in signals, it is essential that the selected scale levels reflect the extent of the underlying structures in the signal. This paper presents a theory and in-depth theoretical analysis about the scale selection properties of methods for automatically selecting local temporal scales in time-dependent signals based on local extrema over temporal scales of scale-normalized temporal derivative responses. Specifically, this paper develops a novel theoretical framework for performing such temporal scale selection over a time-causal and time-recursive temporal domain as is necessary when processing continuous video or audio streams in real time or when modelling biological perception. For a recently developed time-causal and time-recursive scale-space concept defined by convolution with a scale-invariant limit kernel, we show that it is possible to transfer a large number of the desirable scale selection properties that hold for the Gaussian scale-space concept over a non-causal temporal domain to this temporal scale-space concept over a truly time-causal domain. Specifically, we show that for this temporal scale-space concept, it is possible to achieve true temporal scale invariance although the temporal scale levels have to be discrete, which is a novel theoretical construction. The analysis starts from a detailed comparison of different temporal scale-space concepts and their relative advantages and disadvantages, leading the focus to a class of recently extended time-causal and time-recursive temporal scale-space concepts based on first-order integrators or equivalently truncated exponential kernels coupled in cascade. Specifically, by the discrete nature of the temporal scale levels in this class of time-causal scale-space concepts, we study two special cases of distributing the intermediate temporal scale levels, by using either a uniform distribution in terms of the variance of the composed temporal scale-space kernel or a logarithmic distribution. In the case of a uniform distribution of the temporal scale levels, we show that scale selection based on local extrema of scale-normalized derivatives over temporal scales makes it possible to estimate the temporal duration of sparse local features defined in terms of temporal extrema of first- or second-order temporal derivative responses. For dense features modelled as a sine wave, the lack of temporal scale invariance does, however, constitute a major limitation for handling dense temporal structures of different temporal duration in a uniform manner. In the case of a logarithmic distribution of the temporal scale levels, specifically taken to the limit of a time-causal limit kernel with an infinitely dense distribution of the temporal scale levels towards zero temporal scale, we show that it is possible to achieve true temporal scale invariance to handle dense features modelled as a sine wave in a uniform manner over different temporal durations of the temporal structures as well to achieve more general temporal scale invariance for any signal over any temporal scaling transformation with a scaling factor that is an integer power of the distribution parameter of the time-causal limit kernel. It is shown how these temporal scale selection properties developed for a pure temporal domain carry over to feature detectors defined over time-causal spatio-temporal and spectro-temporal domains.     

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