Ground mobile observation system for measuring multisurface microwave emissivity

Large microwave surface emissivities with a highly heterogeneous distribution and the relatively small hydrometeor signal over land make it challenging to use satellite microwave data to retrieve precipitation and to be assimilated into numerical models. To better understand the microwave emissivity over land surfaces, we designed and established a ground observation system for the in situ observation of microwave emissivities over several typical surfaces. The major components of the system include a dual-frequency polarized ground microwave radiometer, a mobile observation platform, and auxiliary sensors to measure the surface temperature and soil temperature and moisture; moreover, observation fields are designed comprising five different land surfaces. Based on the observed data from the mobile system, we preliminarily investigated the variations in the surface microwave emissivity over different land surfaces. The results show that the horizontally polarized emissivity is more sensitive to land surface variability than the vertically polarized emissivity is: the former decreases to 0.75 over cement and increases to 0.90 over sand and bare soil and up to 0.97 over grass. The corresponding emissivity polarization difference is obvious over water (>0.3) and cement (approximately 0.25) but reduces to 0.1 over sand and 0.05 over bare soil and almost 0.01 or close to zero over grass; this trend is similar to that of the Tb polarization difference. At different elevation angles, the horizontally/vertically polarized emissivities over land surfaces obviously increase/slightly decrease with increasing elevation angles but exhibit the opposite trend over water.

Critical Evaluation of Various Spontaneous Polarization Models and Induced Electric Fields in III-Nitride Multi-Quantum Wells

In this paper, ab initio calculations are used to determine polarization difference in zinc blende (ZB), hexagonal (H) and wurtzite (WZ) AlN-GaN and GaN-InN superlattices. It is shown that a polarization difference exists between WZ nitride compounds, while for H and ZB lattices the results are consistent with zero polarization difference. It is therefore proven that the difference in Berry phase spontaneous polarization for bulk nitrides (AlN, GaN and InN) obtained by Bernardini et al. and Dreyer et al. was not caused by the different reference phase. These models provided absolute values of the polarization that differed by more than one order of magnitude for the same material, but they provided similar polarization differences between binary compounds, which agree also with our ab initio calculations. In multi-quantum wells (MQWs), the electric fields are generated by the well-barrier polarization difference; hence, the calculated electric fields are similar for the three models, both for GaN/AlN and InN/GaN structures. Including piezoelectric effect, which can account for 50% of the total polarization difference, these theoretical data are in satisfactory agreement with photoluminescence measurements in GaN/AlN MQWs. Therefore, the three models considered above are equivalent in the treatment of III-nitride MQWs and can be equally used for the description of the electric properties of active layers in nitride-based optoelectronic devices.

Ground mobile observation system for measuring multisurface microwave emissivity

Large microwave surface emissivities with a highly heterogeneous distribution make it challenging to use satellite microwave data to retrieve precipitation and to be assimilated into numerical models over land. To better understand the microwave emissivity over land surfaces, we designed and established a ground observation system for the in situ observation of microwave emissivities over several typical surfaces. The major components of the system include a dual-frequency polarized ground microwave radiometer, a mobile observation platform, and auxiliary sensors to measure the surface temperature and soil temperature and moisture; moreover, observation fields are designed comprising five different land surfaces. Based on the observed data from the mobile system, we preliminarily investigated the variations in the surface microwave emissivity over different land surfaces. The results show that the horizontally polarized emissivity is more sensitive to land surfaces than is the vertically polarized emissivity: the former decreases to 0.75 over cement and increases to 0.90 over sand and bare soil and up to 0.97 over grass. The corresponding emissivity polarization difference is obvious over water (> 0.3) and cement (approximately 0.25) but reduces to 0.1 over sand and 0.05 over bare soil and almost 0.01 or close to zero over grass; this trend is similar to that of the Tb polarization difference. At different elevation angles, the horizontally/vertically polarized emissivities over land surfaces obviously increase/slightly decrease with increasing elevation angle but exhibit the opposite trend over water.

Ground-based Assessment of Snowfall Detection over Land Using Polarimetric High Frequency Microwave Measurements

This paper explores the capability of high frequency microwave measurements at vertical and horizontal polarizations in detecting snowfall over land. Surface in-situ meteorological data were collected over Conterminous US during two winter seasons in 2014–2015 and 2015–2016. Statistical analysis of the in-situ data, matched with Global Precipitation Measurement (GPM) Microwave Imager (GMI) measurements on board NASA/JAXA Core Observatory, showed that the polarization difference at 166 GHz had the highest correlation to measured snowfall rate compared to the single channel high frequency measurements and the polarization difference at 89 GHz. A logistic regression model applied to the match-up data, using the polarization difference at 166 and 89 GHz as predictors, yielded an overall snowfall classification rate of 69.0%, with the largest contribution coming from the polarization difference at 166 GHz. Logistic regression using the four single channels as predictors (at 89 and 166 GHz, horizontal and vertical polarizations) further indicated that the horizontal polarization at 166 GHz was the most important contributor. An overall classification rate of 73% was achieved by including the 183.31 ± 3 GHz and 183.31 ± 7 GHz vertical polarization channels in the final logistic regression model. Evaluation of the final algorithm demonstrated skill in snowfall detection of two significant events.

Implementing the coherence matrix of the partially polarized wave to enhance efficiency of radar observation of the objects

В статье обоснована возможность радиолокационного наблюдения объектов при наличии атмосферного фона. Радиолокационное наблюдение объектов при наличии мешающего фона основано на выделении эхо-сигнала объекта из суммарного эхо-сигнала (объект+фон) по их поляризационному различию. При этом использована матрица когерентности частично поляризованной волны, позволившая установить структуру ее флуктуирующей компоненты. Элементами матрицы когерентности являются действительные параметры Стокса, которые измеряются на выходе приемника судовой РЛС. Отраженная от навигационного объекта и атмосферного фона электромагнитная волна является частично поляризованной и ее полная интенсивность равна сумме интенсивностей стабильной и флюктуирующей компонент. Элементы флюктуирующей компоненты матрицы когерентности отражают поляризационную структуру частично поляризованной волны и представляют дисперсии случайных поляризационных параметров Стокса и их статистическую связь. Для неполяризованной волны матрица когерентности является диагональной в любом поляризационном базисе. Суммарная матрица когерентности позволяет получить информацию о поляризации частично поляризованной волны, отраженной от навигационного объекта и атмосферного фона. Необходимым условием дистанционного радиолокационного наблюдения навигационных объектов, находящихся в зоне атмосферного фона, является разделение эхо-сигнала на эхо-сигнал навигационного объекта и эхо-сигнал атмосферного образования. Отраженная от навигационного объекта и атмосферного фона электромагнитная волна является частично поляризованной и ее полная интенсивность равна сумме интенсивностей стабильной и флюктуирующей компонент. Элементы флюктуирующей компоненты матрицы когерентности отражают поляризационную структуру частично поляризованной волны и представляют дисперсии случайных поляризационных параметров Стокса и их статистическую связь. Для неполяризованной волны матрица когерентности является диагональной в любом поляризационном базисе. Суммарная матрица когерентности позволяет получить информацию о поляризации частично поляризованной волны, отраженной от навигационного объекта и атмосферного фона. Необходимым условием дистанционного радиолокационного наблюдения навигационных объектов, находящихся в зоне атмосферного фона, является разделение эхо-сигнала на эхо-сигнал навигационного объекта и эхо-сигнал атмосферного образования. This article substantiates the possibility of radar observation of objects in the presence of atmospheric background. In the presence of an interfering background the radar observation of objects is based on the separation of the object's echo signal from the general echo signal (object + background) in accordance with its polarization difference. Therefore, the coherence matrix of a partially polarized wave is used, which allows to establish the structure of its fluctuating component. The elements of the coherence matrix are the actual Stokes parameters, which are measured at the output of the ship's radar receiver. The electromagnetic wave reflected from the navigation object and the atmospheric background is partially polarized and its total intensity is equal to the sum of the intensities of the stable and fluctuating components. The elements of the fluctuating component of the coherence matrix reflect the polarization structure of the partially polarized wave and represent the variances of the random Stokes polarization parameters and their statistical relationship. For an unpolarized wave, the coherence matrix is ​​diagonal in any polarization basis. The total coherence matrix provides information on the polarization of a partially polarized wave reflected from the navigation object and the atmospheric background. A necessary condition for remote radar observation of navigation objects located in the atmospheric background zone is the separation of the echo signal into the echo signal of the navigation object and the echo signal of the atmospheric formation. According to the Stokes theorem, the echo signal of a partially polarized wave is decomposed into polarized and unpolarized components. A fully polarized component of a total partially polarized wave has only one type of polarization — linear, circular, or elliptical. The unpolarized component does not have any predominant polarization. The echo signal of the total partially polarized wave is considered as a result of the addition of the intensities of two independent fully polarized components. The polarization of the first component corresponds to the echo signal of the navigation object, and the polarization of the second component corresponds to the echo signal of the atmospheric formation.