超寬頻Ka波段於樹之散射量測及研究分析;Ultra-Wide Band Backscattering Measurement and Analysis of Tree at Ka-Band

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题名:	超寬頻Ka波段於樹之散射量測及研究分析;Ultra-Wide Band Backscattering Measurement and Analysis of Tree at Ka-Band
作者:	莊惟安;Chuang,Wei-An
贡献者:	太空科學研究所
关键词:	散射;backscattering
日期:	2015-08-24
上传时间:	2015-09-23 10:50:45 (UTC+8)
出版者:	國立中央大學
摘要:	中文摘要本篇論文是利用高解析度Ka波段電磁波在國立中央大學校園裡針對闊葉樹和針葉樹的散射特性量測，藉由回波資料獲得在不同頻率、不同極化、不同入射角度之下電磁信號之強度與相位變化，這些資料的統計分析可作為高解析度波段電磁波對闊葉樹和針葉樹背向散射特性在雷達影像分類上之驗證。同調雷達量測系統是以向量式網路分析儀(N5245A PNA-X 10MHz-50GHz)為主體，在量測過程中使用頻率為28 至38GHz，傳輸及接收天線為Ka波段連續波窄波束寬天線，天線入射角度可調整30至80度，而極化可調整為VV, HH, HV不同的極化。寬頻高解析度Ka波段電磁波信號無法深入植被內部，因此忽略了樹木內部的多重散射，我們以金屬球為點目標校正器，點目標校正器的量測是在無回波室內及戶外量測，校正目的就是要得到待測物(闊葉樹和針葉樹)回波資料與其散射係數之間的關係。量測系統經校正之後為符合要求之狀態，其量測到的每筆回波資料平均獲得2000個樣本數，根據雷達方程式原理、相關矩陣之簡化及窄波束寬圓盤天線之特性可推導出與頻率、入射角有關的窄波束寬待測物背向散射係數方程式，求得闊葉樹(榕樹)和針葉樹(松樹)的背向散射係數。並且更進一步的統計分析及比較兩者之特性，作為高解析度波段電磁波對闊葉樹和針葉樹背向散射特性在雷達影像分類上之驗證作為本篇論文之主要目的。網路分析儀具有同調性及連續波量測之功能，獲得時域及頻域上的量測數據，由時域的量測數據標訂出待測目標物與網路分析儀量測系統的距離，由頻域的量測數據可獲得量測數據的強度與相位變化。我們將量測系統安置在校園裡高度將近15公尺四周空曠的教學大樓屋頂，選擇枝葉繁密、樹冠成傘狀、葉片呈橢圓形或倒卵形的榕樹，及葉片呈針狀的松樹為待測樹木。由時域的量測數據標訂出待測樹木的樹冠與量測系統的距離，將與其毗連而非待測物的回波予以屏除。在不同頻率、不同極化、不同入射角度之下電磁回波信號之強度與相位會隨之變化，尤其在高解析度Ka波段之下，其響應會更敏銳，這些資料經過統計分析及比較之後大致獲得一個結論：同調雷達量測系統若以交互偏極化量測時，闊葉樹(榕樹)及針葉樹(松樹)的背向散射係數動態範圍大於同偏極化之量測。松樹針葉的表面積很小，遠小於榕樹葉片，因此松樹針葉的覆蓋也遠小於榕樹葉片，顯而易見的交互偏極化量測時，針葉樹(松樹)的背向散射係數動態範圍大於闊葉樹(榕樹)交互偏極化之量測。 ;ABSTRCT In this thesis, we present high resolution Ka-band backscattering measurement of deciduous and coniferous tree in National Central University (NCU) campus for ground-truth validation in vegetation monitoring. The coherent radar system, designed and configured based on a vector network analyzer N5245A PNA-X (10MHz-50GHz) is operated in a continuous-wave mode, was used to generate the transmitting signal and to detect scattered signals both in amplitude and phase for VV, HH, HV polarizations at incident angles between 30 and 80 degrees. In measuring process, the radar system has typical trace of the frequency ranges from 28 to 38GHz of the received power for a given footprint of tree canopy. For this ultra-wide bandwidth, the wind-induced Doppler effects can be ignored. By this feature, the propagating depth through the tree can also be precisely determined so that the effective propagation constant and attenuation factor can be estimated. A reference measurement was taken at normal incidence using a conducting sphere as a calibration target, showed that the measurement capabilities of this system are satisfactory. The calibration was conducted both in anechoic chamber and in-situ measurements. Ensemble average of 2000 samples was taken to estimate the mean return power and later by inverting the radar equation to obtain the backscattering coefficient as functions of frequencies and incident angles are then estimated. To simplify the matrix inversion, a narrow beam is realized with dish antenna. Based on the measured results, backscattering characteristics and statistical properties were analyzed for the cases of deciduous (banyan) and coniferous (pine) trees. To obtain backscattering coefficient for tree canopy, measuring system was mounted on the top of a 15 m- height roof. From this platform, the radar had an unobstructed view of the selected tree targets. The banyan and pine trees were substantially larger than the footprint of radar with 2.6-degree antenna beam-width. Target trees were selected where adjacent unwanted scatters (ground and other tree) could be conveniently rejected by range gating in time domain. In the proposed study, frequency, polarization, incident angle, and canopy parameters are the primary factors affect backscattering properties. The results indicated that cross-polarized returns generally presents higher dynamic ranges over the frequency and angular behavior. At Ka-band, its wavelength (0.7cm to 1.1cm) is much smaller than leaves and branches. For banyan tree, leaf stems constitute a very small proportion of the target areas, and the corresponding return signal may be reasonably ignored. However, leaf stems of the pine trees constitute a larger portion of the beam areas. The contributions from multiple backscattering are larger than those of banyan trees. The dynamic range of cross-polarization is also larger than that of banyan trees.
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