VHF陣列雷達大氣回波來向角之定位偏差　雷達波束場型之不均勻加權效應

Location Bias of Incident Angles of Atmospheric Echoes Observed with Arrayed VHF Radar Non-uniform Weighting Effect of Radar Beam Pattern

陳振雄(Jenn-Shyong Chen) ； 蔡榮鋒(Rong-Feng Tsai) ； 鄭守漢(Shou-Han Cheng) ； Marius Zecha ； Peter Hoffmann

《建國科大學報》 27卷2期 (2008/01) Pp. 97-112

https://doi.org/10.6995/JCTU.200801.0097

摘要

利用多接收機觀測技術，脈波式特高頻(VHF)陣列雷達可解析出雷達體積內多個大氣回波中心之來向角，此技術在雷達大氣研究領域中稱爲「同相雷達成像法」。由於陣列雷達電波輻射和接收場型都有一個主波瓣與數個旁波瓣，合成之雷達波束場型具有高度不均勻的強度，而且大氣回波並非侷限於一小範圍區域（例如飛機或飛彈之反射回波），因此，大範圍分布的大氣回波會受到不均勻波束場型的權重效應而導致所觀測得之回波中心會偏離原來的位置。本篇研究以高斯函數之回波分布爲例，利用德國53.5MHz(OSWIN)和中壢30MHz雷達的波束場型，探討回波中心的偏差問題。模擬計算顯示：(1)當原始回波在雷達波束場型之凹槽處(null or notch)附近，通常會產生多個回波中心，並且明顯偏離原來的回波中心，偏離角度大小與波束場型、原始回波分布範圍有關；(2)當回波中心位於波束場型之主波瓣和旁波瓣最大強度之處，計算得之回波中心偏差最小；(3)雷達波束場型之凹槽處附近沒有計算得回波中心；(4)當計算得之回波中心偏差越大，其回波強度越低，不易爲雷達所偵測出來。
分析OSWIN雷達之中氣層觀測資料，可以發現第(3)、(4)點特性，並且觀察到位於較大天頂角度之回波是來自於波束場型之第一或第二旁波瓣。這些分析結果有助於適當地應用「同相雷達成像法」於大氣之研究。

關鍵字

VHF陣列雷達 ； 大氣回波中心 ； 雷達波束場型

並列摘要

Pulsed and arrayed VHF atmospheric radars can resolve the angles of arrival of multiple echo centers in the radar volume with multiple-receiver technique. Multiple-receiver technique is termed coherent radar imaging (CRI) in the radar study of the atmosphere. Both radiating and receiving beam patterns of an arrayed radar consist of a mainlobe and several sidelobes, leading to a synthetic beam pattern with highly non-uniform intensity. Besides, the atmospheric echoes are not extremely localized (as compared with the reflecting echoes of airplane or missile, for example). As a result, the widespread atmospheric echoes suffer different weightings from the non-uniform radar beam pattern and then biased locations of echo centers result from such different weightings. We examined this bias problem of echo centers for the German 53.5MHz radar (OSWIN) and Chung_Li 30MHz radar with the model of Gaussian-distributed echoes. Simulated estimates demonstrate: (1) the estimated echo centers of which the locations differ largely from the original echo centers are obtained when the original echoes locate around the notches/nulls of the radar beam pattern. The angular difference between origional and estimated echo centers depends on the radar beam pattern and original echo distribution; (2) the estimated echo center has smallest angular bias against the original echo center when the original echo center locates around the maximum of mainlobe and sidelobes of the radar beam pattern; (3) there are no echo centers estimated around the notches of radar beam pattern; and (4) the echo intensity is lower when the estimated echo center has larger angular bias against the original echo center, which may be undetectable by the radar. Characteristics (3) and (4) can be found in the mesosphere observation made by the OSWIN radar. The observations of OSWIN radar also reveal that the echoes appearing at large zenith angles return from the first or second sidelobes of the radar beam pattern. All of these results are beneficial for applying the CRI technique appropriately to atmospheric studies.

並列關鍵字

arrayed VHF radar ； atmospheric echo center ； radar beam pattern