應用遙測方式獲取高時間與空間分辨率之海洋環境資訊為環境監測與防災之新趨勢，雷達遙測為非接觸式監測，雷達訊號強度會受外在環境影響而衰減，例如降雨與高濕度環境之影響，本研究第一部分即在討論降雨對現在廣為應用之X與S頻航海雷達之影響，並討論此類航海雷達於海洋環境監測能力之優劣．內文第二部分為改善現行廣為使用之影像雷達，使其具有同調性雷達之訊號，以增加其對海洋環境流場與坡場監測之應用性． 本研究第一部分研製 S-band 雷達進行波浪量測，不同於以往採用的 X-band 觀測系統，S-band 雷達不受降雨的影響，可進行全天候不間斷觀測。為了解 S-band 雷達觀測波浪的能力與限制，本研究在東北與西南季風期間於中央大學臨海工作站新屋海域佈放離岸不同距離位置之底碇式都卜勒流速波浪量測儀(Acoustic Doppler Current Profiler, upgrade to wave mode)，並在岸上架設 X-band 與 S-band 雷達測波系統，進行兩次為期 18 與 25 天的同步聯合觀測實驗，實驗期間包含降雨與高相對濕度天候，觀測時間間距為每 20 分鐘一次。觀測比對結果顯示 S-band 雷達可有效於降雨期間進行海面波浪觀測，其回波訊躁比 SNR 與波高之對數比對，成線性正相關，其迴歸曲線隨觀測位置依雷達入射角度而有一截距偏移。由 MTF 曲線顯示 S-band 雷達觀測結果能量較偏低頻，這與使用的天線長度與S-band 雷達電磁波特性造成之觀測 Footprint 大小有關。 本研究第二部分採用商用航海微波雷達主機與天線等硬體配置為基礎，設計中頻訊號處理電路，開發出一套移動式岸基同調微波雷達海象觀測系統，實現海面波浪與流場等的觀測。與非同調性微波雷達不同，本研究所開發之雷達系統，採用單頻式脈衝發射電磁波，通過獲取發射訊號與海面回波訊號之間的相位差，來估算海表徑向流速，再採用流函數模型反演得到觀測海域之矢量流場。為驗證雷達觀測結果之準確性，本研究設計了一套極小化可拋棄式漂流浮球陣列系統：球體體積小、重量輕，避免了對海表流場的干擾；成本低廉、大量佈放，可對測區內空間多點進行同步連續觀測。;The content of this thesis is divided into two parts. The first part is the development of S-band radar and the second part is the development of coherent-on-receive radar.
The first part: The high spatial resolutions of sea-clutter image sequences from X-band radar offer a means of deriving individual waves and wave field at low-cost. However, the performance of X-band radar is impaired under rainy conditions, which are usually accompanied by the severe weather at sea. In the present study, we examine the effectiveness of S-band radar for wave measurements under precipitation. The results of comprehensive comparative studies with sea-truth data show that S-band radar is capable of carrying out wave measurements in rainy conditions. Although the longer wavelength of the S-band leads to a coarser resolution of radar imagery, the S-band radar features at least the equivalent performance of the X-band system in non-rainy conditions, in terms of wave height measurement. The results suggest that the S-band and X-band could be complementary systems. In rainy conditions the S-band is more efficient but in the non-rainy periods the X-band gives more confident results. The relationship of significant wave height with radar signal-to-noise ratio (SNR), and the modulation transfer function (MTF) between radar spectrum and wave spectrum for the used X-band and S-band radars are established and discussed in this paper.
The second part: In present study, an X-band marine radar was installed at the northwestern coast of Taiwan. The IF signal from the super-heterodyne receiver of the marine radar was retrieved, amplified, band-pass filtered and digitized to be the raw dataset, from which the phase of transmitted and echo radar EM wave could be determined. Based on Doppler theory, the time rate change of the phases between succeeding pulses could be used for estimating the water particle velocity of wave orbital motion. Due to the fact that the magnetron in the marine radar produces random frequency jump, and leading to mis-estimate of the phase differences. We adopted the Ensemble Empirical Mode Decomposition method as a filter to exclude the effects of frequency jump.
