水下目標被動測距是水中聲學中的重要技術，可應用於船舶航行監測、水下目標追蹤、海洋環境觀測及國防監測等領域。傳統水下定位系統多仰賴多水聽器陣列，雖能取得較完整的空間資訊，但通常伴隨較高的設備成本、同步需求與海上布放複雜度，因而限制其應用。相較之下，單水聽器具有硬體架構簡單與維護成本低等優點，適合部署於近岸、港區、浮標或自主式水下載具。本研究探討以單一固定水聽器估測淺海移動聲源範圍之可行性，並建立一套由窄頻訊號處理、徑向速度估測至範圍反演的完整分析流程。研究分別採用場差分法（Field Dif-ferencing Method, FDM）與合成孔徑波束形成法（Synthetic Aperture Beamforming, SABF），利用移動聲源與水聽器間的相對運動，由單通道窄頻訊號建立徑向速度－時間能量圖，並擷取能量主脊以取得徑向速度曲線。接著，結合等速直線運動與最近通過點（Closest Point of Approach, CPA）模型，透過非線性最小平方法估測 CPA 時間、CPA 範圍及範圍－時間曲線，使單一接收器亦能由時間序列中的相位與干涉變化完成被動測距。數值模擬針對時間間隔、短時傅立葉轉換窗長及處理階數進行分析。結果顯示，時間間隔需同時考量窄頻訊號之相位週期條件與最大可解析徑向速度；窗長則須在頻率解析度與時間局部性之間取得平衡；適度提高階數可改善能量集中程度與主脊連續性，但階數過大時，其改善效果將逐漸趨緩。實驗部分採用 SWellEx-96 S5 垂直線列陣中的單一通道資料進行驗證。結果顯示，窄頻訊號能量越穩定，徑向速度主脊越清楚，範圍反算結果亦越可靠；在本研究分析條件下，以 127 Hz 及接收深度 94.13 m 的測距表現較佳，且 FDM 整體估測穩定性優於 SABF。上述結果亦說明，分析頻率、接收深度及訊號處理參數均會影響單水聽器測距之準確性。綜合而言，本研究證實單一水聽器可藉由淺海聲場干涉特徵與移動聲源之徑向速度資訊完成被動範圍反演。在不需大型接收陣列與完整環境聲傳模型的條件下，本方法可降低系統建置與海上部署需求，並提升水聲監測系統之應用彈性。未來可進一步結合多頻率資訊、自動化主脊追蹤、即時訊號處理及複雜航跡模型，應用於近岸船舶交通監控、港區與重要海域警戒，以及自主式水下載具之環境感測系統。

Passive ranging of underwater targets is important for vessel monitoring, target tracking, environmental observation, and maritime surveillance. Conven-tional localization systems generally rely on hydrophone arrays, resulting in high equipment costs, synchronization requirements, and deployment complexity. This study develops a passive ranging framework for a moving underwater source us-ing only a single hydrophone. Narrowband acoustic signals are processed using the Field Differencing Method (FDM) and Synthetic Aperture Beamforming (SABF) to generate radial-velocity–time energy distributions. Energy ridges are extracted to estimate radial velocity and are subsequently fitted to a con-stant-speed, straight-line Closest Point of Approach model using nonlinear least squares. The CPA time, CPA range, and range–time curve can therefore be esti-mated from single-channel measurements. Numerical simulations investigate the effects of time interval, STFT window length, and processing order. Experimental validation is conducted using a single channel selected from the SWellEx-96 S5 vertical line array. Results indicate that stable narrowband energy produces clearer radial-velocity ridges and more reliable ranging results. Under the examined con-ditions, the 127 Hz signal and a receiver depth of 94.13 m provide better ranging performance, while FDM exhibits greater overall stability than SABF. The results demonstrate the feasibility of passive range inversion using shallow-water inter-ference characteristics and radial-velocity information without a large receiver array or a complete environmental propagation model.

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