Robust and continuous distance estimation of a rapidly approaching threat is a significant concern in underwater technology. Although frequency modulated continuous waves (FMCWs) have been widely used in near-range situations, FMCWs are challenging to use in a specific underwater situation. If a high-speed underwater target appears, the target echo can no longer be modeled as a simple Doppler-shifted version of the wave, which increases the distance estimation error of conventional FMCW methods. This situation also requires frequent updates of distance estimates. Thus, to obtain the distance of a high-speed underwater target, we propose a signal format of a frequency coded continuous wave (FCCW) and develop an FCCW-based distance estimation scheme. The proposed distance estimation algorithm has three steps: group correlators and accumulated sum (GCAS), time-delay map (TDM), and Doppler line fitting (DLF). The structure of the proposed FCCW enables the detection of each frequency hopping time (FHT) within a shorter period than the observation time. The GCAS algorithm detects FHTs in the FCCW and is designed to easily extend the matched filter length, resulting in an extension of the acquisition time. The TDM and DLF provide robust distance estimates and enable the classification of multiple targets. Simulation results show that the proposed method can detect FHTs and estimate the target distance more stably in low signal-to-noise-ratio environments by increasing the matched filter's acquisition time.
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