Line Spectrum Enhancement Method for Shaft-Rate Magnetic Field Signals Based on OCSSA-NLMS

The shaft-rate magnetic field signal is an extremely low-frequency electromagnetic emission generated by ships navigating in seawater. It is formed by the modulation of anti-corrosion currents within the ship's hull through the rotating propeller shaft. Centered around the main shaft's rotational speed as the fundamental frequency, the signal contains multiple harmonics and exhibits distinct line spectrum features. However, due to interference from marine environmental noise, the signal suffers from a low SNR (signal-to-noise ratio), making it difficult to extract these spectral features and significantly hindering subsequent detection and recognition. To address this issue, we propose a line spectrum enhancement method based on OCSSA-NLMS, specifically designed for shaft-frequency magnetic signals. To overcome the lack of adaptability in the NLMS (Normalized Least Mean Squares) algorithm—which requires frequent adjustment of filter parameters—we integrate OCSSA (Osprey-Cauchy-Sparrow Search Algorithm) to optimize both the iteration step size and filter coefficients. In different intensity of simulation noise, it can eliminate noise and enhance line spectrum characteristics. In the sea test data, the line spectrum enhancement of the shaft-rate magnetic field signal can be realized under the real ocean environment noise, which has significant engineering application value.