# Underwater Low-Frequency Magnetic Field Detection Based on Rao’s Sliding Threshold Method

## TL;DR

A new method for detecting low-frequency magnetic fields underwater improves performance at very low signal-to-noise ratios.

## Contribution

A joint time–frequency analysis method combining Rao detector with dynamic sliding thresholds is proposed for improved low-SNR detection.

## Key findings

- The proposed method achieves 90% detection probability at −13 dB SNR.
- The Rao detector outperforms energy detectors by 15 dB in SNR threshold reduction.
- The method remains effective in complex non-Gaussian noise environments.

## Abstract

This paper proposes a joint time–frequency analysis method that combines Rao detector with dynamic sliding thresholds to enhance the detection performance of electric source axial frequency magnetic field signals. For each signal-to-noise ratio (SNR) point, 1000 Monte Carlo simulations were independently conducted, with SNR ranging from 15 dB to −30 dB. The results show that the proposed method maintains high detection rates even at extremely low SNRs, achieving about 90% detection probability at −13 dB, significantly outperforming traditional energy detectors (with a threshold of 2 dB). Under conditions where the detection probability is ≥90% and the false alarm probability is 10−3, the SNR threshold for the Rao detector is reduced by 15 dB compared to energy detectors, greatly improving detection performance. Even at lower SNRs (−30 dB), the Rao detector still maintains a certain detection rate, while the detection rate of energy detectors rapidly drops to zero. Further analysis of the impact of different frequencies (1–5 Hz) and CPA distances (45–80 cm) on performance verifies the algorithm’s robustness and practicality in complex non-Gaussian noise environments. This method provides an effective technical solution for low SNR detection of ship axial frequency magnetic fields and has good potential for practical application.