## 2 System Description  
This paper first introduces the general design of the long-wave wideband direction-finding system, which primarily monitors long-wave communication signals globally. To achieve global coverage of submarine-to-submarine communication signals, it is necessary to leverage the transmission characteristics of electromagnetic waves in the Earth-ionosphere waveguide. Thus, the operating frequency must fall within the VLF/LF bands. Considering the operating frequencies of global long-wave radio stations, the system's frequency coverage was selected to range from 10 kHz to 300 kHz. The LPF-B0K3+ low-pass filter was chosen to meet the bandwidth requirements and cover the primary target radios. To effectively sample long-wave signals within this frequency band, the sampling frequency, according to the sampling theorem, must be greater than twice the signal bandwidth. The system selects a 1 MHz sampling rate, providing slight redundancy without imposing excessive demands on subsequent processing and storage. The long-wave wideband direction-finding system consists of two long-wave receiving subsystems and one data processing subsystem, as illustrated in the system block diagram in Figure 1. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Reference** & **Year** & **Bandwidth** &
\begin{tabular}{c} **Electric/Magnetic** \\ **Sensitivity** \\ \end{tabular} & **Antenna Size** & **Time Precision** \\ \hline Ref1 [19] & 2009 & 4 Hz–400 kHz & N/A \({}^{*}\) & 1.55 m & \(\sim\) 12 ns \\ \hline Ref2 [25] & 2010 & 300 Hz–30 kHz & \(10\,fT/\sqrt{\mathrm{Hz}}\) & 2.6 m & \(\sim\) 40 ns \\ \hline Ref3 [23] & 2018 & 17–120 kHz & \(1\,fT/\sqrt{\mathrm{Hz}}\) & 56 cm & 15–20 ns \\ \hline Ref4 [26] & 2019 & 1–500 kHz & \(7\,\mathrm{nV/m}\sqrt{\mathrm{Hz}}\) & 1 m & 15–20 ns \\ \hline Ref5 [24] & 2021 & DC-470 kHz & \(45\,\mathrm{nV/m}\sqrt{\mathrm{Hz}}\) & 2 m & 15–20 ns \\ \hline This work & 2025 & 10–300 kHz & \(3.2\,\mathrm{\SIUnitSymbolMicro V/m}\sqrt{\mathrm{Hz}}\), & 20 cm(E-sensor), 35 \(8\,fT/\sqrt{\mathrm{Hz}}\) & \(<\)10 ns \\ \hline \hline \end{tabular} \({}^{*}\) Not mentioned in the literature.  
\end{table} 
Table 1: List of parameters for existing long-wave broadband receivers. The Longwave Receiving Subsystem is primarily used for long-wave broadband signal reception. Longwave Receiving Subsystem 1 and Subsystem 2 denote the main and ref receivers, respectively, with each subsystem comprising an antenna unit and a receiver unit. The antenna unit comprises one vertical electric field sensor and two horizontal magnetic field sensors. The electric field sensor is oriented vertically, while the magnetic field sensors are aligned in the north-south and east-west directions, respectively. For vertically polarized electromagnetic waves, the vertical electric field sensor exhibits high sensitivity and effectively captures weak signals. The horizontal magnetic field sensor measures the horizontal magnetic field, and the magnetic direction-finding method determines the orientation of long-wave signals, thereby providing more information [23; 24]. The receiver unit collects the electromagnetic signals received by the antenna and transmits them to the data processing subsystem. The data processing subsystem comprises both hardware and software components. The hardware includes high-performance computers, storage devices, communication equipment, and display and control units. The software component is mainly utilized for wideband detection of long-wave signals, downconversion, outputting wideband power spectra, time-difference direction finding, display, storage, and transmission. As illustrated in Figure 2, which shows the schematic distribution of the long-wave receiving antennas, the vertical electric antenna uses parallel-plate capacitors and a low-noise amplifier to capture the vertical electric field component of long-wave signals. The horizontal magnetic antenna employs two orthogonal inductive magnetic antennas to separately receive the east-west and north-south components of the magnetic field.