## 3 Method Description
### Skywave Interference Resistance  
This section assesses the impact of skywave interference on the PSSD algorithm and demonstrates its effectiveness in countering such interference.  
Skywave interference, a prevalent form encountered in eLoran system operation due to signals reflected from the ionosphere with highly uncertain intensities, can pose challenges. When intense skywave interference is close to ground wave signals, the mixed signal can be difficult for eLoran receivers to separate, potentially leaving residual interference that accurate realization of the subsequent signal processing module functions.  
Theoretically, beyond environmental noise, given that the test and reference signals are subjected to skywave interference of identical intensity and position, the waveform  
Figure 8: DAR results of multi-group experiments.  
Figure 7: Feature comparison under different processing methods: (**a**) amplitude spectrum; (**b**) phase difference curve.  
distortion remains consistent. Therefore, it can be deemed that skywave exerts negligible influence on the demodulation process, a concept that has been confirmed within EPD algorithms. The PSSD algorithm analyzes signals in the frequency domain via FFT processing. Assuming the test and reference signal are subjected to the same degree of skywave interference, it can be expected that the characteristics of phase difference \(\Delta\Phi\) of the mixed signals \(x_{ref,mix}(t)\), \(x_{text,mix}\) should be similar to those without skywave, thus theoretically eliminating the influence of skywave interference on demodulation.  
To substantiate the theoretical rationale outlined previously and the effectiveness of PSSD algorithm in handling skywave interference, we analyzed the impact of different intensities of skywave on the phase difference. First, in a noise-free environment, we studied its effects on the PSSD theory through experiments. Specifically, delay difference: 40-160 \(\upmu\)s, step size: 0.5 \(\upmu\)s; and sky-to-groundwave ratio (SGR): 6 dB and 12 dB.  
Figure 9 depicts the variations in \(\Delta\Phi\) under different skywave interferences. In the two subfigures, the mean values of \(\Delta\Phi\) for \"+\", \"0\", and \"\(-\)\" modulations are 0.1998\(\pi\), 0, \(-\)0.2005\(\pi\) and 0.1999\(\pi\), 0, \(-\)0.2003\(\pi\) respectively. The deviations from the no-skywave scenario (0.1997\(\pi\), 0, \(-\)0.2007\(\pi\)) do not exceed \(\pi\)/250. The results indicate that, even under skywave interference, after phase spectrum smoothing, the PSSD algorithm can still accurately capture the original characteristics of the signal. Regardless of its position and SGR, the impact of strong skywave interference on the trend of \(\Delta\Phi\) is negligible, thus ensuring the correctness of demodulation. In addition, more experiments found that skywave brings a power gain effect to the mixed signal, changing the SNR. When skywave and groundwave are in phase, the SNR and demodulation accuracy increase. Conversely, when out of phase, they decrease. This phenomenon occurs in both the PSSD and EPD algorithms. However, as shown in Figure 9, skywave interference does not impact the phase difference or the algorithm itself. In reality, there are also skywave separation techniques to minimize its impact on eLoran systems [13].  
Drawing from the above theoretical discussions and empirical results, it can be concluded that the PSSD algorithm can effectively resist skywave interference and safeguard the accuracy of the demodulation process. This discovery indicates that in subsequent simulation experiments, skywave interference is no longer a critical factor requiring special attention. Thus, we can focus on optimizing the algorithm's performance and evaluating its performance in different practical scenarios. Combined with the PSSD algorithm's ability to deal with in-band noise and CWIs, this makes it an all-encompassing and potent demodulation method for eLoran systems.