## 8 Discussion  
For the simplicity of discussion, only Beidou Navigation Satellite signals are applied as the GNSS sources in this research; however, the inter-satellite link signals of other navigation satellites (including global ones such as GPS, GLONASS, and GALILEO and regional ones such as QZSS and IRNSS) can also be used in the proposed method, but only through adjustment of the CEI receiving ends for different signal frequency and signal modulation regimes, with substantially equivalent tracking and measurement accuracy for the high-value cislunar space targets. Furthermore, the impact of monitoring and measuring cislunar targets when there are interference signals on the GNSS Inter-Satellite Links should be noted. Previous studies have proven that the majority of the interference signals that the GNSS Inter-Satellite Link faces belong to narrow-band interference (NBI), followed by single frequency interference [57; 58]. Currently, the technology of spread spectrum (SS) is mostly applied to cope with those interference signals, which can be categorized into four types [59; 60]:  
(i) adaptive finite-impulse response (FIR) filtering method, usually consisting of a linear predictive filter and a linear interpolation filter, whose suppression effect will substantially deteriorate on NBI signals, compared with that on single frequency interference ones. (ii) frequency domain adaptive filtering method, which requires the fast Fourier transform (FFT) operation that greatly increases the computation complexity, and in some cases, spectral leakage may occur. (iii) infinite impulse response (IIR) notch filtering method, which also increases the computational complexity through FFT and spectrum analysis, and its final interference suppression effect tends to be far from satisfactory as the notch filter bandwidth can hardly be precisely controlled by bandwidth parameters. (iv) code-aided technique, which requires no prior information on interference, interference detection, and FFTs, as it calculates the antijamming weight based on the received signal and processes the signal directly in the time domain. Figure 27: Final measurement accuracy for cislunar spacecraft through proposed method (BD-3 in GEO/IGSO). Based on the above analysis, the code-aided technique could be a more applicable method to deal with interference signals on the BeiDou-3 Inter-Satellite Link, and the relevant research could be a reference for future lunar missions, especially for the monitoring and measuring of high-value lunar space targets. To suppress narrowband interference (NBI) in direct sequence spread spectrum (DSSS) systems, in [61], [PERSON] first applied the code-aided interference suppression technique to a direct sequence (DS) UWB system, and then improved the NBI suppression ability of the original code-aided technique by introducing a certain form of spreading sequence. Furthermore, it was suggested in [62] to use an adaptive code-aided technique based on the RLS algorithm, and an improved method was proposed to modify it for massively parallel signal processing. Meanwhile, [57] proposes an adaptive narrow-band interference (NBI) suppression approach for the inter-satellite links (ISLs) of GNSS based on a code-aided technique, and the concept of interference influence coefficient is proposed to evaluate the influence of residual interference on the carrier-to-noise ratio (CNR) at the receiving end. It should also be confirmed that the proposed method in this research uses high-frequency radio frequency inter-satellite links, as they are the most commonly accessible ISL sources among all of the GNSS systems. However, with the emerging advancement of laser communication technology, laser inter-satellite links have appeared in some of the latest generations of GNSS ISL systems, which are expected to be the dominant signal forms for future GNSS ISL systems and could provide enhanced navigation and timing services.