# A Review of High-Sensitivity Tracking Techniques for Satellite Navigation Signals
## 2 Methods
### Vector Tracking Loops
#### 2.3.1 Evolution of Vector Tracking Technique  
The concept of VT was first proposed by [PERSON] et al. [28]. Later, [PERSON] introduced the idea of the vector delay-locked loop (VDLL) [98]. [PERSON] [20] then further expanded on tight and deep combination VT structures, which are shown in Figure 6. The primary distinction between tight and deep combinations lies in their relationship with the tracking loop. The tight combination enhances the performance of the tracking loop by integrating auxiliary data, while the deep combination completely replaces the traditional tracking loop, using the navigation solution directly to control signal processing. The deep combination offers greater stability and sensitivity, especially in challenging signal conditions, as it predicts signal parameters based on the navigation solution, reducing reliance on real-time signal tracking. [PERSON] et al. [31] demonstrated the advantages of VT in highly sensitive, highly dynamic environments through mathematical derivations and theoretical analysis. [PERSON] [99] developed discrete parameter models and transfer function models for VT, including VDLL and vector frequency lock loop (VFLL). [PERSON] [100] categorized the tight combination approach, which integrates the pseudo-range/velocity/time (PVT) solution with signal tracking, as a cascaded VT structure, while the deep combination approach was classified as a centralized VT structure. [PERSON]'s research on centralized VT algorithms led to several important findings. He implemented the VDLL algorithm using a standalone EKF and showed through simulations that VDLL could rapidly recover signals after masking. [PERSON] also investigated the VFLL algorithm and concluded that a VT-based receiver could provide continuous location services, even in dense forests [29]. He further demonstrated the thermal noise performance of VDLL and VFLL using Monte Carlo methods and established their empirical tracking thresholds [30]. Additionally, he compared the observational models of VDLL and the standard delay-locked loop (SDLL), noting that VDLL performance depends on the number and geometric distribution of visible satellites [101; 102]. Building on [PERSON]'s work, [PERSON] proposed a deeply coupled multi-receiver vector tracking (MRVT) architecture to enhance the reliability and robustness of GPS signal tracking and position estimation. MRVT addresses GPS signal intermittency by jointly tracking signals from multiple GPS receivers, thereby improving positioning reliability during weak or intermittent signal conditions. Several key studies have examined cascaded VT structures. [PERSON] proposed an unweighted cascaded VT structure in which local filters are retained in each channel. This structure degenerates into a scalar tracking system when the locally replicated signals are controlled solely by local filters and into a VT system when controlled exclusively by the PVT solver filters. The local filter serves two main functions: (1) filtering high-frequency signal processing results (such as the output of the signal discriminator) and feeding them into the PVT decoder filter at a lower rate, thus completing the VT algorithm and reducing computational load, and (2) performing signal environment detection. The local filter can identify when the signal environment is unsuitable for VT, at which point local signal generation is directly controlled. Simulation results show that this tracking structure outperforms traditional cascaded VT in terms of environment adaptability [103]. Although much of the research so far has focused on the VT of the carrier frequency or pseudo-code phase, recent developments have shifted toward carrier phase tracking due to increasing demands for higher-accuracy positioning [19; 33; 39; 104]. Pseudo-code positioning typically provides accuracy on the order of meters, while carrier phase positioning achieves accuracies of about 19 cm. The first vector-based carrier phase tracking technique was Co-OP tracking, proposed by [PERSON] [19].