# A Review of High-Sensitivity Tracking Techniques for Satellite Navigation Signals
## 2 Methods
### Vector Tracking Loops
#### 2.3.4 Ultra-Tight Combining
Both centralized and cascaded ultra-tight integration directly use the I/Q values from each channel as filter measurements, making them types of coherent ultra-tight integration. The centralized ultra-tight integration system, first proposed by [PERSON] [13], uses a single combining filter to process tracking and data fusion information from all channels. This combined output helps correct accumulated INS errors and supports carrier tracking. [PERSON] [120] derived key mathematical relationships essential for designing centralized ultra-tight systems. [PERSON] [121] introduced a novel centralized ultra-tight system by integrating centralized VT with the Reduced Inertial Sensor System (RISS), enhancing GPS receiver robustness and sensitivity in challenging environments. [PERSON] proposed two ultra-tight coupling approaches based on different observation matrices and introduced an improved deep coupling structure with a multipath error estimator (MPEE), which significantly improves tracking robustness [122; 123; 124]. Later, [PERSON] [125] proposed an integrated error modeling scheme for GNSS tracking loops across the entire VT system, derived in an Earth-Centered Fixed-Earth (ECF) framework. This model addresses the error propagation from the combined navigation filter to INS, particularly focusing on acceleration errors caused by the rotation of the LOS direction, a critical factor for dynamic receivers or medium-orbit satellites. The cascaded ultra-tight integration system, developed by [PERSON] [126] and others, employs a two-stage filtering approach. The baseband I/Q information is first processed by individual pre-filters in each channel, and the output is passed to the main filter. The pre-filter processes the raw GNSS data, estimates errors in the main filter's code phase and  
Figure 8: Deep combining methods for INS-assisted vector tracking. carrier frequency predictions, and periodically corrects the main filter. The combined filter output then corrects both the INS and adjusts the pseudo-code NCO and carrier NCO by incorporating ephemeris data with the INS output. This system improves the high-dynamic performance of the receiver by measuring velocity and position changes at each update cycle of the VT filters and feeding the data back into the tracking loop [32; 127; 128]. [PERSON] optimized the KF algorithm in cascaded ultra-tight systems using fuzzy logic and the FLAS mechanism, integrating these innovations into the Adaptive Unscented Kalman Filter (AUKF) to develop the FASTUKF algorithm, which maintains excellent estimation accuracy and tracking capability [129; 130]. [PERSON] [131] and [PERSON] [132] focused on optimizing conventional pre-filters to reduce errors caused by pseudo-code tracking in cascaded ultra-tight systems. [PERSON] [133; 134] proposed a GPS/MEMS-SINS ultra-tight integration method using differential carrier phase velocity (TDCP) for loop tracking, reducing carrier loop noise and improving code correlator spacing [86; 135]. [PERSON] [135] introduced an ultra-tight integration approach that eliminates the tracking loop in the receiver's baseband, relying entirely on INS to track GNSS code signals, employing a moving window-based discontinuous tracking control method for signals of different wavelengths. In summary, the ultra-tight combination system has become a major area of research in GNSS receiver technology since the 1980s. Continuous innovations in the integration of GNSS and INS have enabled high-performance navigation in diverse environments. However, key technological challenges remain, particularly regarding performance degradation in highly dynamic situations and the use of low-cost MEMS sensors [136]. Table 7 summarizes the two principal approaches to ultra-tight GNSS/INS integration, emphasizing their advantages, challenges, and key references. Future research should focus on improving performance in dynamic environments and integrating other sensors to enhance ultra-tight combination systems.