# A Review of High-Sensitivity Tracking Techniques for Satellite Navigation Signals
## 2 Methods
### Scalar Tracking
#### 2.2.1 Extending Coherent Integration Time
Long-term effects are typically not a concern for carrier tracking, as any frequency deviations can be absorbed into the satellite Doppler estimate, and higher-order drifts are negligible compared to the satellite Doppler drift. Therefore, long-term effects are often ignored, and the short-term random effects of the oscillator are the primary factors affecting the receiver [23]. The impact of oscillator instability on coherent integration time has been extensively studied. Consumer-grade oscillators are typically limited to coherent integration times of up to 500 ms, while ultra-stable oscillators can support integration times of several seconds, which is critical for high-sensitivity GNSS applications [23; 70; 71; 72]. However, due to cost constraints, consumer-level GNSS receivers cannot afford the high-performance ultra-stable oscillators required for extended integration times. As a result, PLLs must be carefully designed to mitigate the instability of the local oscillator [73]. Currently, techniques to address GNSS receiver oscillator instability are broadly classified into two categories: difference-based methods and estimation-based methods. Difference-based methods were first introduced by [PERSON] and eliminate common clock errors by performing single and double differences at the correlator level between the rover and the base station. This technique achieves coherent integration times of up to 100 s for static applications, with a single-channel carrier tracking sensitivity of \(-14\,\mathrm{dB}\cdot\mathrm{Hz}\) and an indoor positioning accuracy of 1 m [74; 75]. [PERSON] achieved more robust tracking by appropriately modeling oscillator noise and using the Wiener filter as a causal filter applied to the carrier phase tracking problem [23]. Similarly, [PERSON] utilized Kalman filtering algorithms to track oscillator noise, thereby improving carrier phase tracking [26]. [PERSON] proposed a different approach, simultaneously estimating oscillator noise using multiple channels by tracking receiver position errors and oscillator biases with four filters [19]. Building on this, [PERSON] developed a carrier phase tracking method that estimates oscillator phase noise using information from multiple correlators [76]. Subsequently, [PERSON] introduced the quartz phase-locked loop (QLL) system, which optimizes the tracking loop into a common loop and multiple individual loops. This approach addresses the problem of oscillator instability caused by receiver vibrations, thereby improving the vibration immunity of GNSS receivers [77]. [PERSON] further advanced this approach by proposing a long, coherent integration method for improved carrier phase tracking. This method uses a multi-channel cooperative loop to track the receiver's oscillator error and combines it with a local loop for ultra-long coherent integration cycles. However, a key limitation is that strong satellite signals are required to achieve long coherent integration for weak satellite signals [12]. In summary, while there are mature solutions for problems like data code flipping and user dynamics when extending coherent integration time, the issue of oscillator instability remains an ongoing challenge. Although various methods have been proposed to address oscillator instability, each has its limitations, and a comprehensive solution is yet to be found. As a result, oscillator instability continues to be a primary focus of future high-sensitivity GNSS tracking research. Table 2 summarizes the major techniques proposed for extending coherent integration time, including their strengths, challenges, and representative works. This comparative overview highlights the evolving landscape of high-sensitivity GNSS tracking innovations.