# Research on Tracking Technique Based on BPSK-CSK Signals
## 1 Introduction  
With the development of modern GNSS signals, the future will see a surge in the number of users, a diversification of user ranges, an increase in information content, and a higher demand for real-time high accuracy. In order to achieve localisation in urban building environments, a higher information transfer rate is required [1; 2; 3; 4]. The advantages of BPSK modulation are the simple modulation process and mature capture and tracking algorithms. However, with the rapid development of GNSS, the limitations of BPSK modulation are becoming more and more prominent. Firstly, the rapid increase in the number of navigation satellites leads to more and more congestion in the frequency band of navigation signals, and the problem of mutual interference of signals becomes more and more serious. Secondly, BPSK signals have limited ranging accuracy and poor multipath resistance, which cannot meet the demand for high-precision positioning. In order to increase the bit rate, the traditional methods are to increase the Pseudo Random Noise (PRN) code rate or reduce the PRN code length. However, the former increases the spectral width and the latter affects the orthogonality of PRN codes [5; 6; 7]. The emergence of CSK modulation can effectively solve the above problems and can effectively increase the bit rate to make the information rate flexible and variable [8; 9; 10]. CSK modulation has the following advantages over traditional BPSK modulation: firstly, it can realise non-coherent demodulation; secondly, there is no need to modify the symbol rate of the reference signal, the code rate, or the length of the PRN code when the original code bit rate is increased; thirdly, the flexibility of the code bit data rate is much higher; fourthly, the CSK modulation is a shift of the pseudo code sequence, which does not need to modify the information rate. The sequence is shifted without extensive hardware modifications. However, there are also problems such as that the synchronisation process is difficult to implement, the receiver complexity is high, and the number of correlators is much larger than BPSK [11; 12; 13]. CSK modulation can flexibly adjust the information transmission rate while keeping the main code period unchanged, which can satisfy a variety of application scenarios [14; 15]. The CSK modulation used in the L6 signal of Japan's Quasi-Zenith Satellite System (QZSS) achieves a symbol rate of 2000 bit/s, which is much higher than the 50 bit/s information transmission rate of the L1C/A signal of the Global Positioning System (GPS) information transmission rate [16]. In addition, scholars have investigated the use of CSK modulation in optical communications [17; 18] and underwater acoustic communications [19] to achieve high-rate message transmission without carrier synchronization. Compared with BPSK signals, CSK-modulated signals have a higher transmitted information rate and even achieve demodulation gain within the same master code period [20; 21; 22]. However, compared with BPSK, CSK consumes more resources and has higher computational complexity. For CSK signals, scholars have mainly studied the improvement of CSK modulation and demodulation algorithms, as well as the combination of coding techniques to improve the reliability of transmission [23; 24; 25; 26]. The tracking algorithms for BPSK signals are relatively mature, and the research on CSK signal tracking is not sufficient, mainly focusing on reducing the number of correlators and the demodulation complexity. Literature [27] extends the definition of CSK signals and proposes a CSK capture tracking method, but this method is not applicable to standard CSK signals. Literature [28] proposed an improved tracking method for CSK signals, which reduces the implementation complexity at the expense of tracking accuracy.