## 1 Introduction  
The global navigation satellite system (GNSS) has become a fundamental component of the position, navigation, and timing (PNT) service, providing high-precision positioning and timing services. Meanwhile, the GNSS provides L-band microwave signals with high temporal resolution, extending its applications from positioning/navigation to remote sensing. However, the GNSS core constellations, such as the global positioning system (GPS) in medium Earth orbit (MEO), exhibit significant signal attenuation after long-distance propagation. This limits the applicability of the GNSS in environments with high levels of deep attenuation and interference, including dense urban areas and indoor settings, which presents a challenge for the further application of the GNSS [1]. In recent years, many proposals have been put forward regarding the construction of numerous large constellations of low Earth orbit (LEO) satellites, which accelerates the development of LEO constellations. It is anticipated that the integration of communication and navigation functions in LEO satellites will result in cost savings, as the construction of multiple systems will no longer be necessary [2]. The navigation system based on a LEO constellation offers several advantages, including strong signal strength, good dilution of precision (DOP) factor, fast geometric changes that help improve precise point positioning (PPP) performance, etc. [3, 4, 5]. Signal modulation is the key technology for satellite navigation systems and has a significant impact on the performance of navigation signals [6]. Some modulations have been employed in the GNSS, starting with binary phase shift keying (BPSK) [7]. To separate the spectrums of different signals on the same center frequency, binary offset carrier (BOC) modulation was adopted [8]. In the context of the modernization of the GNSS, two new modulation techniques were proposed, multiplexed BOC (MBOC) and alternate BOC (AltBOC) [9]. AltBOC is capable of providing a greater degree of flexibility in terms of the services it can offer than the former [10]. Nevertheless, the existing navigation modulation cannot be employed directly for LEO satellites for two primary reasons. On the one hand, the high Doppler caused by the LEO high-mobility channel significantly increases the acquisition complexity of traditional navigation modulation; despite chirp spread spectrum-like modulations effectively reducing the acquisition complexity, the measurement accuracy decreases [11; 12]. On the other hand, the LEO constellation is primarily utilized for providing communication and Internet services [13; 14]; existing navigation modulation techniques face challenges in meeting the requirements of broadband communication, despite their inherent data transmission capacity [15]. It is therefore evident that the implementation of navigation based on broadband communication modulation has become an important research area [16]. Orthogonal frequency division multiplexing (OFDM) modulation is widely adopted in communication systems, primarily due to its high spectral efficiency and strong anti-multipath performance. Furthermore, the Cramer-Rao lower bound (CRLB) for OFDM delay estimation is lower than that of BPSK [17], indicating that OFDM can improve the ranging accuracy of LEO satellite signals for future high-precision navigation applications. The navigation performance study of OFDM modulation showed that the code-tracking performance of OFDM was superior to that of the other GNSS signals with a pre-filtering bandwidth greater than 2 MHz [18]. Both the long-term evolution (LTE) and new radio (NR) systems successfully use the positioning reference signal (PRS) based on OFDM modulation for independent positioning [19; 20; 21; 22]. As an OFDM-compatible modulation technique, orthogonal time frequency space (OTFS) has been considered a candidate for the next-generation communication system (6G) due to its excellent performance in high-mobility channels [23; 24].