## 1 Introduction  
During the past decades, global navigation satellite systems (GNSS) have served as the foundation for positioning, navigation, and timing (PNT) services that are critical for agriculture, industry, and daily life [1]. Existing GNSS satellites are typically deployed in the medium Earth orbit (MEO) to provide extensive signal coverage [2]. Consequently, due to their extremely weak power levels, GNSS signals are highly susceptible to interference and spoofing while suffering severe degradation in urban canyons and indoor environments [3, 4]. The burgeoning low Earth orbit (LEO) constellations present a new opportunity to overcome the limitations of existing GNSS and enhance the quality of PNT service [5; 6; 7]. Numerous studies have demonstrated the advantages of LEO-based PNT services. (1) Compared to the existing MEO GNSS, the received signal power of LEO satellites enhances by 25-30 dB [4; 8; 9]. (2) Rapid orbital motion of LEO satellites induces faster geometric changes, reducing the convergence time of precise point positioning (PPP) from 30 to just 1 min [10]. Some studies focus on dedicated LEO satellites for PNT services. The first dedicated LEO navigation satellite was the TRANSIT system designed by the US Navy [11]. Luojia-1A, developed by Wuhan University, broadcasts a dual-band ranging signal with its orbital data, significantly improving GNSS positioning performance [12]. The Centispace system, an LEO navigation augmentation system designed for GNSS, operates in the same radio frequency bands as GNSS (L1 1575.42 MHz and L5 1176 MHz) [13]. The Pulsar constellation, designed by Xona Space, not only provides enhanced GNSS services but also offers encrypted signals to support authentication [14]. Furthermore, by retrofitting existing Iridium satellites, the satellite time and location (STL) service aims to provide assured time and location estimates, even in challenging and strongly attenuated environments [15]. The limited coverage of LEO satellites [16] requires massive constellations to ensure continuous navigation services, imposing prohibitively high costs that render dedicated LEO navigation systems economically unfeasible. As most existing LEO constellations are designed for broadband communications [17], integrating navigation functions into communication signals has become a feasible solution. Three multiplexing methodologies are used to integrate communication and navigation: _time division multiplexing (TDM), frequency division multiplexing (FDM), and code division multiplexing (CDM)_[18]. In TDM, navigation signals are inserted into communication signal as a frame [19; 20]; FDM employs binary offset carrier (BOC) modulation to achieve spectral separation between communication and navigation signals [21; 22; 23; 24]; and CDM directly superimposes spread-spectrum navigation signals onto communication waveforms. But these multiplexing approaches inherently introduce mutual interference between communication and navigation signals. This fundamental conflict motivates the exploration of opportunity navigation, which directly leverages original reference signals in communication waveforms as navigation signal [25; 26; 27]. Due to the spectral efficiency and anti-multipath capabilities [28], orthogonal frequency division multiplexing (OFDM) has been adopted as the core physical modulation method in Starlink [29; 30] and 5G NTN networks [31; 32] The OFDM-based opportunity navigation is an increasingly vital and attractive paradigm [33; 34; 35]. Studies in [36; 37] investigate the feasibility of OFDM-based positioning; for non-cooperative LEO OFDM signals, cognitive approaches have been adopted for localization [38; 39]. However, there are some limitations for LEO OFDM navigation. (1) Cyclic prefixes (CPs) have been universally adopted in OFDM, and they are a part replica of the OFDM signal. These CP signals are used to mitigate inter-symbol interference caused by multipath effects, but no work analyzes their adverse impact on the navigation performance.