###### Abstract  
Experts and scholars from various nations have proposed studying low Earth orbit (LEO) satellite signals as the space-based signals of opportunity (SOPs) for navigation and positioning. This method serves as a robust alternative in environments where global navigation satellite systems (GNSS) are unavailable or compromised, providing users with high-precision, anti-interference, secure, and dependable backup navigation solutions. The rapid evolution of LEO communication constellations has spurred the development of SOPs positioning technology using LEO satellites. However, this has also led to a substantial increase in the number of LEO satellites, thereby reintroducing the traditional challenge of satellite selection. This research thoroughly examines three critical factors affecting positioning accuracy: satellite observable time, satellite elevation, and position dilution of precision (PDOP). It introduces a strategic approach for selecting satellites in LEO SOPs navigation and positioning. Simulation outcomes confirm that this satellite selection strategy effectively identifies visible satellites, ensuring precise positioning through LEO SOPs. G 2020 2020 19 December 2020 2 January 2020 2 January  
sources, SOPs positioning encompasses Wi-Fi positioning [2], broadcast signal positioning [3], acoustic positioning technology [4], geomagnetic positioning [5], and light pattern recognition positioning [6]. From a positioning principles perspective, SOPs positioning includes time of arrival (ToA) [7], time difference of arrival (TDoA) [8], angle of arrival (AoA) [9], signal strength (SS) [10], and doppler shift (DS) positioning [11]. Particularly in recent years, the rapid development of low Earth orbit (LEO) constellations such as Iridium-NEXT [12] and Starlink [13] in the United States; Hongyan, Hongyun, and China Satellite Network in China; and OneWeb [14] in the United Kingdom has accelerated advancements in SOPs positioning technology for LEO communications satellites. In 2019, [PERSON] et al. proposed a positioning technology based on Iridium SOPs. They obtained doppler shift (DS) information through the single tone signal of Iridium and established an instantaneous doppler positioning model. The experimental results demonstrated positioning accuracy better than 200 m [15]. In 2020, [PERSON]'s team proposed a positioning technology based on Orbcomm satellite SOPs, achieving an accuracy better than 140 m in the final experimental results [16]. In 2022, the team completed research on a positioning method using Starlink SOPs. By analyzing the pilot signals of Starlink, they extracted the instantaneous doppler shift using the 11.325 GHz and 11.575 GHz frequency signals of Starlink. The experimental results showed a two-dimensional (2D) positioning accuracy better than 15 m with an observation time of less than 5 min [17]. In 2021, [PERSON], [PERSON], and [PERSON] proposed a doppler integrated navigation framework for Iridium NEXT and Orbcomm satellites. The experimental results indicated that using one Orbcomm satellite and four Iridium NEXT satellites resulted in a positioning error of 22.7 m [18]. In the same year, [PERSON]'s team published the first carrier phase tracking and positioning results with Starlink LEO satellite signals. They proposed a Starlink signal carrier phase tracking algorithm based on an adaptive Kalman filter. The experimental results clearly indicated a two-dimensional error of 7.7 m for known altitude information and two-dimensional and three-dimensional errors of 25.9 m and 33.5 m, respectively, for unknown altitude information [19]. Moreover, in 2022, the team proposed a framework for positioning with carrier phase difference low Earth orbit (CD-LEO) measurements. This framework uses a base and a rover to navigate using the Orbcomm LEO constellation without prior knowledge of the rover's position. The impact of ionospheric and tropospheric delays on carrier phase and CD-LEO measurements was also discussed, with experiments showing a final positioning error of 11.93 m [20].