Correction: Survey on positioning technology based on signal of opportunity from low earth orbit

Since the advent of the Global Navigation Satellite System (GNSS), it has played a pivotal role in both military and civilian domains, making irreplaceable contributions to national defense and economic construction. As its application scope continues to expand, the demands on GNSS have far exceeded the initial design specifications. The most prominent issue is the inability of traditional satellite navigation receivers to meet positioning requirements in complex environments [1]. Firstly, the signal strength of satellite signals diminishes with increasing propagation distance during space transmission, resulting in weak signal power reaching the ground and limiting its application in urban areas and canyons. Secondly, GNSS operates on a single, transparent frequency point, making it vulnerable to malicious interference and deception, which can lead to service unavailability. The limitations of Global Navigation Satellite Systems have been significantly exacerbated in recent battlefield scenarios observed during the Russia-Ukraine conflict, where such systems have demonstrated critical vulnerabilities and operational unreliability in combat environments. In stark contrast, LEO satellite constellations exemplified by Starlink have emerged as resilient alternatives. These advanced LEO systems not only maintain robust communication capabilities but also demonstrate enhanced positioning potential in complex battlefield conditions, presenting a paradigm shift in tactical navigation solutions. Over the past decade, an increasing number of researchers have demonstrated the potential of signals of Opportunity (SOPs) in Positioning, Navigation, and Timing (PNT), which can effectively compensate for the shortcomings of GNSS.SOPs positioning technology offers a viable alternative for positioning services when GNSS signals are unavailable or denied. SOPs encompass all potential radio signals in the environment from which location and time information can be extracted for navigation purposes. These signals are categorized into land-based and space-based SOPs. Ground-based SOPs, such as radio, mobile communication, and WIFI signals, primarily cover urban areas but lack coverage in deserts, oceans, and remote regions. In contrast, space-based SOPs utilize Earth-orbiting satellites as radiation sources, including non-cooperative/non-navigation satellite signals, non-cooperative navigation satellite signals, and cooperative non-navigation signals. Compared to ground-based SOPs, space-based signals offer the advantage of extensive coverage, enabling seamless global positioning. Among these, Low Earth Orbit (LEO) satellite signals are a typical example of space-based SOPs emitters. Compared with GNSS satellites in Medium Earth Orbit (MEO), LEO satellites exhibit significant advantages, such as rapid geometric changes, stronger received signal strength, and larger Doppler frequency shifts [2]. Additionally, many LEO constellations possess rich spectral resources and strong anti-interference capabilities. Moreover, Two-Line Element (TLE) data for LEO satellites is readily available, allowing for precise satellite position calculations through models like the Simplified General Perturbations No. 4 (SGP4). These advantageous properties ensure the PNT capabilities of LEO satellites in GNSS-denied environments. Consequently, LEO constellations are considered a promising alternative for PNT services. Currently, numerous countries are planning or have already launched a large number of LEO satellites, providing abundant radiation sources for space-based SOPs [2-4]. Table 1 lists the main LEO satellite constellations that have been deployed or are planned both domestically and internationally.