More and more satellites are populating the sky nowadays in the Low Earth orbits (LEO). Most of the targeted applications are related to broadband and narrowband communications, Earth observation, synthetic aperture radar, and internet-of-Things (IoT) connectivity. In addition to these targeted applications, there is yet-to-be-harnessed potential for LEO and positioning, navigation, and timing (PNT) systems, or what is nowadays referred to as LEO-PNT. No commercial LEO-PNT solutions currently exist and there is no unified research on LEO-PNT concepts. Our survey aims to fill the gaps in knowledge regarding what a LEO-PNT system entails, its technical design steps and challenges, what physical layer parameters are viable solutions, what tools can be used for a LEO-PNT design (e.g., optimisation steps, hardware and software simulators, etc.), the existing models of wireless channels for satellite-toground and ground-to-satellite propagation, and the commercial prospects of a future LEO-PNT system. A comprehensive and multidisciplinary survey is provided by a team of authors with complementary expertise in wireless communications, signal processing, navigation and tracking, physics, machine learning, Earth observation, remote sensing, digital economy, and business models.

Logistics is an important driver for the competitiveness of industries and material supply. The development of smart logistics, powered by precise positioning and communication technologies can significantly improve the efficiency of logistics. The emerging technology of ultra-wideband (UWB) precision positioning has attracted significant attention throughout the previous decade owing to its promising capabilities over other radio frequency-based indoor localisation systems. In addition, UWB is characterised by large bandwidth and data rate, short message length, low transmission power and high penetration capability, which are all favourable for indoor positioning applications. However, UWB localisation technology faces several challenges that are somewhat similar to other technologies, such as mitigating errors that originate from non-line-of-sight (NLOS) situations and tackling signal interference in dense environments, and when required to operate in extreme conditions. This paper reviews the most recent advances made in UWB positioning systems over the last five years, with a focus on high-ranking articles. In addition to going through more conventional solutions to UWB challenges, modern solutions, which involve the use of machine learning and sensor data fusion, are discussed. We highlight the most promising findings of the recently implemented and foreseen UWB positioning systems by providing a summary of each reviewed article. Additionally, we address a major challenge that faces the UWB positioning technology: NLOS situations, focusing on some proposed remedies such as multi-sensor fusion and machine learning. As an application, this study introduces how UWB technology promotes smart logistics by offering indoor positioning to improve efficiencies in the delivery of goods from the source to the customer. Furthermore, it demonstrates the benefits of UWB technology for accurate positioning and tracking of both stationary and moving items, and machinery in an indoor logistics environment.

Capital expenditures and indoor challenges are two of the main obstacles towards equal-access positioning services worldwide. Global Navigation Satellite Systems (GNSS) are not well functioning indoors and in some outdoor challenging scenarios, such dense forest canopies, or hilly terrains rich in vegetation, due, for example, to multipaths and low carrierto-noise ratios. Terrestrial solutions can be nowadays used to complement GNSS, but they are typically costly to deploy with high coverage and do not offer equal access, for example in some low-revenue countries, in regions forbidding wireless 5G access due to health concerns, or in areas hard to reach with terrestrial infrastructure, such as deep jungle, desert areas with sandy dunes, or deep valleys/deep canyons. As many Low Earth Orbit (LEO) mega-constellations are emerging and their satellites are significantly closer to Earth than GNSS satellites, solutions based on LEO could complement GNSS. LEO-based communications are expected to be widespread in the next decade, and they will offer a global and-easy-to-access infrastructure, with the main costs to the end user coming from the receiver equipment. It is our assumption that future wireless receivers will support the integration of terrestrial and satellite infrastructure, and thus, the LEO-based positioning tasks could be mainly implemented as software adds-on on existing future receivers. Nevertheless, a closer proximity to Earth does not automatically mean stronger received signals or acceptable positioning accuracy, especially when the carrier frequencies of the new LEO signals are higher than those in GNSS. In here, we present a feasibility study of LEO-based equal-access localization, by looking at the current opportunities, benefits, and challenges of LEO megaconstellations used as signals of opportunities (SoO). We show that there is an unharnessed-yet potential of future LEO megaconstellations for equal-access localization, although several challenges are still to be overcome.

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