# The Potential of LEO Satellite-Based Opportunistic Navigation for High Dynamic Applications
## 1 Introduction  
High dynamic vehicles require resilient navigation solutions. Indeed, the Global Navigation Satellite System (GNSS), which is the leading technology of positioning navigation and timing, is sensitive to jamming and spoofing. To cope with such a situation, the GNSS community has developed interference mitigation techniques, but another complementary solution is that which uses an alternative technology to GNSS when it becomes inoperative. This work deals with the navigation using signals of opportunity (SOP), more specifically the signals of the Iridium Next Communication System. SOP are all radio frequency (RF) signals that are not intended for navigation purposes. They are signals of communication systems (mobile networks, satellite-based communication systems), TVs, AM/FM radios, radar, etc. The advantages of SOP in navigation are the existing infrastructure that is free of use, the high signal power level compared to that of GNSS signals, and the high frequency diversity if many systems are used. The disadvantages of SOP stem from the fact that the signals are not optimized for navigation purposes. That is, the signals' availability is not guaranteed everywhere, signals from different transmitters (including those of the same system) are not synchronized in time and their clock stability is lower than that of GNSS satellites, and transmitter positions are unknown. In addition, if different systems are coupled, one needs multi-band antennas, multi-band RF front-end and a sufficient computing power. Despite these constraints, finding an alternative or an augmentation technology to GNSS is crucial. The research in the topic of navigation based on SOP essentially started two decades ago, and has gained interest over the last few years, especially with the announcement of the advent of future new mega LEO satellite (LEO SV) constellations,Starlink and OneWeb, and the modernization of the Iridium constellation (which became Iridium Next in 2017). The popularity of LEO SV systems for navigation is mainly thanks to their global coverage not only on Earth, but also in its surrounding space, which allows high-altitude vehicles to be potential users. In this paper, we study the potential of the Iridium Next LEO SV signals for the navigation of a high dynamic vehicle. The Iridium Next system offers voice and data communication. The company Satelles used the messaging to transmit bursts designed specifically for the navigation in a PNT solution called Satellite Time and Location (STL) [1]. Since positioning and namely timing capabilities have been demonstrated based on the STL technology, in this work, we will use the Iridium Next signals as signals of opportunity--that is, no navigation specific data are extracted from the signal in space. Navigation using LEO SV signals as SOP has been the focus of many papers. In [2], a differential positioning using time difference of arrival (TDOA) and frequency difference of arrival (FDOA) is implemented. Experiments showed a positioning accuracy of 25 m with 5 min data collected in a static location. In [3], the authors studied a positioning using LEO SV Doppler shifts that are loosely fused with an altimeter height. Experiments, using 1 min data from two Orbcomm satellites and by assuming a known altitude, exhibited a 2D positioning error of 358 m for a static observer. In [4; 5], the authors derived the performance of LEO SV measurements fused with an inertial navigation system (INS). A range of information from LEO SVs is assumed to be available in addition to the Doppler shifts. The algorithm refines the satellite position and velocity estimates by including them into the state in addition to satellite clock biases and drifts. The global positioning system (GPS) is only used at the beginning, and then the navigation filter is updated by LEO SV and altimeter height measurements. An experiment using 4 min data of 2 Orbcomm SV led to a positioning accuracy with a root mean square error (RMSE) of a few hundred meters for a land vehicle.