Orbital configurations for large LEO constellations providing navigation services

Several very large constellations in Low Earth Orbit (so called BigLEOs), with hundreds up to thousands of satellites, are currently operational, generally providing high bandwidth internet services. Use of these systems to provide navigation services in addition to telecommunication ones, either in cooperation or as sources of opportunity signals, has been repeatedly suggested: the huge advantage of such a proposal stays with the signal to noise ratio far stronger than traditional GNSS constellations in MEO, potentially enabling indoor positioning. Many technical issues are involved, including the signal identification and availability, the time synchronization of the sources, the accurate knowledge of their ephemerides and their broadcast. However, the concept is gaining momentum, and there is the motivation for in-depth analyses and trade-offs among different possible solutions. This contribution focusses on the space segment's configuration of BigLEOs, evaluated with respect to the provision of navigation services. The aim is to help in identifying the possible systems' performance, which is knowledgeably affected by the relative geometry between user and sources and measured by the dilution of precision parameters (DOPs). DOPs actually depend on the nature of the intended observable, so that analyses well known for systems as GPS, working on the basis of the time-of-arrival observable, cannot be automatically extended to the new case. Specifically, this paper presents results related to the instantaneous Doppler shift observable, a promising solution in terms of the use of signals of opportunity from LEO existing systems. In addition to the analysis of numerical simulations carried out for BigLEOs, a discussion on service availability at different latitudes - quite depending on the orbit configuration of the constellations but not at all intuitive due to their complex, heterogeneous sets of orbital parameters - is included. The results obtained are deemed useful for a possible mission design of these novel, emerging navigation services. A brief note about technical issues not dependent on the orbital design yet instrumental to possible applications of the concept is proposed.