LEO Satellites for PNT, the Next Step for Precise Positioning Applications

Abstract

All the current GNSS constellations comprise L-Band satellites orbiting the Earth within different types of orbits. Although the widely used approach for decades comprised Medium Earth Orbits (MEO), - as it was the case for GPS, Galileo and GLONASS - QZSS, NavIC and BeiDou added Inclined Geosynchronous (IGSO) & Geostationary (GEO) Orbits satellites, with the aim of improving satellite visibility over certain areas. On the other hand, one of the main commercial GNSS-related applications over the past years has been high accuracy positioning, both differential (RTK) and absolute (PPP). As widely known, the main traditional drawback of PPP (Precise Point Positioning) versus RTK (Real-Time Kinematic) as a precise positioning solution, is the convergence time. Whereas RTK convergence is practically instantaneous, standard PPP processing (relying solely on orbit and clock information) requires between 30 to 40 minutes to achieve a fully converged solution. In order to bridge the aforementioned convergence gap, techniques such as PPP ambiguity fixing (by providing code and phase biases in addition to orbit and clocks) and multi-constellation processing have been used, reducing the PPP convergence time down to 10 minutes. Further techniques such as PPP-RTK have managed to reduce the convergence time close to the pure RTK solution, but with the cost of needing to provide precise atmospheric corrections to the end user. The aforementioned cost is paid in terms of required bandwidth, and reference network density, which needs to be largely increased within the area of interest, partially losing the advantage against RTK in terms of required ground infrastructure. Few years ago, the concept of bringing Low Earth Orbit (LEO) satellites within the GNSS satellites’ portfolio, was introduced. The MEO, IGSO and GEO satellites are far away to the Earth, and therefore their dynamics evolve in relatively slow motion. This implies that a relatively long time (minutes) is needed for achieved centimeter level accuracy (due to the time required for solving the carrier-phase ambiguity values). By using LEO satellites within the GNSS constellations, these aforementioned issues can be tackled, and therefore potentially removing the precise atmospheric corrections’ dependency for the RTK-PPP users.