User location determination using delay and Doppler measurements in LEO satellite systems

Although an external Global Navigational Satellite System (GNSS) solution is often relied upon for fixing the location of user terminals (UTs), fixing the UT position autonomously in LEO satellite systems is a tractable problem which has a system benefit. An autonomous solution comes to use if the GNSS solution becomes unavailable either temporarily (e.g., due to a delay in initial GNSS position fix) or for prolonged periods (due to major outages or regulatory constraints), or if the system security policies do not allow transmission of the user position over the air. In this paper, we describe two nonlinear root-finding algorithms (applicable either at the UT or at the satellite Gateway (GW)) that take as the inputs the delay and Doppler offset measurements from the physical layer receiver and generate an estimate of the user location (and optionally velocity) as the output. Algorithm simulation results suggest that a high accuracy estimate (in which position error exceeds 1 km in less than 10% cases) is possible when UT has visibility to more than one satellite, while less accurate results (e.g., ∼ 20% chance of position errors exceeding 2 km) are achievable in a single satellite scenario.