LEO Doppler-aided GNSS position estimation

Positioning systems based on the global navigation satellite system (GNSS) face significant problems in areas with severe obscuration or GNSS interference, where most GNSS signals are blocked or jammed by interference sources causes reducing the number of available satellites. Signals of opportunity (SOOPs) from a large number of future low-earth-orbit (LEO) satellites are expected to enhance the GNSS system. We propose an aided joint position algorithm to track the receiver position, clock bias, and clock drift by using LEO Doppler together with pseudorange and Doppler measurements obtained from the GNSS. Taking the combination of 2 GPS and 2 ORBCOMM satellites as an example, the sensitivity inequality of the positioning accuracy to different measurement errors is analyzed. Since LEO satellites may have large errors after long-term orbit extrapolation, we analyze the impact of LEO orbit errors on positioning accuracy and possible positioning results if there are more GPS/LEO satellites. Since pseudorange and Doppler measurements are two types of heterogeneous measurements and dilution of precision cannot be used as a unified indicator, we present the relationship between the Cramer–Rao bound (CRB) and position error. Simulation and experimental results show that under the combination of 2 GPS satellites and 2 ORBCOMM satellites, the positioning accuracy is approximately several hundred meters or kilometers for static users and sensitive to GPS Doppler error. With more LEO satellites, the positioning accuracy can reach about 250 m. The CRB can be used as an index to evaluate the position accuracy of the joint system.