Elevation Angle Redundancy from Barometric Altitude in Multipath-affected Phased Array Radio Navigation of UAVs

Phased Array Radio Systems (PARS) are a promising alternative or backup to Global Navigation Satellite Systems (GNSS) based positioning, offering higher signal-to-noise ratio (SNR), narrow beam communication and robust encryption to mitigate these risks. However, PARS systems face multipath challenges, particularly when radio signals are reflected off horizontal surfaces such as flat fields, lakes and oceans, affecting the accuracy of elevation angle measurements. The proposed solution introduces the concept of a recalculated elevation angle, inspired by grazing angle determination, as an alternative to the potentially uncertain elevation angle provided by PARS. Derived from PARS range measurements, barometric altitude, and the effective Earth radius, the recalculated elevation angle aims to overcome the limitations of previous methods that failed to fully consider the Earth's curvature, leading to inaccuracies in elevation angle estimates. Our approach uniquely incorporates the recalculated elevation angle into the PARS-aided inertial navigation system (INS), enhancing positioning accuracy, especially when the UAV is operating in close proximity to the ground antenna. The paper evaluates the performance of the navigation system using the recalculated elevation angle using field test data. The root mean square vertical position error was improved by a factor of 7.5 with the proposed method compared to using multipath affected elevation measurement. The results show that the recalculated elevation angle is a viable alternative to the multipath affected measured elevation angle in PARS-based navigation.