Robust UAV Relative Navigation With DGPS, INS, and Peer-to-Peer Radio Ranging

This paper considers the fusion of carrier-phase differential GPS (CP-DGPS), peer-to-peer ranging radios, and low-cost inertial navigation systems (INS) for the application of relative navigation of small unmanned aerial vehicles (UAVs) in close formation-flight. A novel sensor fusion algorithm is presented that incorporates locally processed tightly coupled GPS/INS-based absolute navigation solutions from each UAV in a relative navigation filter that estimates the baseline separation using integer-fixed relative CP-DGPS and a set of peer-to-peer ranging radios. The robustness of the dynamic baseline estimation performance under conditions that are typically challenging for CP-DGPS alone, such as a high occurrence of phase breaks, poor satellite visibility/geometry due to extreme UAV attitude, and heightened multipath intensity, amongst others, is evaluated using Monte Carlo simulation trials. The simulation environment developed for this work combines a UAV formation flight control simulator with a GPS constellation simulator, stochastic models of the inertial measurement unit (IMU) sensor errors, and measurement noise of the ranging radios. The sensor fusion is shown to offer improved robustness for 3-D relative positioning in terms of 3-D residual sum of squares (RSS) accuracy and increased percentage of correctly fixed phase ambiguities. Moreover, baseline estimation performance is significantly improved during periods in which differential carrier phase ambiguities are unsuccessfully fixed. Note to Practitioners-This paper was motivated by the need to enhance the robustness of CP-DGPS/INS relative navigation. In particular, small UAVs exhibit fast dynamics and are often subjected to large and quickly changing bank angles. This in turn induces missed satellite observations and changes in the phase ambiguity. This paper suggests leveraging the emergence of Ultra Wideband ranging radios to directly observe the baseline separation. In this paper, we outline the details of the algorithm implementation. We then use a simulation to show that adding UWB greatly helps to enhance the robustness of the carrier ambiguity integer-resolving algorithm, which is necessary for improved solution accuracy. This work has extensions to ground vehicles, ocean buoys, and space vehicles. In future work, we will experimentally validate results.