Vision-aided Guidance and Navigation for Close Formation Flight

Close formation flight can extend an unmanned aerial vehicle's (UAV) range and endurance by utilizing lift from a wingman's wake vortices and by autonomous midair refueling or recharging. The prohibitive challenge in each of these applications is the highly accurate and reliable relative positioning that is required to station-keep in the wingman's wake and to dock, amid external disturbances. Global navigation satellite systems are well-suited to reliable absolute positioning, but they fall short for accurate relative positioning. This work proposes a relative positioning solution for UAV rendezvous and close formation flight that has been verified in multiple flight tests. A nonlinear estimation framework uses precise air-to-air measurements to correct onboard sensor measurements and produce an accurate relative state estimate that is resilient to intermittent relative measurement outages and degrades gracefully during extended outages. A guidance strategy compensates for wingman turn dynamics, acts explicitly on the estimated relative state, and is applicable to both rendezvous and formation flight. Ground testing showed a relative position estimate accuracy that is 2% of the separation distance, with successful detection and correspondence at up to 36m. Autonomous close formation flight tests verified the relative positioning solution over extended periods, as close as two wingspans, in winds that were 30%-40% of the cruise airspeed, and at altitudes as low as 15m. Root-mean-square relative position errors were 1.2m horizontally and 0.44m vertically during flights at the closest separation.