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 36﾿m. 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 15﾿m. Root-mean-square relative position errors were 1.2﾿m horizontally and 0.44﾿m vertically during flights at the closest separation.