L1 guidance logic extension for small UAVs: handling high winds and small loiter radii

L1 guidance logic is one of the most widely used path following controllers for small fixed-wing unmanned aerial vehicles (UAVs), primarily due to its simplicity (low-cost implementation on embedded on-board processors, e.g. micro-controllers) and ability to track both circles and lines, which make up the vast majority of a typical fixed-wing vehicle's flight plan. The logic was later extended for speed independent dynamic similarity with an adaptive L1 distance, a formulation currently used on common open-source autopilot platforms. Two primary drawbacks remain, specific to small, slow flying fixed-wing UAVs; namely, 1) the combination of low operator defined gains and high ground speeds may violate the bounds of the algorithms convergence region for the case of loiter circles with small radii and 2) L1 logic breaks down when wind speeds exceed the vehicle's airspeed, another common predicament for small, slow-flying UAVs. This brief presents simple extensions to this extensively field tested algorithm, allowing legacy operators to keep existing controller tunings while taking advantage of the enhanced performance and safety features developed within.