  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.
