  Modular robots have been rising in popularity for a variety of applications,
and autonomous midair docking is a necessary task for real world deployment of
these robots. We present a state estimator based on the extended Kalman filter
for relative localization of one multirotor with respect to another using only
onboard sensors, specifically an inertial measurement unit and a camera-marker
pair. Acceleration and angular velocity measurements along with relative pose
measurements from a camera on the first multirotor looking at a marker on the
second multirotor are used to estimate the relative position and velocity of
the first multirotor with respect to the second, and the absolute attitude of
the first multirotor. We also present a control architecture to use these
onboard state estimates to control the first multirotor at a desired setpoint
with respect to the second. The performance of the estimator and control
architecture are experimentally validated by successfully and repeatably
performing midair docking -- a task that requires relative position precision
on the order of a centimeter.
