  We present the first decentralized multi-copter flock that performs stable
autonomous outdoor flight with up to 10 flying agents. By decentralized and
autonomous we mean that all members navigate themselves based on the dynamic
information received from other robots in the vicinity. We do not use central
data processing or control; instead, all the necessary computations are carried
out by miniature on-board computers. The only global information the system
exploits is from GPS receivers, while the units use wireless modules to share
this positional information with other flock members locally. Collective
behavior is based on a decentralized control framework with bio-inspiration
from statistical physical modelling of animal swarms. In addition, the model is
optimized for stable group flight even in a noisy, windy, delayed and
error-prone environment. Using this framework we successfully implemented
several fundamental collective flight tasks with up to 10 units: i) we achieved
self-propelled flocking in a bounded area with self-organized object avoidance
capabilities and ii) performed collective target tracking with stable formation
flights (grid, rotating ring, straight line). With realistic numerical
simulations we demonstrated that the local broadcast-type communication and the
decentralized autonomous control method allows for the scalability of the model
for much larger flocks.
