  Miniature multi-rotors are promising robots for navigating subterranean
networks, but maintaining a radio connection underground is challenging. In
this paper, we introduce a distributed algorithm, called U-Chain (for
Underground-chain), that coordinates a chain of flying robots between an
exploration drone and an operator. Our algorithm only uses the measurement of
the signal quality between two successive robots as well as an estimate of the
ground speed based on an optic flow sensor. We evaluate our approach formally
and in simulation, and we describe experimental results with a chain of 3 real
miniature quadrotors (12 by 12 cm) and a base station.
