  This paper focuses on the stability analysis of a formation shape displayed
by a team of mobile robots that uses heterogeneous sensing mechanism. Depending
on the convenience and reliability of the local information, each robot
utilizes the popular gradient-based control law which, in this paper, is either
the distance-based or the bearing-only formation control. For the two and three
robot case, we show that the use of heterogeneous gradient-based control laws
can give rise to an undesired invariant set where a distorted formation shape
is moving at a constant velocity. The (in)stability of such an invariant set is
dependent on the specified distance and bearing constraints. For the two robot
case, we prove almost global stability of the desired equilibrium set while for
the three robot case, we guarantee local asymptotic stability for the correct
formation shape. We also derive conditions for the three robot case in which
the undesired invariant set is locally attractive. Numerical simulations are
presented for illustrating the theoretical results in the three robot case.
