  We consider a single non-holonomic Dubins-like robot traveling with a
constant longitudinal speed in an a priori unknown and unsteady planar
environment. The robot should detect, locate, and track the boundary of a
dynamic environmental scalar field. The field is measured by an on-board sensor
in a point-wise fashion at the robot's location. The focus is on unsteady
boundaries that evolve over time in an arbitrary fashion, including
deformations, i.e., changes of shapes and sizes. We present a sliding mode
control method for localizing and tracking such boundaries: the robot is
steered to the boundary and circulates in its close proximity afterwards. The
proposed control algorithm does not require estimation of the spatial gradient
of the field and is non-demanding with respect to both computation and motion.
The paper offers the proofs of technical facts required for rigorous
justification of non-local convergence of the proposed control law, as well as
theoretical illustrations of its performance in specific scenarios.
