  The isoline tracking of this work is concerned with the control design for a
sensing robot to track a given isoline of an unknown 2-D scalar filed. To this
end, we propose a coordinate-free controller with a simple PI-like form using
only the concentration feedback for a Dubins robot, which is particularly
useful in GPS-denied environments. The key idea lies in the novel design of a
sliding surface based error term in the standard PI controller. Interestingly,
we also prove that the tracking error can be reduced by increasing the
proportion gain, and is eliminated for circular fields with a non-zero integral
gain. The effectiveness of our controller is validated via simulations by using
a fixed-wing UAV on the real dataset of the concentration distribution of PM
2.5 in Handan, China.
