  In certain systems which are subject to significant constant external forcing
such as an airplane in wind or an underwater glider in ocean currents, the
ability to detect the forcing depends on both the measurements available and
whether appropriate control is being applied. Using analytical nonlinear
observability techniques, we define the necessary characteristics required of a
measurement function for systems with unicycle dynamics subject to constant
forcing to be observable. We further consider the necessary associated motion
characteristics required of this class of systems to enable the desired sensing
capabilities. We then apply these results in combination with the empirical
Gramian to quantify relative observability, optimal sensor selection, and
sensing quality for different motion primitive modes for a Dubins path.
