  Path planning algorithms for unmanned aerial or ground vehicles, in many
surveillance applications, rely on Global Positioning System (GPS) information
for localization. However, disruption of GPS signals, by intention or
otherwise, can render these plans and algorithms ineffective. This article
provides a way of addressing this issue by utilizing stationary landmarks to
aid localization in such GPS-disrupted or GPS-denied environment. In
particular, given the vehicle's path, we formulate a landmark-placement problem
and present algorithms to place the minimum number of landmarks while
satisfying the localization, sensing, and collision-avoidance constraints. The
performance of such a placement is also evaluated via extensive simulations on
ground robots.
