Positioning information is important for cooperative swarm robotics in many applications. Yet the localization for robot swarm in harsh environments is often challenging due to the lack of external global positioning systems and guaranteed relative distance sensing capability. In this paper, we consider developing a cooperative localization method for robot swarms in structured uniform coverage configurations, which is functioning under no global positioning system and restricted inter-robot communication. We introduce the gradient mechanism that appears in natural embrionic systems. With the gradient mechanism, the robots can obtain long-range distance information through gradient propatation. With three gradient sources, robots can triangulate their own position. The method has been tested in uniform coverage configurations in simulations.