This paper examines sensor scheduling problems for multi-sensor systems when only one sensor is allowed to take a measurement at each time. In particular, this paper formulates an optimal sensor scheduling problem for mobile robot exploration and navigation using a sensor network whose sensors are placed on the ground. The sensors are position sensors, and each measurement noise depends on the distance between the positions of the sensor and the robot. This paper provides an optimal sensor selection policy which minimizes a given quadratic cost function. In addition, we characterize the set of points where the available sensor is switched under the optimal sensor scheduling.