In previous chapters, sensors are all based on omni-sensing model and the sensing area is a disk. In this chapter, we consider the directional antennas or camera sensors and the sensing area is a sector, which can be described by its center s, a sensing angle α, a sensing radius r, and a sensing direction which is a vector f→ at s (Fig. 15.1). The vector f→ is the bisector of angle α, i.e., divides angle α evenly. The sector may be rotatable, i.e., the sensing direction is adjustable. There are two types of rotations, discrete and continuous. For the discrete rotation, there are a finite number of fixed orientations (Fig. 15.2). For the continuous rotation, the sector can be freely rotated and stops at any orientation.