A low-cost mobile sensor network structure with a sensing mechanism is presented to explore the border of unknown target areas effectively. At first, mobile sensors with GPS receivers are organized into groups called SensorGroup (SG) which works well for the detecting jobs. After that, an advanced structure called Centralized SensorGroup (CSG) is proposed to further reduce the device cost since only the center node of the group is equipped with a GPS receiver. To detect the target area, an S-shaped sensing mechanism is also presented to cooperate with SG and CSG. The operation processes and detection phases are provided and verified. The detection performance is evaluated by detection completeness and accuracy in different scenarios in a square area of 100m*100m sensing environment. The simulation results show that CSG has good detection accuracy and the same performance as that of SG.