In this paper , the gdop ( geometric dilution of precision ) expression of area navigation system is deduced . using the matrix theory the lowest gdop and that gdop is smaller with more navigation stations are proved in theory 本论文推导了区域导航定位系统的几何精度因子,并用矩阵理论对其最小值及其随导航台个数增加而减小进行了证明。