measuring is achieved by double-difference observed quantities, which is based on “Position-Velocity Integrated System”. It is proved that PWCS theory is appropriate for this kind of system's observability analysis. Observability of the system under different N Asset Coverage is studied, and their ability of attitude measuring, sun as precision, is compared by simulation. The results show that, while the N Asset Coverage is more than 4, the system become completely observable. In such cases, the filtering parameter of navigation system can get convergent rapidly with a good precision. While continued increasing the N Asset Coverage and observed quantities of attitude measuring on this basis, the precision and convergence rate will not improve accordingly.