Abstract: Positioning dilution of precision(PDOP) availability of navigation constellation is one of the important performance indexes in the satellite navigation system.PDOP availability is affected by satellite inherent reliabilities,orbit regulation,single particle overturn and constellation formation and so on.In this paper,satellite availability model is set up by using Markov chains firstly,then the constellation PDOP availability model is established by using probability and combination method.Finally,simulation and analysis of PDOP availability around our country is achieved under a given faults probability and faults restoring probability of satellites.The simulation results indicate that the constellations,the number and types of faults,faults probability and faults restoring probability have notable effects on PDOP availability.In order to improve PDOP availability,all the faults should be avoided or reduced,especially the single particle overturn fault.The proposed model of PDOP availability is impersonal and rational,which can evaluate the performance of navigation constellation and have significant reference to enhance Chinese navigation system's international competence.