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Service availability for accuracy is a basic element in the performance frame of the GNSS services.In the system designing phase,service availability is a key link to the satellite constellation and maintenance strategy,the distribution of internal reliability performance and the decision of service accuracy.In the system operation monitoring phase,service availability is one of the emphases of system service performance assessment and evaluation.In this paper,the understanding for the concept of service availability in the field of satellite navigation is introduced firstly.Then,some key problems in the design of satellite navigation service availability are studied,a calculation formula of service availability is proposed based on multiple constraints such as single satellite availability,ranging error and user operating conditions,and the accuracy threshold for the service availability is established which is different form the GPS SPS.Finally,according to two configuration alternatives of the BeiDou global constellation,simulation analysis the service availability of the Asia-Pacific region is made,pointing out the difference between the two alternatives,providing a reference for the future BeiDou global system optimization.