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With the development of global navigation satellite system (GNSS), research on evaluation theory of GNSS availability has drawn increasing attention.The current studies about GNSS availability mainly focus on long-term fault monitoring and global navigation constellation, however, the availability evaluation theory with respect to regional instantaneous situations is still unestablished. This article extends the global availability of GNSS to the availability in a regional area. The GNSS availability model under the influence of navigation signal fault is reviewed. Based on that, the number of visible satellites and PDOP (Position Dilution of Precision) values in the service area are considered and analysed. The method of regional instantaneous availability evaluation is then proposed. The regional availability is simulated and verified, which shows the feasibility of the proposed evaluation method