Space navigation users of GNSS can be classified into four categories: low orbit users,medium orbit users,high orbit users and deep space users.The GNSS space navigation capability can be analyzed by visibility,GDOP and received EIRP.The transmit antenna pattern and space receiver antenna orientation are important factors which should be considered carefully.In this paper the navigation requirements are put forward respectively for different types of space user.And the related factors are studied theoretically.At last,the navigation capability for space users of one MEO constellation is simulated with “satellite navigation mission analysis simulator (SNMAS)”.The simulation illustrate that: the constellation can provide almost the same navigation service for the low orbit users and the land users; the “continuous positioning” requirement of medium orbit users can be reached; the “intermittent positioning” and “continuous timing” requirement of high orbit users can also be realized; but the “continuous timing” requirement of deep space users can not be realized such as lunar orbit users.