Space Rubidium Atomic Frequency Standard for BeiDou Navigation Satellite System

China’s navigation satellite system BeiDou is developed according to a three step strategy [1]. The first step construction begun in 1994 and ended in 2000, as the BeiDou Navigation Demonstration System, as known as BeiDou I, provided radio determination satellite service (RDSS). The second step covers from 2004 to 2012, and is to construct the BeiDou navigation satellite System (BDS, also called BeiDou II sometimes), providing radio navigation satellite service (RNSS) for users in China and nearby regions. The third step involves functional enhancement of the system, making the BDS system able to provide high precision RNSS service for the worldwide users in 2020. As result of the second step construction, the current BDS system consists 16 satellites, including 5 MEO, 6 GEO and 5 IGSO satellites, and could provide the service with positioning precision of 10m in both horizontal and height, velocity measurement precision of 0.2m/s, and timing precision of 50ns. The service has covered the whole Asia Pacific region.