I found the source from the signal strength requirements, the ardupilot wiki:
TIME Mode of base station requires: 5 GPS satellite signals +2 GLONASS satellite signals in the strength of 40 or more; or 5 GPS satellite signals +3 Beidou satellite signal in the strength of 40 or more. As shown in the figure below, only one satellite strength is higher than 40, the signal condition does not meet the RTK standard.
http://ardupilot.org/copter/docs/common-here-plus-gps.html?highlight=here