Common view means two observers at two different locations observe the same GPS/BDS satellite at the same time. GPS/BDS common-view method is one of the main time comparison methods for long distance on earth so far. It is a system firstly used to make the time comparison between the atomic oscillators in international timing labs in 1983. With this method, the uncertainty of time transfer can reduce to very small value, may be as low as 3-5ns. Therefore, it can be used by the users who need time service accuracy as good as nanoseconds level in the field of aerospace and missile, and those who need to take ultra-high precision time/frequency synchronization measurements in 5G networks.