In ntp, what stratum consists of atomic devices or gps clocks?

In a network of computers the computers and their time is synchronized with the Network Time Protocol (NTP) protocol. NTP is a hierarchical protocol that uses Coordinated Universal Time (UTC) to synchronize computer clock times to a millisecond, and sometimes to a fraction of a millisecond. The NTP protocol consists of several stratums: Stratum 0 is the reference clock (the gps clock). The devices in Stratum 1 are synchronized with the clock in stratum 0 and have most accurate time.