Which NTP concept is used to measure the distance from a device to its authoritative time source?

Stratum is a hierarchical concept in NTP, where stratum 0 are high-precision time sources, stratum 1 devices are directly connected to stratum 0, and subsequent strata indicate the number of NTP hops from the primary source. A lower stratum number signifies a closer and more reliable time source.

An NTP peer is another NTP server that a device exchanges time information with, not a measure of distance to the authoritative source.

GPS is a type of authoritative time source (e.g., connected to a stratum 0/1 server), not a concept for measuring distance in NTP itself.

An atomic clock is a highly accurate type of clock used as a primary time source, but it is not an NTP concept for measuring distance.