No reference clock s 7.1.1. Therefore it should never be used, except when the system time is synchronized by some means not visible by xntpd . On an isolated network one might set the time manually from time to time. GPS Receivers 7.2.1. GPS receivers have high accuracy, are simple to install, rather immune against electromagnetic noise, and they only need an antenna with free view to the sky.
Most calibration happens automatically.
Unfortunately many of these receivers were not designed with accurate time-keeping in mind. Fortunately there are also receivers that were designed for precision timekeeping. Contributors for missing entries are very much invited. These parameters qualify the accuracy of a GPS receiver. Small values express high precision.