 often used interchangeably. There is one technicality to note here because stratum 1 and PRS are not the same. The difference is that a stratum 1 clock, by definition, is autonomous (meaning it requires no input signal). This means that the normal mode of operation for a stratum 1 clock is free-running rather than locked, as it has no reference signal to acquire or lock to (being autonomous). However, a PRS can be either autonomous or not. For example, a PRS based on a GNSS receiver is not autonomous (when the GNSS signal disappears, then the quality degrades below stratum 1), and it is not stratum 1 without the GNSS guidance (because it likely does not have a cesium oscillator). But it is traceable to both a stratum 1 clock and a source of UTC (in the GNSS system), and it is as accurate as a stratum 1 clock, even though it is not a stratum 1 clock. On the other hand, a highly accurate clock based on a cesium oscillator that can meet the stratum 1 performance requirement is a combination of a PRS, a stratum 1 clock, and an autonomous clock. T1.101 defines the stratum levels based on clock performance parameters such as Free-run accuracy: Maximum long-term deviation from the normal frequency without the use of any external reference (such as the 1 X 10-1 11 from PRS). Holdover stability: Maximum change in frequency with respect to time when the clock loses connection with its reference clock and tries to maintain its maximum accuracy (using data learned while being locked). Not applicable for the PRS, because it is always free-running. Pull-in range: Maximum offset between the reference frequency available to a slave and a spe- cified nominal frequency that will still allow the slave to achieve a lock state. Not relevant to a PRS, because it has no input reference. Hold-in range: Maximum offset between the reference frequency available to a slave and a specified nominal frequency that will still allow the slave to maintain a lock state as the refer- ence signal varies throughout its allowed range of frequency. Al