e characteristics of a PRTC-B (the best PRTC class without having to buy a cesium atomic clock for tens of thou- sands of dollars). These characteristics are taken from a sample of currently available devices with the option of two different oscillators. Table 9-11 Typical Accuracy and Holdover Specifications for a PRTC Class B Metric Rubidium 10 MHz frequency accuracy over 24 hours (GNSS locked) 11x10-12 Frequency holdover at constant temp (per day) 4 X 10 10 2 X 10-11 1 PPS phase accuracy to UTC over 24 hours (GNSS locked) +40 ns +40 ns Phase holdover at constant temp (after 8 hours) 200 ns Phase holdover at constant temp (after 48 hours) 10 us The oscillators listed in Table 9-11 are using either a high-quality OCXO (better than stratum 3E) or a rubidium oscillator. All the holdover numbers are taken after a minimum of at least 48 hours sta- bilization time (locked to GNSS). Note that the specific numbers might vary a little (or even a lot) depending on the manufacturer, the cost, and the conditions of testing-but the table gives good estimates. Another thing to consider is the accuracy of the PRTC itself. When going into holdover with a PRTC-A (for example), the initial error (when holdover starts) is quite likely to be significantly higher (100 ns versus 40 ns) than if it was starting with a PRTC-B source. There is a similar advant- age to using eSyncE that is explained a bit later in this section. So, if the scenario requires 1 microsecond of holdover, then it will require good equipment, such as PRTC-B devices with rubidium oscillators in a temperature-controlled environment. Normally, this level of performance is not found in normal network elements outside devices specifically manufac- tured for specialized timing applications. There are also some operational factors to consider when comparing the performance of any clock in holdover: The rate of oscillator aging (even how new it is from the factory). The length of time the clock has already been running and aligned with the GNSS receiver, which impacts 