on autonomous atomic clocks, the PRTC normally provides (and requires) traceability to some source of UTC. This means that the PRTC is always relying on some form of signal that aligns it to UTC. Primary Reference Time Clock: PRTC There are two common implementations of a PRTC, either a standalone GNSS receiver or an atomic clock calibrated and aligned to UTC by an external signal. This signal can be from either a local GNSS receiver or an external source, typically an atomic clock at a national time laboratory (which is tracking UTC). See the section "White Rabbit" in Chapter 7 for details on one method to do this. The time signals output by these devices (beyond the frequency requirements) are expected to be accurate to within +100 ns of UTC. Up until a few years ago, this was the only level of perform- ance, adopted only because it aligned with the accuracy specified by GPS. With better technology (and improvements to GNSS systems) becoming available, the ITU-T has now defined additional categories for the accuracy of PRTC devices. Currently, ITU-T defines two levels of PRTCs in G.8272, and new ePRTCs in G.8272.1. The ori- ginal PRTC specification of +100 ns was retained but renamed to class A performance (PRTC-A). A new class B (PRTC-B) was added, improving that accuracy to +40 ns. This improvement is achieved (chiefly) by using better GNSS receivers, typically ones that have the capability of dual- band reception. For example, with GPS, this means a device that receives signals on both the L1 and L2 (or other) bands, but all GNSS systems have an equivalent facility. Note the difference between multiband and multi-constellation. To minimize environmental factors, such as temperature influence on the clock, the PRTC-B is only intended for locations where it is possible to guarantee satisfactory environment conditions, which means controlled temperature or air-conditioned indoor locations. Note that there are no long-term holdover requirements in G.8272 for either PRTC-A or PRTC-B. Once the PRTC loses all its i