g of the phase. For a 10-m fiber cable, this could be a significant error, in the order of 50 ns. Therefore, testers allow for the length of the Eth- ernet cable to be entered SO that the phase can be adjusted to compensate. Similar logic applies to the 1PPS cable, both from the DUT and from the reference PRTC (about 5.1 ns per meter). Another important point in understanding the source of TE also arises in the right-hand use case from Figure 12-11. Remember that according to G.8272, the accuracy of a Class A PRTC (PRTC A) combined with a T-GM "should be accurate to within 100 ns or better" when verified against UTC. But the time tester is also connected to a PRTC-A to get a reference 1PPS phase signal, and that signal is also allowed to be up to 100 ns off from UTC. And that is with a good installation and accurate compensation of the GNSS antenna cable (see Chapter 11 on GNSS installation). So, the issue is that the tester might be getting a 1PPS reference signal that differs by up to 200 ns from the signal being output by the T-GM! Of course, if the two receivers are seeing the same satel- lites from the same GNSS constellation and experiencing the same propagation conditions for the GNSS signal, then it is highly likely that the error would be less than this. But theoretically, an error of that significance could arise, SO it is futile to try to measure the phase offset to the last 10 ns or SO for this particular case. If getting a much better measure is important, then the tester should take its 1PPS reference from the same PRTC to compensate for this error. The difference in phase alignment between the 1PPS output of the PRTC and the PTP output of the T-GM integrated inside the same PRTC should be only a few nanoseconds. 12.3.1 Testing Metrics Table 12-1 is a summary of the metrics that are possible from different types of testing. Please con- sult Chapter 5 (for physical frequency) and Chapter 9 (for packet methods) for details on these metrics. Table 12-1 Metrics According to Test Types and Topology Topo-