d a time (or it can take that information from another GNSS device). It then impersonates the combined signals of a fleet of GPS satellites and make the receiver think that it is at that location starting at that time. The simulator will continue to (accurately) roll the time forward (guided by the 10-MHz reference input), and the GPS receiver will recover the artificially produced time and phase. The simulator also has a 1PPS output that is in phase with the time being generated (of course, the relative accuracy of these two signals needs validation). Besides the normal 10-MHz reference con- nected to the tester, the 1PPS out from the simulator is connected to the timing tester as a reference phase signal. Then the phase output by the GNSS receiver can be measured against the phase from the GPS simulator. Tester Reference Tester Reference inputs inputs PRC/PRS Time standard 10 MHz signal signal signal signal simulator PRTC T-GM PRTC/T-GM RF signal 10 MHz PTP SyncE Figure 12-12 Testing a PRTC and T-GM One piece of advice if you are interested in testing any of the highly accurate enhanced clocks (like the ePRTC and ePRC): because these devices have amazing amounts of stability over very long in- tervals, be prepared for extremely long test runs to verify their performance. Phase Accuracy For phase accuracy, the tester receives either or both of two test signals, PTP from the T-GM (or it could be 1PPS as well) and/or 1PPS from the PRTC (if separate devices). The tester compares the phase alignment between the signal from the GPS simulator and the output of the PRTC or the T- GM (or both). For the PTP output, the appropriate metric on the tester is the two-way time error (2wayTE; see Chapter 9 for details). Compare the 2wayTE for the PTP messages, or the 1PPS max|TE| against that class of PRTC (PRTC-A, PRTC-B, ePRTC-A, or ePRTC-B). This measurement tells the engin- eer if the PRTC and/or the T-GM is within the required level of accuracy (100 ns for PRTC-A, 40 ns for PRTC-B, and 30 ns for the enhanced PRTC Classes A