re delayed when they travel the path from a terrestrial transmitter to a terrestrial receiver, and the accuracy of the received signal can be no better than the knowledge of the path delay. As seen with GNSS systems, these path delays are difficult to measure due to continuously changing ionospheric conditions. Despite all this, the recovered time accuracy for a clock with good reception could be around 5-10 us. Of course, this does not take account of propagation delay, which would need to be corrected to gain high accuracy. In practical use, instrumented and calibrated receivers can readily achieve in the low milliseconds of accuracy for phase/time. For frequency, one example is that a receiver tuned to WWVB, with one day of averaging, could achieve a frequency uncertainty of less than 1 part in 10-11 across most of the United States. Various consumer electronic products like wristwatches and wall clocks can receive and decode the time signals, as can public radio-controlled clocks, such as those at railway stations. There are also many other uses such as synchronizing all manner of industrial devices. However, as has been shown, time and frequency are now distributed globally using GNSS systems. At almost any location on the planet, GNSS and other radio receivers provide a source of frequency and time, in a topology sometimes referred to as a distributed PRTC. Figure 3-20 illustrates how this works in practice. Distribution of time, phase, and frequency - GNSS or other radio Receiver Receiver Receiver performance limit Timing distribution Timing distribution Timing distribution network network network application application application application application Figure 3-20 Radio Waves Used for Timing Distribution One of the main advantages of the distributed-PRTC approach is that the reference time is available globally, and it is a flat distribution method without any hierarchy and hence it provides a simple and loop-free design. The main disadvantages of this approach are that it requires infrastructure every