ers of time and navigation rely on it, including countless mobile operators. The only major disadvantage to these GNSSs is that the signal received on the ground is very weak and therefore quite susceptible to interference and jamming. Because the information transmitted (at least for civilian users) is not encrypted, there is also an increasing danger from bad actors spoofing the signals, thereby fooling the receivers into accepting fake time data. Many experts fear that everybody is far too complacent in thinking that GPS will always be there, when in fact it is quite vulnerable. This topic is revisited when discussing the options for deploying synchronization across a wide-area network. "Chapter 11. 5G Timing Solutions", covers the issues regarding the resilience of the GNSS systems in much more detail. There is a lot more information on GNSS systems in "Chapter 3. Synchronization and Timing Con- cepts", and an interesting background paper for using GNSS for timing is found in the ITU-T tech- nical report GSTR-GNSS, referenced at the end of the chapter. 1.5 Accuracy Versus Precision Versus Stability Accuracy is defined as how close a measured value is to the actual (or accepted) value. For ex- ample, accuracy would indicate how close a clock has stayed to real UTC time or how close an os- cillator has stayed to its nominal frequency. As shown previously, time accuracy can be expressed in terms of a time error over a period (1 second per day) or a frequency error compared to the nom- inal frequency (20 Hz for a 20-MHz oscillator, or 1 ppm). Precision is defined as how close the values measuring some quantity are to each other-in other words, how repeatable and reproducible the measured values are. Another word for precision might be stability in that it indicates how well a measurement continues to reveal the same value-no matter whether that value is accurate (meaning reflecting the real value). Imagine that a 20-MHz oscillator is measured to be oscillating at 20.200000 MHz every time you measure it. Because i