DEC 12 DEC 17 Laboratory k [UTC-UTC (k) I/ns AUS (Sydney) -369.4 -381.2 -378.4 SU (Moskva) USNO (Washington DC) This snippet of Circular T shows the difference between three versions of UTC(k) and "real" UTC. The three versions are from laboratories based in Sydney, Moscow, and Washington, DC. The three values show the difference (in nanoseconds) between UTC and UTC estimated by that labor- atory on three separate dates in December 2019. Note the last line; that version of UTC, namely UTC(USNO), is shown to be within a nanosecond of UTC, which is a good thing, because UTC(USNO) is the time that is distributed by the GPS con- stellation (USNO is the United States Naval Observatory). 1.4 How Can GPS Provide Timing and Synchronization? A very powerful tool in the distribution of time information (frequency, phase, and time) is the global navigation satellite system (GNSS), the most well-known of which is GPS. Many docu- ments on timing refer to GPS specifically, but it is important to know that there are numerous sim- ilar systems, and SO this book uses the generic term, GNSS, except when specifically referring to any one system. Table 1-1 shows the most common GNSSs, including some regional systems. Table 1-1 Major Global Navigation Satellite Systems Country United States of America GLONASS Russian Federation Galileo European Union Beidou People's Republic of China INRSS Republic of India Japan Conceptually, GNSSs are quite simple, although there is a lot of complexity behind their success that is outside the scope of this book. You can think of a GNSS as a network of atomic clocks flying around in orbit, which is absolutely what they are (for GPS, at 20,200 km above the earth). Trans- mitters on the satellites use the onboard clocks to broadcast timing signals toward the earth such that receivers below can determine time, location, and speed. Controlling these satellites are ground and monitoring stations that ensure the satellites are in the correct position, that they are transmitting the correct data, and that 