their clocks are accurately timed. The timescale used by GPS is based on the UTC(USNO), which as you read previously is the version of UTC from the U.S. Naval Observatory and is aligned to within a nanosecond or SO of the "real" UTC. Consequently, there is little difficulty in using a suitable receiver to recover a very accurate representation of UTC time from GPS. The GPS receiver in your mobile phone, which commonly now also contains a Galileo receiver, simultaneously locks onto GPS signals from several satellites and quickly determines position and speed. It would also be possible to get UTC(USNO) time from GPS, although most phones get their time synchronization from the cellular network or the Internet. However, there are more specialized receivers that are not constructed to determine location or help you navigate around the world but are designed specifically to deliver an accurate source of time. To recover accurate time information, these devices only need a connection to an antenna located outside (because GNSS signals will not penetrate walls). Normally, these receivers also have up to three physical connectors that allow cables to carry timing information away to any device that needs it. The three signals consist of Frequency (some sort of sine or square wave) Pulse (representing the start of the second or phase) Time of day (a string of characters that conveys the UTC date and time) These three signals can then be used to provide frequency, phase, and time synchronization to any nearby equipment. Because GPS is a worldwide system, and it consists of many satellites (currently 31) that are in overlapping orbits, the coverage is global and will work anywhere on the earth. Be- cause the antenna for the receiver is placed outside or on the roof, there is almost nothing to get in the way between the receiver and the transmitter (except in dense urban canyons). Consequently, everybody takes for granted that GPS is extremely reliable and "always there" because it has worked SO well for decades, and many us