ime distribu- tion too. All GNSS services work by using a set of orbiting satellites, referred to as a constella- tion, orbiting in a middle-earth orbit, meaning that the position of any satellite with reference to a point on the ground changes over time, unlike communication satellites, which appear fixed. Each satellite in the constellation carries a very accurate clock, based around a rubidium oscillator, which is, in turn, synchronized to TAI be uplink ground stations. The satellites transmit their time to earth in a wide-area signal, as well as providing information about their orbit. GNSS receivers on the ground can receive the signals from multiple satellites Chapter 8 Synchronization in open radio access networks 271 Galileo: Glonass: 3 orbital planes 3 orbital planes 6 orbital planes 27 satellites + 3 spares 21 satellites + 3 spares 24 satellites + spare 56° inclination angle 64.8° inclination angle 55° inclination angle Altitude 23616km Altitude 19100km Altitude 20200km FIGURE 8-4 A GNSS constellation. GNSS, Global Navigation by Satellite Systems. simultaneously and use a comparison of the transmitted times, along with the orbital para- meters, to determine not only a precise location on the surface of the Earth but also a precise time. To achieve this, in general, requires reception from a minimum of four satellites, though additional ones can be used for redundancy. However, if the exact position of the receiver is already known, it is in fact possible to recover precise time using just a single sat- ellite-something that can be used to improve reliability in fixed-location applications, such as macrocells. Since GNSS signals are out there in the ether for anyone to receive (though the Galileo service does have the option for subscription-based encryption), it really is a free source of time for anyone to use. It is also a relatively precise source of time-modern, so-called multi- band, receivers can provide a time output to within better than 30 ns of actual time (TAI). Sadly, GNSS is not without its