e signals from multiple satellites. Knowing the position of each satellite in the sky and being able to triangulate the signals of at least four satellites based on the time of arrival, the receiver can solve equations to determine position (latitude and longitude), height, and time. The time distributed is based on a predefined timescale, which can vary for each system, but is somehow related to UTC. This timescale is kept aligned by networks of atomic clocks, both on the ground and on the satellites. So, from the satellite signals, the PRC/PRS or PRTC with an embedded GNSS receiver can con- stantly determine a value of time that is traceable to a source of UTC (or allows UTC to be determ- ined by some simple conversion). Seeing that a PRC/PRS is normally in a fixed location, further optimizations are possible to gather time from fewer satellites once the receiver position has been fixed. Like all atomic clocks, the long-term accuracy of the received time is very stable, but over shorter observation intervals, the receiver can see significant variability (due to atmospheric effects on the radio signal reception). For this reason, GNSS receivers being used as a PRC/PRS or PRTC also contain a good quartz oscillator (such as an OCXO) to filter out (smooth) the short-term variability. Sometimes, this combination is referred to as a GDO (GNSS or GPS disciplined OCXO). For further information on GNSS systems in general, and their use as sources of time for telecom- munications networks, a reference source is the ITU-T technical report on GNSS systems, "GSTR- GNSS Considerations on the use of GNSS as a primary time reference in telecommunications." However, the following sections cover the major systems in more than enough detail to gain a good understanding of the characteristics of each. Because of the popularity and long history of use of GPS in telecommunications, the next section covers GPSs in finer detail as an example of how all these systems work. In 1973, the U.S. Department of Defense (DoD) approved the Glob