itself is quite a complicated piece of hardware capable of receiving the spread-spectrum signals from the satellites, pro- cessing and decoding them, and using the extracted data to regenerate a local time source. Luckily, fully integrated GNSS receiver modules are available these days from multiple ven- dors for anything from a few to a few tens of US dollars depending on accuracy, features, etc. A lot of these modules can support multiconstellation operation, meaning that they can use the signals from more than one GNSS service-GPS, Glonass, etc.-to achieve improved reli- ability and accuracy. The output interface of such a GNSS module typically consists of these elements- conveying frequency, phase, and time, as shown in Fig. 8-5. Chapter 8 Synchronization in open radio access networks 273 Frequency 10MHz Clock Phase 1 PPS Receiver Serial time-of-day & status NMEA 0183 @ 4800 baud FIGURE 8-5 GNSS receiver interfaces. GNSS, Global Navigation by Satellite Systems. The clock output provides a frequency that is syntonized to time from GNSS. The long-term stability of this clock should meet the requirements of what is called a Primary Reference Clock, meaning that it is effectively the same as having a local cesium frequency reference. The 1PPS, or one pulse per second, output provides phase alignment-remember, time to the subsecond level without consideration for which second. The serial interface, at least the transmit portion, is used to convey a whole host of information including time-which second the 1PPS refers to, position information, and status indications, such as whether the received information should be consid- ered reliable. There are various protocols used for this information, some proprietary and some standard. However, by far the most common is based on a standard called NMEA-0183. Under this, the information is packaged into defined textual messages called sentences. The serial port can also be used bidirectionally for module configuration in a vendor-specific manner. O-RAN synchronization Based