il recently as the MTSAT Satellite Augmentation System (MSAS). However, with the re- cent decommissioning of MTSAT-2, the MSAS signals have been migrated to the QZSS Michibiki geostationary satellite (QZS-3) and SO now MSAS stands for Michibiki-Satellite-based Augmenta- tion System. There are plans from 2023 to add MSAS to a further two QZSS GEO satellites and plans in the fu- ture to eventually integrate IGSO QZSS satellites and be able to augment all the major GNSS systems. Russia has a system called System of Differential Correction and Monitoring (SDCM) that uses three geostationary satellites to provide an augmentation service for GLONASS as well as GPS. These last three systems, GAGAN, MSAS, and SDCM, provide a similar service to WAAS and EGNOS. There is currently very limited coverage over Africa and South America. There are several other systems in various stages of development to cover other regions of the planet including the Korea Augmentation Satellite System (South Korea), Agency for Air Navig- ation Safety in Africa and Madagascar (ASECNA) for the Indian ocean and Africa, BeiDou SBAS for China, and Southern Positioning Augmentation Network for Oceania. 3.3.1 Sources of Frequency This chapter has gone into considerable detail about the sources of time and frequency and how it is distributed via satellite systems. So, the next step to cover is how time is received and shared at the place it is needed, such as inside the core of an operator's network, or even on top of a radio cell site. PRC/PRS As previously described, a source of frequency is the PRC/PRS or stratum 1 clock. This device is either an atomic clock combined with an OCXO or a GNSS receiver combined with an OCXO. Sometimes the two are combined to allow autonomous operation and long holdover performance in case of GNSS failure. Figure 3-16 shows the typical frequency sources (photos courtesy of Microchip). © Microchip Technology Inc. Microchip Technology Inc. Figure 3-16 Examples of Frequency Sources, Cesium Clock (L) and GPS Receiver (R) T