rings and a backup control center at Vandenberg). There are also monitoring stations (some run by other agencies) that track the GPS satellites, process navigation signals, make measurements, and collect atmospheric data. Some other stations, with uplink capability, transmit corrections, navigation data, and control information up to the satellites. User segment: This segment includes the many devices being used for PNT. GPS is free and open for civilian use and has led to the development of hundreds of applications touching every aspect of modern life. New receivers that can take advantage of the new civilian signals are now becoming commonplace, as well as multi-constellation devices that can simultaneously receive signals from other constellations in combination with GPS. Other GNSS Systems (Galileo, GLONASS, etc.) As previously discussed in Chapter 1, besides the GPS system, there are another five major GNSS satellite constellations in orbit providing PNT services. This section discusses the basic features of them in turn. These systems are used, either globally or regionally, to provide timing services and are increasingly combined with one of the other constellations (normally GPS at least). GLONASS The Soviet Union started launching satellites for the GLObalnaya NAvigazionnaya Sputnikovaya Sistema (GLONASS, or Global Navigation Satellite System) in 1982. Like GPS, it is designed for global coverage and was the second GNSS system deployed after GPS. It is now controlled by the Russian Federation and achieved a full operational constellation in 1995. Following the collapse of the Soviet Union, GLONASS fell into disrepair owing to a lack of re- sources, SO that by the mid-2000s it was barely functional. With renewed investment in the system, it was slowly rebuilt back to a full constellation and has remained operational since 2011. It is prob- ably the second most popular GNSS system after GPS (although that may change with Galileo). GLONASS uses 24 MEO satellites in three different orbital planes offering gl