re satellites to the orbit by repositioning six satellites. As a result, GPS now effectively operates as a 27-slot constellation that improves the coverage in most parts of the world. In case of any active satellite failure, there are still more satellites reserved to provide redundancy to the system. For that reason, over the past few years, there has been a total of 31 operational satellites in the constellation. The generations of satellites from oldest to newest are IIA (now all retired), IIR, IIR-M, IIF, and III. Figure 3-14 shows how the satellites are laid out in orbit based on their positions on February 4, 2021. Slant Chart (GPS Satellite Locations) 180 SVN64 AS SVN57 SVN68 SVN58 SVN47 SVN69 E1 Geographic Longitude of SVN62-B2 the Ascending SVN45 D2F SVN65 Node (deg) SVN63 SVN66 C2 SVN43-AFFF SVN71 SVN72->33 SVN56/ SVN55 SVN50 SVN52 SVN67 D4 SVN53 C4F SVN617/D1 SVN70 F1 SVN59 GPS IIR SVN76:14E4 GPS IIR-M SVN77 SVN51 SVN44 GPS IIF SVN73 E2 GPS III SVN48 SVN75 February 1, 2021 SVN74 F4 GPS Orbital Planes Distribution A. Approved for public release; distribution unlimited. SMC-2021-2113, 4 February 2021 Figure 3-14 GPS Constellation in Six Orbital Planes The GPS system uses its own version of time, known as GPS time, but it also carries key informa- tion in its navigation messages to allow the receiver to determine UTC from the received GPS time. The UTC steering the GPS system is based on the UTC(USNO) from the U.S. Naval Observatory (see Chapter 1 for details on UTC and UTC(k) and how GPS can give us time). Technically, the GPS system (and most others) consists of three segments: Space segment (the satellites): At the time of writing, there are 31 operational satellites in the GPS constellation, with new Block III satellites currently being integrated into the system. Control segment and monitoring stations: There are six monitoring stations at U.S. Air Force bases around the world (at Kwajalein Island, Diego Garcia, Ascension Island, Hawaii, and Cape Canaveral, with a master control center at Colorado Sp