The current plan of GPS modernization is focused on improving the quality of civilian uses of GPS, primarily through the implementation of a new L2C code on L2 frequency and a new civilian signal on L5 frequency of 1227.6 MHz (Spilker and Van Dierendonck, 2001). In addition, a new M-code (encrypted) has been implemented exclusively for military use, ensuring that military and civilian users will have entirely separate signals and codes. Consequently, AS policy will be abandoned. (SA has already been turned off.) The new dual-frequency civilian tracking capability is available on the Block IIR-M GPS satellites (currently, five satellites in orbit, with the most recent launch of Decembr 20, 2007), and more improvements are planned for Block IIF satellites. As of February 2008, the first GPS Block IIF launch was projected by 2010 (http://facility.unauco.org/science_ tech_gnss_modernization.html) . The GPS Block IIF spacecraft represents the next-generation GPS system designed to meet both military and civil customer requirements. Among other features, the GPS Block IIF spacecraft will feature extended design life up to 15 years, modular design, 3 m spherical error probable (CEP), options for L5 civil signals, increased autonomy from ground segment, and rapid on-orbit reprogrammability (www.deagel.com/pandora/?p=pm00371004).
In terms of GPS instrumentation, hardware miniaturization, further development of software receivers, improvements in reliability, faster sampling rates, lower noise and more multipath resistance, more real-time operations, and ubiquitous dual-frequency measurement capability are expected (Rizos, 2002c). The current trend of integrating GPS with other sensors such as inertial, vision systems, laser scanners, and pseudolites will continue to serve more specialized, customized applications. It is expected that the increasing number of CORS-based local services will serve real-time users.
Other GNSS that are complementary to GPS are the Russian Glonass system originally developed for the Russian military, and the European Galileo system (civilian), currently under implementation (first satellite was launched on December 28, 2005). Glonass became operational with a full twenty-four-satellite constellation in 1996. However, as of February 2008, the number of operational satellites is sixteen, with the most recent launch of December 25, 2007, where four new Glonass satellites were placed in orbit. Still, the system's long-term stability might be questionable. Nevertheless, GPS/Glonass receivers, which take advantage of the extended constellation created by the additional satellites in view, were developed. For more information on Glonass and Galileo, the reader is referred to http://www. gpsworld.com/; http://www.insidegnss.com/.
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