The United States was the first country in the world to develop a satellite navigation system,with rich experience in system management,R&D,operation,and satellite applications industry.It started the construction of the Global Positioning System(GPS)in 1973 and deployed the first satellite in 1978.It has successfully developed and deployed three series of GPS satellites with a total of seven models.The United States is now focusing on the research and development of cutting edge navigation technologies and constellation modernization,replacing old ones with new GPS III series of satellites and actively exploring and verifying the frontier navigation technologies represented through the Navigation Technology Satellite 3(NTS-3).It is now upgrading the original ground-based operation and control system,actively developing and deploying the GPS Next Generation Operational Control System(GPS OCX),and upgrading the military user equipment supporting Military Code(M-Code).The U.S.attaches importance to multiple measures to improve the service performance of the GPS system and enhance the resilience of the system to provide positioning,navigation,and timing capabilities.In this context,the progress of the construction of GPS and the related technological innovations are separated out and analyzed,which will help the Global Navigation Satellite System(GNSS)solutions summarizing their experience and learning from each other’s development to better serve social progress and economic development.
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