GPS development mode

　　Before the appearance of satellite positioning system, remote navigation and positioning mainly used wireless navigation system.

　　1. Radio navigation system

　　Roland -- C: working at 100KHZ, composed of three ground navigation stations, the navigation working area is 2000KM, and the general accuracy is 200-300M.

　　Omega (Omega): working at more than ten kilohertz. It is composed of eight ground navigation stations, which can cover the whole world. Accuracy several miles.

　　Doppler system: based on the principle of Doppler frequency shift, the parameters of moving objects (ground speed and drift angle) can be obtained by measuring their frequency shift, and the aircraft position can be calculated. It is a self-contained dead reckoning system. The error accumulates as the voyage increases.

　　Disadvantages: small working area; Radio wave propagation is affected by atmosphere; Low positioning accuracy

　　2. Satellite positioning system

　　The earliest satellite positioning system is the transit system of the United States, which was developed in 1958 and officially put into use in 64. Because the number of satellites in this system is small (5-6), the operating height is low (1000 km on average), and the time interval between observation of satellites from ground stations is long (1.5 h on average), it cannot provide continuous real-time three-dimensional navigation, and the accuracy is low. In order to meet the urgent requirements of military and civil sectors for continuous real-time and three-dimensional navigation. In 1973, the US Department of Defense formulated the GPS plan.

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