GPS system architecture and analysis

　　The working mode of GPS positioning system is to use the basic triangulation positioning principle of satellite. The GPS receiver first finds the location of more than three satellites, and then calculates the distance between each satellite and the receiver to obtain the coordinates of the receiver in three-dimensional space. More than 20 satellites of the GPS system will transmit signals with L1 and L2 frequencies of 1575.42MHz and 1227.60MHz respectively. Generally, GPS receivers for civil use only need to receive L1, that is, 1575.42MHz frequency signals.

　　The GPS device at the customer receiving end will receive the positioning signal from the navigation satellite, which is a one-way GPS signal receiver. First of all, the GPS antenna will first receive GPS satellite signals, and then convert high-frequency signals into medium and low-frequency digital signals through the RF RF front end and transmit them to the GPS baseband module. The core technology of baseband is the design of correlator. The correlator is mainly used to compare and find the correct satellite number, and then obtain the Almanac, BroADCast Ephemeris and other data of multiple satellites by comparison. The more channels of the correlator, the faster the satellite position can be found. At present, general GPS receivers provide at least 12 channel correlators, while higher order receivers have 16 or even 32 channel correlators.

　　The control function of GPS receiver is realized by microprocessor or microcontroller. The processor can be an independent unit or integrated with baseband. At present, low-end GPS receiver products usually use ARM7 as the core; High end products are generally ARM9. In addition, such components will also have microprocessor support functions, such as UART and Real Time Clock (RTC).

　　The broadcast ephemeris data will be output to the main processor in NMEA 0183 or RTCM format, and further integrated with the GIS map engine to display the location of the street; It can also send location information through the wireless communication interface, so that remote servers can further provide relevant location services. NMEA 0183 is a standard communication protocol commonly used by GPS. It uses simplified ASCII serial communication protocol to define the format of data transmission.

　　The WAAS system in the United States or the EGNOS system in Europe adopt the auxiliary positioning mode of differential positioning (DGPS). When GPS adopts this mode, RTCM or NTRIP 1.0 protocol format should be output.

　　In addition, since the original data formats provided by different receivers are usually different, a universal GPS data exchange format must be established to uniformly process the data collected by different receivers. At present, the format commonly used in the industry is RINEX.

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