GPS system architecture and analysis

　　The GPS positioning system works by using the basic triangulation principle of satellites. The GPS receiving device first finds the location of three or more satellites, and then calculates the distance between each satellite and the receiver, and then the coordinate value of the receiver in the three-dimensional space can be obtained. More than 20 satellites of the GPS system will transmit L1 and L2 signals with two frequencies of 1575.42MHz and 1227.60MHz respectively. General civilian GPS receivers only need to receive L1, that is, 1575.42MHz frequency signal .

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

　　The control function of the GPS receiver is realized by a microprocessor or a microcontroller. The processor can be a stand-alone unit, or it can be integrated with the baseband. At present, low-end GPS receiver products usually use ARM7 as the core ; high-end products generally use ARM9. In addition, these 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 the format of NMEA 0183 or RTCM, and further integrated with the GIS map engine to display the location of the street; the location information can also be sent through the wireless communication interface, so that the remoteserver can further provide the relevant location. Serve. NMEA 0183 is a standard communication protocol commonly used by GPS, which uses a 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 adopts the differential positioning (DGPS) assisted positioning mode. When the GPS adopts this mode, it needs to output the protocol format of RTCM or NTRIP 1.0.

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

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