A typical GPS receiver contains the functional block shown in Figure 3, which consists of: RF part (GPS-RF front end), GPS signal processor and main processor.

　　The RF part includes GPS antenna, RF filter and GPS RF front end. The RF part receives the satellite signal, separates the pseudo-random code from the carrier frequency, and sends it to the GPS signal processor. In most existing receivers, the RF front part plus the GPS signal processor can process four to 12 satellite signals simultaneously. This parallel processing capability provides higher positioning accuracy and reduces the time to output data. The main processor provides data to the user through a GUI(graphical user interface), a display screen, or some other operating system approach (shown on the left side of Figure 4), depending on the requirements of the application. Synchronization and timing are important characteristics of GPS receiver. To achieve the purpose of synchronization and timing, the CNC temperature compensated crystal oscillator TCXO (such as DS4000) is used.

　　In the block diagram of FIG. 3, there are two oscillation sources, including REF(reference) crystal (or oscillator) and RTC(real time clock) crystal. REF crystals or oscillators can be fairly accurate or imprecise, depending on the receiver used. A typical range is between 13MHz and 30MHz, depending on the manufacturer. In applications where GPS receivers are used as precision time benchmarks,REF oscillators can be rubidium sources, OCXO, or even TCXO. In this case, the main processor will correct any timing slip between the satellite and the receiver.

　　The RTC crystal provides real-time clock information for the signal acquisition process of the GPS receiver to capture different satellites in the cluster of 27 satellites. RTC helps provide a starting point for locking all visible satellites through a lookup table of satellite location information.

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