GPS dynamic positioning

　　The positioning accuracy can be investigated under static and dynamic conditions, and the effect of dynamic positioning is better than that of static positioning. The nominal positioning parameters of the GPS module are measured in a completely open sky with good satellite signals. Therefore, it is difficult to achieve the nominal positioning time and positioning accuracy in conventional tests. There are two common description methods for horizontal positioning accuracy: one is ?m CEP, that is, the circle probability error, which means that the measured point has a 50% probability of being located in a circle with the real coordinates as the center and ?m as the radius; The second is ?m 2DRMS, that is, 2 times the horizontal mean square root error, which means that the measured point has a probability of about 95.5% to be located in a circle with the real coordinates as the center and ?m as the radius.

　　The positioning accuracy of the GPS module depends on many aspects, such as the satellite clock error and orbit difference from the GPS system, the number and geometric distribution of visible GPS satellites, solar radiation, atmosphere, multipath effects, etc. In addition, the same GPS module will have different positioning errors due to antenna and feeder quality, antenna position and direction, test time period, open sky range and direction, weather, and PCB design. Even if different GPS modules of the same manufacturer and the same model use the antenna diversity to test at the same time, the static drift will be different.

　　GPS module is often used as a time reference in practical applications, supplemented by the RTC inside the module, a very high-precision time reference can be obtained, which provides great convenience for product design. As for GPS speed measurement, it is just an extended application of simple calculation based on obtaining longitude and latitude.

　　The common antenna of GPS is a ceramic panel antenna. This antenna has a low cost and an external active amplifier circuit. The direction of the received signal is single and the gain is relatively high, so it is used the most. But its disadvantage is that it is bulky and susceptible to frequency drift due to temperature. If the ceramic area is made small, it will affect the receiving gain; if it is made thin, it will affect the receiving bandwidth of the receiving antenna, and it will also be affected by the active amplification part. The size that works well at present is 25×25×4mm3. Ceramic patch antennas work best when placed vertically upwards in actual use.

　　The signal transmission line of the GPS antenna is also very important, including the external feeder and PCB traces. The maximum output power is possible only when the impedance is matched. Therefore, the entire transmission line must ensure a high-frequency impedance of 50Ω. For how to design the RF trace impedance on the PCB, some small software can help you easily calculate.

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