GLOBAL PosiTIoning System, referred to as GPS, or global satellite positioning system, has been widely used in recent years and has generated considerable demand for GPS products. And with the advancement of technology and the continuous development of application directions, GPS will inevitably penetrate into people's daily lives. The GPS positioning system we often refer to is designed and controlled by the US military. In addition, China's Beidou Double Star Positioning System is quietly contributing to China's modernization; Russia's GLONASS system has also had a glorious history; the Galileo satellite positioning system designed by the European Union is compatible with the widely used GPS system. In a few years, it will add a more glorious page to the global positioning system. The GPS system consists of three major components: the space component, the control component, and the user component. The space part is the general term for GPS satellites. The average height of the artificial satellite is about 20200Km, the running orbit is an ellipse, and the earth is at a focus of the ellipse; the operating period is about 12 hours. Nearly 30 navigation satellites are distributed unevenly on the orbital planes with 6 inclinations of about 55°, some of which are spare satellites. The US military can adjust the number of working satellites through the ground control section. In the GPS system, the GPS satellite is a dynamic known point, and all navigation and positioning information of the user terminal is calculated based on the "ephemeris" sent by the dynamically known point. The GPS ephemeris is actually a series of real-time status parameters that describe the motion and orbit of the GPS satellite. The broadcast ephemeris received by the civil GPS module is a forecast ephemeris for real-time data processing directly broadcasted by the GPS satellite to the user through the navigation message, and broadcasts the pseudo-personality at different rates on different carriers. Random code C/A code ephemeris and military P code ephemeris. For the entire GPS system, the ground control part is actually the core of the entire system. The ephemeris used for navigation and positioning by all GPS satellites is provided by five monitoring stations distributed on the ground. The ground system is responsible for monitoring GPS signals, collecting data, calculating and injecting navigation messages, status diagnostics, orbit corrections. It is the massive data processing of the ground monitoring system that makes the GPS system operate accurately. The GPS positioning module we often say is called the user part. It receives and demodulates the satellite's broadcast C/A code signal like a "radio" with a frequency of 1575.42MHz. The GPS module does not broadcast signals and is passively positioned. By calculating the pseudo-distance with each satellite, the four parameters of the receiver's longitude, latitude, altitude and time correction are obtained by the distance intersection method. The characteristic is that the point speed is fast, but the error is large. The initial positioning module requires at least 4 satellites to participate in the calculation, called 3D positioning, and 3 satellites can achieve 2D positioning, but the accuracy is not good. The GPS module continuously outputs positioning information and auxiliary information in the NMEA format through the serial communication port for the recipient to select an application. The price index of the GPS module performance mainly includes receiving sensitivity, positioning time, position accuracy, power consumption, and time precision. The module's power-on positioning time is very different in different startup modes. Generally speaking, the cold start time means that there is no data saved in the module to help locate the data, including ephemeris, time, etc., which is generally within 1 minute. The warm start time refers to a newer satellite inside the module. Ephemeris (generally no more than 2 hours), but the time deviation is very large, generally nominally within 45 seconds; hot start time refers to the situation where the shutdown is no more than twenty minutes and the RTC time error is very small. Generally nominally within 10 seconds; recapture time is like a car has drilled a tunnel and recaptures the satellite as it exits the tunnel. Generally nominal within 4 seconds. If the module is placed for a long time after positioning, or the module is transported to a few hundred kilometers away after positioning, there is an ephemeris inside the module, but the ephemeris is wrong or not of reference. In these cases, the positioning time may take a few minutes or even longer. Therefore, when the GPS module is shipped from the factory, the ephemeris and other data inside the module should be cleared, so that the customer can quickly locate the cold start mode after getting the module. Positioning accuracy can be investigated under static and dynamic conditions, and dynamic positioning is better than static positioning. The nominal positioning parameters of the GPS module are measured under the condition that the satellite signal is excellent under a completely open sky. Therefore, it is difficult to achieve the nominal positioning time and positioning accuracy in conventional tests. There are two common methods for describing horizontal positioning accuracy: First? m CEP, the circular probability error, means that the measured point has a 50% probability of being at a center with real coordinates. m is inside the circle of radius; second is? m 2DRMS, which is 2 times the average square root error, means that the measured point has a probability of about 95.5% located at a center with real coordinates. m is inside the circle of radius. The positioning accuracy of the GPS module depends on many aspects, such as the satellite clock difference and orbit difference from the GPS system, the number and geometric distribution of visible GPS satellites, solar radiation, the atmosphere, and multipath effects. In addition, the same GPS module will also generate different positioning errors due to antenna and feeder quality, antenna position and direction, test time period, open sky range and direction, weather, PCB design and other reasons. Even when different GPS modules of the same model of the same manufacturer use the antenna splitter to test at the same time, the amount of static drift will be different. The GPS module is often used as a time reference in practical applications, supplemented by the RTC inside the module, which can obtain a very high-precision time reference, which provides great convenience for product design. As for GPS speed measurement, it is only an extended application of simple calculation based on obtaining latitude and longitude. The common antenna of GPS is a ceramic flat panel antenna. This kind of antenna has low cost, externally added active amplifying circuit, and the receiving signal direction is single, and the gain is relatively high, so it is adopted most. However, its disadvantage is that it is bulky and susceptible to temperature drift. If the ceramic area is made small, it will affect the receiving gain; if it is thin, it will affect the receiving antenna receiving bandwidth, and it will also be affected by the active amplification. The currently used size is 25 & TImes; 25 & TImes; 4 mm3. The ceramic chip antenna works best when placed vertically upwards in actual use. The signal transmission line of the GPS antenna is also very important, including external feeders and PCB traces. The output power is only possible when the impedance is matched. Therefore, the entire transmission line must guarantee a high-frequency impedance of 50Ω. For how to design the RF trace impedance on the PCB, some small software can help you to calculate it conveniently.
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