Global PosiTIoning System, or GPS for short, is a global satellite positioning system that has been more and more widely used in recent years and has generated considerable demand for GPS products. And with the improvement of science and technology and the continuous development of application directions, GPS will quickly penetrate into people's daily life undoubtedly.
The GPS positioning system we often mention is designed and controlled by the US military. In addition, China's Beidou double-star positioning system is silently contributing to China's modernization construction; Russia's GLONASS system has also had a glorious history; the Galileo satellite positioning system designed by the European Union is compatible with the currently 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 parts: the space part, the control part and the user part.
The space part is the general term for GPS artificial satellites. The average height of the artificial satellite is about 20200Km, the orbit is an ellipse, and the earth is located at a focal point of the ellipse; the operation period is about 12 hours. Nearly 30 navigation satellites are unevenly distributed on 6 orbit planes with an inclination of about 55 °, some of which are backup satellites. The US military can adjust the number of working satellites through the ground control section. In the GPS system, GPS satellites are dynamic known points, and all navigation and positioning information on the user end is calculated based on the "ephemeris" sent by this dynamic known point. GPS ephemeris is actually a series of real-time state parameters describing the movement and orbit of GPS satellites. The broadcast ephemeris received by the civil GPS module is a forecast ephemeris for real-time data processing that is directly broadcast to users by GPS satellites in a spread-spectrum communication manner through navigation messages, and broadcasts the civilian pseudo-element 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. All ephemeris for navigation and positioning broadcast by GPS satellites are provided by 5 monitoring stations distributed on the ground. The ground system is responsible for monitoring GPS signals, collecting data, calculating and injecting navigation messages, status diagnosis, orbit correction, etc. It is the massive data processing of the ground monitoring system that makes the GPS system operate accurately.
The GPS positioning module that 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 belongs to passive positioning. 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 position is fast, but the error is large. The initial positioning module requires at least 4 satellites to participate in the calculation, which is called 3D positioning. 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 receiver to select the application.
The evaluation indexes of GPS module performance mainly include receiving sensitivity, positioning time, position accuracy, power consumption, and time accuracy. The start-up positioning time of the module is very different in different startup modes. Generally speaking, the cold start time refers to the situation where the module does not save any data that is helpful for positioning, including ephemeris, time, etc., generally nominal within 1 minute; the warm start time refers to the newer satellites inside the module Ephemeris (generally no more than 2 hours), but the time deviation is very large, generally nominal within 45 seconds; hot start time refers to the situation where the shutdown does not exceed twenty minutes, and the RTC time error is very small. Generally, it is nominally within 10 seconds; the recapture time is like a car drilling through a tunnel and recapturing satellites when exiting 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 place hundreds of kilometers away after positioning, then the module has an ephemeris inside, but this ephemeris is wrong or has no reference significance. In these cases, the positioning time may take several minutes or 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 after getting the module.
The positioning accuracy can be investigated under static and dynamic conditions, and the dynamic positioning effect is better than static positioning. The nominal positioning parameters of the GPS module are measured under the condition that the satellite signal is good under a completely open sky. Therefore, it is difficult to achieve the nominal positioning time and positioning accuracy in routine testing. There are two common ways to describe horizontal positioning accuracy: One is? M CEP, which is the circle probability error, which means that the measured point has a 50% probability of being located in a circle with the true coordinates as the center and? M as the radius; The second is? M 2DRMS, which is twice the root mean square error, which means that the measured point has a probability of about 95.5% located in a circle with the true coordinates as the center and? M as the radius.
The positioning accuracy of the GPS module depends on many aspects, such as satellite clock difference and orbit difference from the GPS system, the number and geometric distribution of visible GPS satellites, solar radiation, atmosphere, and multipath effects. In addition, the same GPS module will also produce different positioning errors due to antenna and feeder quality, antenna position and direction, test period, open sky range and direction, weather, PCB design and other reasons. Even when different GPS modules of the same manufacturer and the same model are tested simultaneously using an antenna diversity device, 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, and 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 for simple calculation based on obtaining latitude and longitude.
The common antenna of GPS is a ceramic flat antenna. This kind of antenna has a low cost. With an external active amplifier circuit, the direction of the received signal is single, and the gain is relatively high, so it is used most. However, its disadvantage is that it is bulky, and it is susceptible to frequency drift caused by temperature. If the ceramic area is small, it will affect the receiving gain; if it is thin, it will affect the receiving bandwidth of the receiving antenna, and it will also be affected by the active amplification part. At present, the size with good effect is 25 × 25 × 4mm3. The ceramic patch 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. Only when the impedance is matched can the output power be maximized. Therefore, the entire transmission line should 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.
The GPS positioning system we often mention is designed and controlled by the US military. In addition, China's Beidou double-star positioning system is silently contributing to China's modernization construction; Russia's GLONASS system has also had a glorious history; the Galileo satellite positioning system designed by the European Union is compatible with the currently 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 parts: the space part, the control part and the user part.
The space part is the general term for GPS artificial satellites. The average height of the artificial satellite is about 20200Km, the orbit is an ellipse, and the earth is located at a focal point of the ellipse; the operation period is about 12 hours. Nearly 30 navigation satellites are unevenly distributed on 6 orbit planes with an inclination of about 55 °, some of which are backup satellites. The US military can adjust the number of working satellites through the ground control section. In the GPS system, GPS satellites are dynamic known points, and all navigation and positioning information on the user end is calculated based on the "ephemeris" sent by this dynamic known point. GPS ephemeris is actually a series of real-time state parameters describing the movement and orbit of GPS satellites. The broadcast ephemeris received by the civil GPS module is a forecast ephemeris for real-time data processing that is directly broadcast to users by GPS satellites in a spread-spectrum communication manner through navigation messages, and broadcasts the civilian pseudo-element 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. All ephemeris for navigation and positioning broadcast by GPS satellites are provided by 5 monitoring stations distributed on the ground. The ground system is responsible for monitoring GPS signals, collecting data, calculating and injecting navigation messages, status diagnosis, orbit correction, etc. It is the massive data processing of the ground monitoring system that makes the GPS system operate accurately.
The GPS positioning module that 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 belongs to passive positioning. 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 position is fast, but the error is large. The initial positioning module requires at least 4 satellites to participate in the calculation, which is called 3D positioning. 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 receiver to select the application.
The evaluation indexes of GPS module performance mainly include receiving sensitivity, positioning time, position accuracy, power consumption, and time accuracy. The start-up positioning time of the module is very different in different startup modes. Generally speaking, the cold start time refers to the situation where the module does not save any data that is helpful for positioning, including ephemeris, time, etc., generally nominal within 1 minute; the warm start time refers to the newer satellites inside the module Ephemeris (generally no more than 2 hours), but the time deviation is very large, generally nominal within 45 seconds; hot start time refers to the situation where the shutdown does not exceed twenty minutes, and the RTC time error is very small. Generally, it is nominally within 10 seconds; the recapture time is like a car drilling through a tunnel and recapturing satellites when exiting 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 place hundreds of kilometers away after positioning, then the module has an ephemeris inside, but this ephemeris is wrong or has no reference significance. In these cases, the positioning time may take several minutes or 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 after getting the module.
The positioning accuracy can be investigated under static and dynamic conditions, and the dynamic positioning effect is better than static positioning. The nominal positioning parameters of the GPS module are measured under the condition that the satellite signal is good under a completely open sky. Therefore, it is difficult to achieve the nominal positioning time and positioning accuracy in routine testing. There are two common ways to describe horizontal positioning accuracy: One is? M CEP, which is the circle probability error, which means that the measured point has a 50% probability of being located in a circle with the true coordinates as the center and? M as the radius; The second is? M 2DRMS, which is twice the root mean square error, which means that the measured point has a probability of about 95.5% located in a circle with the true coordinates as the center and? M as the radius.
The positioning accuracy of the GPS module depends on many aspects, such as satellite clock difference and orbit difference from the GPS system, the number and geometric distribution of visible GPS satellites, solar radiation, atmosphere, and multipath effects. In addition, the same GPS module will also produce different positioning errors due to antenna and feeder quality, antenna position and direction, test period, open sky range and direction, weather, PCB design and other reasons. Even when different GPS modules of the same manufacturer and the same model are tested simultaneously using an antenna diversity device, 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, and 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 for simple calculation based on obtaining latitude and longitude.
The common antenna of GPS is a ceramic flat antenna. This kind of antenna has a low cost. With an external active amplifier circuit, the direction of the received signal is single, and the gain is relatively high, so it is used most. However, its disadvantage is that it is bulky, and it is susceptible to frequency drift caused by temperature. If the ceramic area is small, it will affect the receiving gain; if it is thin, it will affect the receiving bandwidth of the receiving antenna, and it will also be affected by the active amplification part. At present, the size with good effect is 25 × 25 × 4mm3. The ceramic patch 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. Only when the impedance is matched can the output power be maximized. Therefore, the entire transmission line should 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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