Analysis of performance indicators and common problems of GPS modules

The GPS module is a GPS signal receiver. It can be connected to a computer or mobile phone by wireless Bluetooth or wired, and transmits the GPS signal it receives to the GPS software in the computer or mobile phone for processing. 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.

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The key to the application of the GPS module is the development of the serial communication protocol, which is the relevant input and output protocol format of the module. It mainly includes data types and information formats, and the data types mainly include binary information and NMEA National Marine Electronics Association data information. These two types of information can communicate with the GPS receiver through the serial port.

The GPS module calculates the four parameters of the longitude, latitude, altitude and time correction of the receiver by calculating the pseudo distance between each satellite and the satellite. 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.

GPS module main performance indicators

GPS module performance indicators mainly include receiving sensitivity, positioning time, position accuracy, power consumption, and time accuracy.

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 GPS 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 the horizontal positioning accuracy: one is CEP, which is the circular probability error, which means that the measured point has a probability of 50% in a circle with the true coordinate as the center and m as the radius; the second is the DRMS. That is, the two-dimensional root mean square error means that the measured point has a probability of about 95.5% in a circle with the true coordinate as the center and m as the 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.

Analysis of problems with GPS modules

1. What is the meaning of the 16 channels of the GPS module?

The GPS module (what is GPS) has a parameter of the number of channels. For example, the latest type is 16 channels. It means that the module can establish communication with 16 satellites at the same time. The module only receives more than 3 satellite signals at the same time. After the operation, the correct positioning data can be obtained. If the number of satellites communicating at the same time is more, the module can obtain the positioning data more quickly and accurately. It is possible to simultaneously and 16 satellites on the surface of the earth only under the ideal geographical conditions. Communication, in a typical urban environment, it is usually possible to simultaneously communicate with 4 to 8 satellites simultaneously.

2. Why does the GPS module take a long time to get the positioning data after booting?

The GPS module has two parameters: cold start and hot start. If the GPS module is powered on for the first time, or if the power is turned on after more than 1000 km, the module must recalculate the ephemeris data, usually takes 1 to 5 minutes, and then stores the ephemeris data in Inside the module, the data is saved by the lithium battery inside the module. This is called cold boot and takes a long time. If the next time you turn it on, because the lithium battery has saved valid ephemeris data, it can be quickly located, usually within 30 seconds, so it is called hot boot. If the module is powered off for more than 10 hours, the lithium battery discharges, then Booting is also equivalent to a cold boot.

3. What should I do if I cannot boot up for a long time?

Only in rare cases will be more than 15 minutes can not be cold boot, then generally just power off the GPS module or turn the GPS back, there is a situation that it protects the tube clamp or damage, in an emergency In the case, the protection diode can be removed first for communication operation, and then the specific experiment.

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.

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