How To Choose GPS Module Performance?

2022-03-05/ By Admin

GPS module performance indicators mainly include receiving sensitivity, positioning time, position accuracy, power consumption, time accuracy, etc. How should we choose when purchasing?

Sensitivity of the GPS module

With the continuous expansion of GPS applications, the industry has higher and higher sensitivity requirements for GPS receivers. The high-sensitivity reception performance enables the receiver to still achieve positioning and tracking indoors or in other scenarios with weak satellite signals. Greatly expanded the use of GPS.

As one of the most important performance indicators of GPS receivers, high sensitivity has always been the goal of every GPS receiver module. For the GPS receiving system, the sensitivity indicators include indicators in multiple scenarios, namely: tracking sensitivity, capture sensitivity, and initial startup sensitivity.

At present, receivers with tracking sensitivity below -160dBm can be realized in the industry, and the initial startup sensitivity and acquisition sensitivity can also reach below -142dBm and -148dBm respectively.

The GPS receiver first needs to complete the acquisition of satellite signals, and the minimum signal strength required to complete the acquisition is the acquisition sensitivity; the minimum signal strength required to maintain satellite signal tracking after the acquisition is the tracking sensitivity.

In order to realize positioning, the GPS receiver also needs to demodulate the navigation message sent by the GPS satellite. Correspondingly, the minimum signal strength required to demodulate the navigation message is the initial startup sensitivity.

According to the above definition, the tracking sensitivity is the highest, the capture sensitivity is the second, and the initial startup sensitivity is the worst.

From a system-level point of view, the sensitivity of a GPS receiver is mainly determined by two aspects: one is the gain and noise performance of the entire signal path at the front end of the receiver, and the other is the algorithm performance of the baseband part.

Among them, the receiver front end determines the signal-to-noise ratio when the received signal reaches the baseband part, and the baseband algorithm determines the minimum signal-to-noise ratio that can be tolerated in the demodulation, acquisition, and tracking process.

Positioning time of GPS module

The module startup positioning time is very different in different startup modes.

Generally speaking, the cold start time refers to the situation that the module does not save any data that is helpful for positioning, including ephemeris, time, etc., generally nominally within 1 minute;

The warm start time means that there is a newer satellite ephemeris inside the module (generally no more than 2 hours), but the time deviation is very large, generally within 45 seconds; the warm start time means that the shutdown does not exceed 20 minutes, and the RTC When the time error is very small.

Typically nominally within 10 seconds; the recapture time is like a car going through a tunnel and recapturing a 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 place hundreds of kilometers away after positioning, there is an ephemeris inside the module, but this ephemeris is wrong or has no reference significance.

In these cases, the positioning time can take several minutes or more. Therefore, the general GPS module should clear the ephemeris and other data inside the module when it leaves the factory so that the customer can quickly locate the module by cold start after getting the module.

Positioning accuracy of GPS module

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 CEP, that is, circular 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 DRMS , that is, the two-dimensional root mean square 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 satellite clock and orbit differences 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 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.

Power consumption and positioning accuracy of GPS module

The influence of power consumption is mainly due to the design of the hardware and the power consumption of the chip itself.

Positioning accuracy The current civilian accuracy is basically 5 meters. The positioning accuracy of the GPS module depends on many aspects, such as satellite clock and orbit differences 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 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 when different GPS modules of the same manufacturer and the same model are tested at the same time using the antenna diversity, the static drift will be different.

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