The table speed and navigation speed which is more accurate, in fact, depends on the specific use of the car, the table shows that the speed is more stable, the navigation speed is not precise, but also depends on the situation, the upper limit is high, and the lower limit is relatively low.
In principle, navigation is more accurate than a speedometer
Navigation is basically positioning + speed measurement
First of all, from the perspective of principle, the speed displayed by the navigation is actually more accurate.
The principle of navigation to obtain the speed of the car is the formula of S=VT (distance equals speed multiplied by time) learned when reading.
For example: navigation every 0.1s to get our location information, and then according to the distance difference of 2 position information, such as 2m, it can calculate that you moved 20m in 1s, and it is 72km/h for another.
Therefore, as long as the mobile phone signal is normal and can be connected to gps or Beidou satellites, the calculated speed is more accurate.
There is an error in the speed of the table itself, and some models are even more inaccurate
The speedometer is different, and the speedometer of some models itself has errors.
GB 7258-2017 "Motor Vehicle Operation Safety Technical Conditions" Point 4 Article 11 has provisions: the speedometer speed can not be lower than the actual speed, the specific error can not be greater than the actual speed divided by 10 and then added 4.
It sounds complicated, but for example, the speedometer says 120km/h, the actual speed will be between 120 and 104km/h, showing 60km/h, and the actual speed is between 60 and 50.
That is to say: when some models leave the factory, the speedometer that comes with it may not be very accurate, and some friends may joke that this is called "happy meter".
And according to the principle of speed measurement of different models, there are also differences, and the error size is not necessarily.
Sun Tao, Ye Jinfei and others of Anhui Jianghuai Automobile Co., Ltd. published a paper in the journal "Practical Technology of Automobiles", "Methods of Acquisition and Transmission of Automobile Speed Signals", which was said above.
Speed acquisition is generally divided into two ways, one is to obtain the speed signal on the specific shaft of the gearbox, and the other is to use ABS to obtain the wheel speed signal.
At present, most OEMs use abs schemes, so some models that use the old method to obtain speed will have more serious apparent errors.
The error will become larger after the car is used for a long time
In addition to the difference in the principle of speed measurement, as we use the car for a long time, it will also cause the error of the apparent speed to become larger.
Or as mentioned in the previous paper: After ABS reads the wheel speed per unit time, it will calculate according to the tire diameter data to get the speed.
This algorithm is set up when the new car leaves the factory, and if the parameters of the tire size of the tire size of the car we use later have changed, it may affect the speed of the table.
For example, the tires of the car are the same as the eraser after 3 or 4 years, and there will be wear and tear after a long time.
The circumference of this wheel will actually become smaller, and when it is worn off, the error of the speedometer will also be generated, of course, this impact amplitude is generally relatively small.
In terms of stability, the speedometer is better than navigation
Unstable navigation where the signal is not good
When driving every day, which point is better to refer to? In fact, the score of this thing depends on what environment it is driving in.
Meng Zhaogang et al. published a paper in the journal "Electronic Components and Information Technology", "Smart Phone GPS Positioning Accuracy Analysis Experiment", which used mobile phone GPS to conduct a test.
In open areas, GPS accuracy can reach 2 meters; in the complex situation of urban buildings, GPS accuracy is only 10 meters.
To put it simply: downtown navigation is not very reliable, the accuracy can be 10 meters worse, and the measured speed is not so accurate.
This is actually closely related to the mobile phone signal, high-speed rail station, business circle such a place, if you use a fruit mobile phone, the mobile phone two-dimensional code may not be opened, the reason is the same.
There are also some in the mountains, inside the tunnel, because the signal is not very good, the navigation speed is also inaccurate, the car into the tunnel, mobile phone navigation general real-time speed is not even displayed.
Navigation is also unstable with frequent acceleration and deceleration
In addition to the driving environment will have an impact on navigation, driving style will also have an impact on navigation accuracy.
Jiang Owe of Shandong University of Science and Technology has a master's thesis, which is shared with you, "Reliability Research on Mobile Phone A-GPS Navigation Performance", which conducts a test on mobile phone navigation performance under vehicle conditions.
It found that when the speed of the vehicle changes rapidly, the positioning error fluctuation will be relatively large.
To put it simply: if you drive a car you often "bombard" such a big foot throttle one by one, the brake is also "ha" to brake to the end, the speed displayed by the navigation should not be referenced, especially inaccurate.
Complex environments are better by speedometer
So, only on open roads, urban expressways, elevated roads, the car is relatively uniform, or you use fixed speed cruising, right?
The speed displayed by the navigation is more accurate and reliable, even if it is driven against the speed limit, there will not be too much of a problem.
If other slightly more complicated environments you rely on navigation, it is better to eat tickets if you do not want to eat tickets, or the speedometer is better.
How navigation software knows how to get caught ahead
Usually when I go home from work, many friends also want to look at the navigation: which road is blocked a little? I pick a better one, go home faster, how does the navigation know the congestion?
Running a 300km high-speed, some navigation says it takes 5 hours, is it broken? Or how?
Why does Navigation provide us with the service of forecasting time? AutoNavi Map, Baidu Map, which navigation is expected to be more accurate? How did they expect that? What features are we more useful for?
Relevant questions, materials, cases, and practical operations are all prepared for friends.
Keyword: navigation
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bibliography
[1] GB 7258-2017.Technical conditions for motor vehicle operation safety[S].
Sun Tao,Ye Jinfei. Method of acquisition and transmission of automobile speed signal[J].Practical Technology of Automobile,2015.
Meng Zhaogang,Zhang Yan,Niu Xuechao,Meng Aoqi. Analysis experiment of GPS positioning accuracy of smart phone[J].Electronic Components and Information Technology, 2019.
[4] Jiang owes. Reliability study of mobile phone A-GPS navigation performance[D].Shandong University of Science and Technology, 2018.