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what does a car's speedometer measure
The speed displayed by the car speedometer is the magnitude of the instantaneous speed. There are three general principles of speed measurement.
Pulse device:There is a pulse device on the tire, which sends out a pulse every time the tire rotates. The number of pulses per unit time multiplied by the circumference of the tire is the speed.
Radar:With pulses, different reflection times represent different distances, and the ratio of the distance difference to the time is the speed.
Radar or radio:In places where there is no reflection condition, using the ground radio to transmit signals with moving objects, there is time to obtain the distance between the two, and the speed can also be obtained by comprehensively calculating the signal's sedan.
How is the navigation speed calculated?
We all know that more and more people rely on map navigation to travel, so whether it is Baidu or AutoNavi, these are displayed by their own speed. So how does the map navigation software realize the speed display through a software? So in fact, the way to calculate the speed of the map navigation is very simple. That is, the satellite navigation system uses multiple satellites as time signal transmitters. Finally, let the navigation software measure the three-dimensional data of the car's longitude, latitude and altitude, and then use the atomic clock on the satellite to obtain the precise time, and then use the travel time and travel distance to finally calculate our actual speed. For example, our real speed is 100km/h, then according to the formula, the maximum displayed speed is 100/10+4, and then the actual speed of 100 is added, that is, the displayed speed is between 100 and 114km/h. It is in line with our national standards. In fact, different manufacturers have different final results, but our displayed speed is basically larger than the actual speed. Then the speed measuring radar of the traffic control department is generally composed of a speedometer and a camera, so the speedometer is actually mainly based on the "Doppler effect principle", that is, when the vehicle approaches the speed measurement area, it will be multi-frequency at this time. The vehicle emits ultrasonic waves, and finally, according to the change of the reflected wave frequency, the speed of our vehicle can be accurately measured. In fact, the general speed measurement radar monitoring range is within 30 meters, so it is not the instantaneous vehicle speed measurement, so we need to pay attention.
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