Technology based questions

Many of you should have the experience of the police radar gun. It is the most common example which is referred for the Doppler shift lesson in A/L physics. The GPS receivers use the same mechanism for velocity measurements. In addition to the calculation through Doppler shift, the GPS system got the distance traveled within a given time period. This time period (tracking interval) can be configured through the settings of the GPS tracker and the minimum tracking interval depends on the advancement and the manufacturer of the tracker. When there is the distance travelled within a given time, the velocity can be calculated.
In the vehicle speedometer, the indicated speed is a result of the driveshaft rotations. This is directly affected by the wheel size. The wheel size changes when the tyre pressure and thread wear changes. When the diameter of the wheel is smaller than the original factory settings, the speedometer indicates a faster speed than the actual speed. Therefore, most of the time, the speed indicated in the speedometer is faster than the GPS speed which is more accurate one. If there are no obstacles like concrete buildings and if no electromagnetic interferences, the GPS speed is the most accurate speed measurement for the vehicles.

GPS mileage is a measurement on a 2D surface. It doesn’t have the knowledge of the nature of the surface which the vehicle travels, like hills and slopes. It calculates the direct distance between the location points of the messages sent to the tracking server.

Vehicle mileage is a calculation based on wheel rotations. Therefore, the problem of not knowing the nature of the surface is not applicable to that mechanism.

There is a possibility of GPS mileage to miss some distance in sharp bends, hills and slopes because of the above reason. Since it is an error associated with the technology, the only way to minimize the error is to make the tracking interval minimum. However, even with the minimum tracking interval, there can be an error in cumulative mileage around 3%. When considering a single trip mileage, there are some times that the GPS mileage is 100% equal to the actual mileage. It happens when the trip is along a route with less bends and hills. Sometimes the error is high for a single trip mileage when the vehicle is traveling up country.