![]() ![]() ![]() Vehicle speedometers also require calibration to maintain perfect accuracy as general wear and tear, changing wheel sizes, and the manufacturers own ‘erring on the side of caution’ and setting the speedometer to read higher than the actual speed, all contribute to inaccuracies. It is dependent however on GPS satellite signal quality but with the use of moving average calculations any errors are minimal. GPS speed calculations are more accurate than a vehicle’s speedometer since it is not affected by the same inaccuracies, including the vehicle’s wheel size or drive ratios. Why does GPS speed differ from the vehicle’s own speedometer? For example, using a standard movement per time calculation, if you have covered 80 feet in one second, the GPS device works out and converts that to MPH, which in this case is 55MPH. It uses frequent calculations to determine the vehicle’s speed. Speeds are updated at short intervals to maintain accuracy at all times. It should also be noted that the speed reading is normalized, and is not an instant speed. The algorithm also uses the doppler shift in the pseudo range signals from the satellites. GPS devices are positional speedometers, based on how far the device has moved since the last measurement. Using even basic time and location data, a GPS unit can quickly calculate the relative speed of the object, based on how much distance it covered in a given time. Using GPS, a device is able to calculate a lot of information about a moving object. ![]()
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