Editing Speedometer (section)

== GPS ==
{{Main|Automotive navigation system}}
[[GPS]] devices can measure speeds in two ways:

# The first and simpler method is based on how far the receiver has moved since the last measurement. Such speed calculations are not subject to the same sources of error as the vehicle's speedometer (wheel size, transmission/drive ratios). Instead, the GPS's positional accuracy, and therefore the accuracy of its calculated speed, is dependent on the satellite signal quality at the time. Speed calculations will be more accurate at higher speeds when the ratio of positional error to positional change is lower. The GPS software may also use a [[moving average]] calculation to reduce error. Some GPS devices do not take into account the vertical position of the car so will under-report the speed by the road's gradient.
# Alternatively, the GPS may take advantage of the [[Doppler effect]] to estimate its velocity.<ref name="ono-sokki-gps-speed">{{cite web|url= https://www.onosokki.co.jp/English/hp_e/products/keisoku/automotive/lc8_principle.htm |title=Principle of speed measurement using GPS |access-date=2020-06-27}}</ref> In ideal conditions, the accuracy for commercial devices is within 0.2–0.5&nbsp;km/h,<ref name="ono-sokki-gps-speed"/><ref>{{cite web|url= https://www.theglobeandmail.com/globe-drive/culture/commuting/whats-more-accurate-the-cars-speedo-or-the-gps/article4348596/ |title=What's more accurate: the car's speedo or the GPS? |work=The Globe and Mail |date=17 November 2010 |access-date=27 June 2020}}</ref><ref>{{cite web|url= https://www.vboxautomotive.co.uk/index.php/en/how-does-it-work-gps-accuracy#gps-factors |title=GPS Accuracy |access-date=28 June 2020}}</ref> but it may worsen if the signal quality degrades.

As mentioned in the [[automotive navigation system|satnav]] article, GPS data has been used to overturn a speeding ticket; the GPS logs showed the defendant traveling below the speed limit when they were ticketed. That the data came from a GPS device was likely less important than the fact that it was logged; logs from the vehicle's speedometer could likely have been used instead, had they existed.