Velocity showing in GPS-Navigation Systems

gpsnavigation

How exact is the velocity that is showing in a GPS Navigation System like Navigon or TomTom?

Best Answer

This depends upon a large number of variables that are imposed by the GPS navigation system in use and by the processing of the data by the SAT-NAV following the determination of the position fix. These include

  • The interval between position fix solutions. This is normally 1 second.
  • The position accuracy of each of the positions used to determine the velocity. This depends upon the quality of the sky view and, to a certain extent, the length of time that the GPS has had a sky view.
  • The number of positions used to calculate the velocity
  • The acceleration of the unit
  • Filtering algorithm used to smooth the velocity value

The velocity value from the GPS receiver is calculated from the difference between position fixes. As these are typically generated at a 1Hz rate you are measuring a distance travelled in 1 second. The position fixes can have an error of a number of metres, with successive positions wandering from the true position, sometimes by a metre or more for a static unit. I have had static units using high end GPS chipsets that have apparently had a velocity of up to 2 m/s while fixed to my office building.

Some units may filter the velocity value or use more than 2 position fixes to calculate the antenna velocity. This will add a lag to the velocity value shown as the antenna accelerates.

Providing a definitive answer is not possible in just the same way that defining position accuracy has to be bounded by many caveats around the sky view and number of satellites visible.

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