* Posts by Chris in North Carolina

2 publicly visible posts • joined 26 Oct 2007

California teen offers GPS challenge to speeding rap

Chris in North Carolina

Speed accuracy

@Anne van der Bom

You would be correct if your assumption about how speed was averaged in the receiver was correct. However it is not correct. In your assumption you imply that the errors are cumulative over a long period. They are not.

GPS does use time and distance to calculate speed but it is done over 30 or fewer position fixes. With the average sample rate of 4hz the time to accumulate 30 samples is only a few seconds and not a whole trip. Most modern receivers use a variable number of previous samples to average into the displayed speed. When speed is changing rapidly fewer samples are used to make the reading more responsive. When you are traveling at a steady speed more samples are averaged together to give a more stable reading.

Chris in North Carolina

Speed is more accurate than you might think.

There needs to be a better understanding on how position accuracy affect speed calculations. Most errors in position accuracy do NOT come into play with speed accuracy. Sounds totally illogical I know but it is true. This is because most position inaccuracies are caused by atmospheric conditions. Another by terrestrial interference. The ones caused by atmospheric conditions are a gentle drift and affect the position relatively slowly and affect all samples by similar amounts. This type of interference has virtually NO impact on speed accuracy.

If you are in a city or have other natural things around like hard mountains, you can suffer terrestrial interference. This tends to happen very quickly and cause a very rapid deviation in position. This one can cause a large spike in speed but tends to only last for a couple of refresh cycles.

Another issue with accuracy is actually satellite geometry. If the satellites the receiver is using are all close together in the sky the math gets much harder because the angles are so small. If the satellites are overhead as well as near the horizon the accuracy in position and speed will be much higher. If you find your speed jumping around more than usual check the DOP reading in your receiver. You will likely find it is high or jumping around itself. This would indicate a geometry issue with bad satellite position relative to where you were.

Newer GPS receivers have filtering for terrestrial interference. They will automatically remove any samples that are considered outliers. The slow drift is also correctable here in the states if the GPS receiver has WAAS (Wide Area Augmentation System) ability. This system enhances position accuracy as well as accuracy speed accuracy.

Expect a GPS to report your speed accurately within about 0.3mph under good satellite geometry and low terrestrial interference conditions. Plenty accurate to beat a traffic radar.