A GPS calculates speed by change of position over the time between position readings. Theoretically if in the time between readings I went like a bat out of hell and turned around and returned to my starting position the GPS would report my speed as zero since I hadn't moved. The point is that the GPS is making some assumptions to calculate speed.
First is that the GPS has no way to know how you got from point to point. It assumes a straight line but you may have been traveling in a curve. The faster you are going the more distance you travel in a sampling period and the more it would effect the difference between a straight line and a curved path. If you are traveling in a straight line the accuracy is going to be higher. BTW, curved paths would result in lower speed calculated than reality.
Next, how accurate is your GPS positioning. Since speed is calculated by position and the sampling periods are relatively short, if the positions are accurate the speed calculation will be as well. Accuracy is a attribute of the design of the unit, the installation on your boat (antenna position, etc) and the ability to receive accurate satellite signals from minute to minute. There can be a lot of variability.
|