Dilution of Precision
Here is my plotter in normal operation
The velocity of the boat is 4.4 knots at 137T. The dotted line predicts the future course over the ground.
Having sailed across to Weston in a very pleasant F3 and whilst still pleasantly underway the plotter decides that we are barely moving at all. The Horizontal Dilution of Precision is 49. Large DOP numbers are not good.. It appears that the arrangement of satellites above could not resolve our postion in two directions mutually at right angles. Hence the machine reports 0.1 knots. A rare occurrence but interesting. Back to the hand bearing compass and dead reckoning!