Re: [Paddlewise] GPS Errors Summary

On 2 Sep 2004 at 21:04, alex wrote:

> This expalins why I sometimes noticed a significant discrepancy
> between the direction shown by my non-compass GPS and actual direction
> to waypoint when paddling.  I thought GPS calculated the direction
> using calculated location of the kayak and known location of the
> waypoint.


The GPS does calculate that way and that bearing's accuracy is 
directly related to the accuracy of the kayak position measurement.  
However, it has to calculate the direction to north (N) from the 
motion of the kayak.  Since slow speed means short distances between 
position determinations and each position can be in error by up to 
15m, the line between two positions can point all over the place.


         x1 <--- 10m ---> y1


      y2 <--6m--> x2

For example, assume your kayak moves from point x1 to point x2.  If 
the error in determination of the first point is right 10m giving y1 
and the error for the second is left 6m giving y2, you can see that 
the apparent direction of motion y1->y2 is very different than true 
direction x1->x2.  If this angular error is used to determine N, 
you'll be way off.  

If you're moving very quickly, the distance from point 1 to point 2 
is larger and the angular error is reduced even if the linear error 
is the same.

Note that the GPS is reading every second or so, so averaging out 
tends to reduce the errors.  The error in estimating N is still 
velocity dependent as is the estimated speed (in the above diagram, 
estimated distance y1-y2 is somewhat larger than true distance x1-x2, 
so the speed will be overestimated).   

Sitting still is the best way to ensure that the estimated position 
of N is bad.  If I turn on the GPS and don't move, the direction that 
my GPS reports it's traveling is usually way off.  The reported speed 
also wanders a lot, rarely stating 0.

Mike

PS - above assumes my usual worst case limits - no WAAS or DGPS and 
GPS turned on briefly.  With WAAS and leaving the GPS running, things 
improve due to smaller errors in position determination and more 
averaging.
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